Regardless of the application, if you’re building or rebuilding an LS engine from scratch and intend to boost horsepower, it just makes sense to purchase a set of forged pistons to eliminate the potential variable. Aftermarket performance piston manufacturers offer a wide selection of forged pistons for the LS engine format, in a variety of popular bore sizes, compression heights, and compression ratios. Check brands such as JE, Wiseco, Diamond, Ross, CP, and others.

Piston Compression Height

Compression height, also called compression distance or piston CD, refers to the wrist pin bore centerline to the piston deck. If you’re planning to build a stroker engine, the combination of crankshaft stroke, block deck height, connecting rod length, and piston compression height is relative to the piston deck’s location to the block deck at top dead center (TDC).

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As an example, let’s say that the block deck height is 9.240 inches, which is the GM factory spec for LS deck height. Block deck height refers to the distance from the main bore centerline to the block deck. However, OEM blocks are rarely machined properly. The raw block may be slightly less or slightly more than 9.240 inches, decks may be finished nonparallel to the main bore centerline, and deck height may differ from the front to the rear of the block. The block should be corrected prior to selecting components to verify the final corrected deck height. But for the purposes of theory, let’s say that our block’s deck height is 9.240 inches.

For the purposes of this example, our crankshaft stroke is 4.000 inches. To determine where our piston dome will be located relative to the block deck, we need only factor half of the crankshaft stroke, from the rod journal’s top-dead-center location to the block deck. So, in this case our stroke factor is 2.000 inches. At this point we know that 2.000 inches of our 9.240-inch deck height is taken up by the crank stroke. The remaining theoretical distance of 7.240 inches must be achieved by the combination of our connecting rod length and our piston compression distance. Rod length refers to the distance from the centerline of the rod big end to the centerline of the rod’s small-end bore. By selecting a rod length of 6.125 inches, this leaves a required piston compression distance of 1.115 inches. Formula:

1/2 Stroke + Rod Length + Piston CD = Block Deck Height

In this example:

2.000 + 6.125 + 1.115 = 9.240

This would theoretically place our piston dome flat flush with the block deck.

With block deck height, crank stroke, and rod length already known, to determine piston CD is easy using the following formula:

Block Deck Height – (1/2 Stroke + Rod Length) = Piston CD

Using the previous example:

9.240 – (2.000 + 6.125) = 1.115

Aftermarket performance forged pistons may be ordered in a range of compression distances. Some piston makers offer a specific range of compression heights, while some makers will custom-machine your pistons to whatever compression height you require, as long as the pin bore will not intersect the second compression ring groove.

If the block decks have been machined to a height less than 9.240 inches, the pistons may protrude a few thousandths above the decks. To obtain adequate piston-to-valve clearance, these clearances must be checked, factoring in total valve lift and cam duration, rocker arm ratio, intake and valve diameters for radial clearance, and thickness of the cylinder head gasket. If during test fitting and measuring, valve clearance is not adequate, a thicker head gasket may be selected to compensate. Another option is to have the piston dome’s valve pockets milled to a larger radius if the valve head diameter is a tick too large for radial clearance. As far as valve-to-piston clearances are concerned, a minimum recommended clearance for intake valves should be .080 inch, with a minimum of .100 inch for the exhaust valves. This clearance is even more of a concern with an aluminum block, which has a greater potential for thermal expansion.

When checking valve clearance with head gaskets installed, the gaskets must be crushed as they would be during final assembly. Multi-layer steel (MLS) head gaskets can easily be measured for thickness by measuring thickness adjacent to the gasket rivets, where the layers are already fully compressed. MLS gaskets are available in a variety of thicknesses, generally in the .041- to .045-inch range. However, some gasket makers, such as Cometic, can supply custom MLS gaskets in a much wider range of thicknesses. For example, if a compressed gasket thickness of .045 inch does not provide adequate clearance, a gasket thickness of .051 inch may be obtained. This is just an example.

Before ordering oversize pistons, it’s wise to measure the cylinder wall thickness in all cylinders using an ultrasonic thickness gauge. This will aid in determining how far the boring/honing oversize can go. Measure at a variety of bore height and clock positions, as wall thickness may vary.

To place the piston at TDC relative to the block’s deck height, with certain crankshaft stroke and rod length variables, the piston compression height may dictate that the pin bore be raised, intersecting the oil ring groove. This requires the use of an oil ring support rail that completes the footprint for the oil ring package over each end of the pin bore.

A dial indicator aids in measuring the precise top-dead-center (TDC) position of the piston. With the piston at TDC, a depth micrometer can be used to measure deck height relative to the block deck, to determine if the piston is below, flush with, or above deck.

The support rail features a male pimple that prevents the rail from rotating and may prevent placing the rail gap at one of the two relief areas. When installing the rail, the male dot must be placed at either relief opening. Support rails are included in the oil ring package for pistons that require these rails.

Depending on stroke and rod length, a shorter piston CD may be required to keep the piston flush or just below the block deck. In many cases, shortening the piston CD, which involves moving the wrist pin bore upward, results in the pin bore intersecting with the oil ring groove. In this case, the oil ring groove will have been machined taller to accommodate both the oil ring package and a “support rail,” which serves to complete the footprint for the oil ring package at each side of the wrist pin bore.

Bore Size, Dome, and Compression Ratio

The available combinations of cylinder bore diameter, piston dome shape, and static compression ratio for LS engine applications are quite extensive. The standard bore size for LS1 and LS6 formats is 3.898 inches. The standard bore size for LS2 is 4.000 inches. Bore size for LS3 is 4.065 inches, and LS7 standard bore size is 4.125 inches. Oversizes are available in a variety of diameters, depending on whether the block is a standard bore and whether it is an iron or an aluminum block. While iron blocks allow a degree of overboring, aluminum blocks with their integrally cast-in sleeves allow for only a slight bit of honing oversize. For example, an aluminum LS1 or LS6 block will accept an oversize up to only around 3.905 inches. Aluminum LS2 blocks will accept honing oversize to about 4.030 inches. LS3 aluminum block cylinder bore oversizing is limited to about 4.080 inches. LS7 blocks may be oversized to about 4.130 inches. Iron blocks may accept a further increase, but cylinder walls should first be measured for thickness using an ultrasonic thickness gauge to verify the minimum available wall thickness. Finished wall thickness generally should be limited to a minimum of about .180 to .200 inch. Again, as a generalization, an LQ9 6.0L iron block should accept an overbore of .060 inch, although an overbore of .030 inch would be safer.

An example of a flattop piston dome. Depending on the cylinder head combustion chamber volume, choosing piston dome volume in combination allows you to tailor the desired compression ratio.

If you plan to use forced induction to boost cylinder pressure, stay away from a theoretically minimum wall thickness. The thicker, the better. Because of the tolerances involved in both aluminum block cast-in cylinder liners and iron block castings, cylinder wall thickness may not be uniform from top to bottom or around the circumference of the bores. Before oversizing, measure existing wall thickness from top to bottom and at a variety of clock positions on each cylinder location.

Off-the-shelf dome configurations include flattop, inverted dome, and domed. Flattop pistons will feature intake and exhaust valve relief pockets. Inverted dome pistons feature a relieved dome “bowl,” while domed pistons feature a raised dome area. Obviously, the volume of the dome affects compression ratio. Inverted domes provide a lower compression and domed provide a higher compression ratio. Obtaining the desired compression ratio is not limited to the piston itself. The combination of piston dome volume and cylinder head combustion chamber volume work in unison to create static compression ratio.

High-dome pistons obviously create a smaller combustion chamber volume, increasing compression ratio compared to a flattop dome.

Piston makers offer comprehensive charts that list the cubic-inch displacement and static compression ratio for any given combination of cylinder bore diameter, crank stroke, rod length, piston compression height, block deck height, piston dome volume, and combustion chamber volume.

While we don’t have the space here to list all possible combinations for all LS platforms, citing an LS2 aluminum block as an example, a cylinder bore oversize of 4.005 inches coupled with a crank stroke of 4.000 inches would provide 403 ci of displacement. To mate with the 9.240-inch block deck height, rod length would be 6.125 inches and piston compression height would be 1.115 inches.

Using a flattop piston with a dome volume of –5 cc and a cylinder head with a combustion chamber volume of 64 cc would provide a compression ratio of approximately 11.6:1. With a combustion chamber volume of 68 cc, compression would be about 11.3:1. With a 70-cc combustion chamber, compression would be about 10.8:1.

An example of an inverted dome piston for an LS3 application. The JE forged piston shown here features a .097-inch-deep inverted dome and 12-degree valve relief pockets. The lines you see here are simply surface shadows left by the CNC-milling under studio lighting. The dome surface is actually flat and smooth. Dome volume is –14.6 cc. Using an inverted dome allows a reduction of compression ratio compared to a flattop piston.

Using the same combination of parts but with an inverted dome piston that has –7.6 cc of volume, using a 64-cc combustion chamber would provide about 10.9:1. With a combustion chamber of 68 cc, compression would be about 10.5:1. With a 70-cc combustion chamber, compression would be about 10.3:1.

Again, using the same above combination but with a domed piston that has 5.0 cc of volume, a 64-cc chamber would provide about 13.2:1. A 68-cc chamber would provide about 12.5:1, and a 70-cc chamber would provide about 12.2:1. Depending on bore size, piston dome configuration, and cylinder head combustion chamber volume, pistons for LS applications may be selected to achieve anywhere from about 8.1:1 to 13.2:1 static compression ratio.

Keep in mind that if you plan to run forced induction, the static build compression ratio will dramatically increase at effective compression when boost is added. For example, with a Roots-style supercharger, a static ratio of 8.0:1 can rise to an effective ratio of 11.3:1 with only 6 psi of boost, and to more than 15.6:1 with 14 psi boost. Generally speaking, for street applications running aluminum heads and 92 octane fuel, you should try to keep effective compression to about 12:1 maximum. Suggested limit examples are as follows:

Piston Selection of KS7 Flattop Pistons

To demonstrate piston selection based on stroke, bore diameter, rod length, piston compression height, and cylinder head combustion chamber volume, following are examples of LS7 flattop pistons that feature a dome volume of –2 cc. Note: “CD” stands for piston compression distance and “CR” stands for compression ratio. All results are based on a block deck height of 9.240 inches. To illustrate one piston maker’s offerings, all examples here are listings by JE Pistons.

The following examples are based on LS2/LS3 builds, with choices of three different cylinder head combustion chamber volumes. Bear in mind that an incredibly diverse selection of pistons is available for these applications. Here I’ve listed only a few. These examples feature flattop pistons with a dome volume of –5 cc.

As noted, the examples from JE Pistons represent only a small number of build variations. Depending on stroke, bore diameter, rod length, piston compression height, combustion chamber volume, and piston dome volume, off-the-shelf forged pistons are readily available to achieve compression ratios of 8.0:1 to 13.2:1. Obviously, flattop or inverted dome pistons and/or larger combustion chambers can achieve lower compression ratios. Whole higher ratios can be had with small combustion chambers and/or higher-domed pistons.

Skirt-to-Wall Clearance

The engine block’s cylinder bores should never be final-finished until you have your intended pistons in hand. The piston skirt diameter dictates the required finish-honed bore diameter. Measure skirt diameter and add the wall clearance recommended by the piston maker to finish-hone the bores. It is critical to measure the piston skirts at exactly the location specified by the piston maker because piston skirts feature a very slight taper. As an example, JE forged pistons are usually measured exactly .500 inch from the bottom of the skirt, unless the measuring location is specified otherwise. The measuring point varies depending on the specific piston design. Add to this the recommended piston clearance to final-hone the cylinders. It’s important to note that piston diameters are not measured at the ring area, but at a specific point on the skirts. A general rule of thumb for forged pistons is .001-inch clearance for every inch of cylinder bore diameter. For example, a 4.000-inch bore would require .004-inch piston-to-wall clearance. Again, this is a generic specification. Always adhere to the clearance recommendation provided by the piston maker.

Before finishing the cylinder bores to size, the piston skirt diameter must be measured to determine the bore diameter required for the recommended piston-skirt-to-wall clearance. It is critical to measure only at the skirt area specified by the piston maker.

Piston and Rod Orientation

The installed direction of pistons to connecting rods is critical. Let’s consider the rod orientation. If the big end of the connecting rod features a larger chamfer on one side, this side must be installed facing the crankshaft’s journal radius fillet. If the rods are designed for use on a crank that does not feature a radi-used fillet, the rods may not feature a large chamfer on one side.

Valve pockets can be used as reference as well when orienting the pistons. The larger intake valve pocket faces the front of the engine on the left-side cylinder head, while the intake valve pocket faces the rear of the right-side head.

Original equipment hypereutectic flattop pistons feature a dot on the dome. The dot indicates that the piston is installed with the dot facing the front of the engine on the right bank and with the dot facing the rear of the engine on the left bank.

If there is no noticeable chamfer on either side of the rod big end, the bearing placement on the side of the rod that faces the fillet should be slightly spaced away from the fillet to prevent the bearing from digging into the fillet radius.

Skirts and Major/Minor Thrust

The shape, area of mass, and weight of a piston’s skirts play a major role in managing friction and in stabilizing the piston during TDC and BDC transitions. Here we’ll discuss the role of the major and minor thrust sides of a piston and the development of asymmetric skirt designs intended to minimize weight while maximizing efficiency.

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Piston skirts are not perfectly round, and each side of the piston experiences different levels of loading relative to the intake and exhaust sides of the cylinders. Skirt design plays a major role in accommodating these forces in ways that improve durability and performance; so does piston weight. The piston skirt area is slightly barrel shaped to provide an adequate surface load against the cylinder wall while reducing friction. The amount of surface area must accommodate the load while providing piston stability to minimize rocking relative to the pin axis as the piston moves down from TDC and back up from BDC. The piston experiences a “major” and “minor” thrust force at opposing sides of the piston skirts. The major thrust face is the side of the piston that receives the thrust on the power stroke. As viewed facing the front of the engine, if the crankshaft is rotating clockwise, the major thrust face is on the left side of the cylinder (the exhaust sides of the right/passenger-side cylinders; and the intake sides of the left/driver–side cylinders). The minor thrust side experiences force on the compression stroke.

This piston’s minor thrust side skirt design provides great contact and support for the thrust side where it’s needed, and a small skirt for the minor side, where it’s less critical, saving weight in the process.

This difference in force at each side of the piston is caused in part by the operating angles of the connecting rod during its travel. During the firing cycle, the load experienced on the major thrust side skirt can be as much as 10 times greater than the load experienced on the minor thrust side skirt. The difference in skirt loading will vary depending on variables such as crankshaft stroke, connecting rod length, and peak cylinder pressures.

Asymmetric pistons are bank specific, and thus each piston is labeled for right or left bank position. The dome may also feature a laser-etched arrow that indicates piston orientation toward the front of the engine.

Asymmetric pistons feature a wider skirt at the major thrust side and a smaller skirt at the minor thrust side. The underside view clearly shows the difference.

Major Thrust Side

When the piston is pushed down during the power stroke, it experiences resistance as it attempts to turn the crankshaft. As load increases, the amount of resistance increases. During this resistance, the piston side load is forced to one side, and that’s the major thrust side. It places more force and subsequently increased friction and potential wear on the thrust side of the cylinder wall. If the piston dome features a reference dot or other orientation mark, it’s critical to install the piston with this mark facing the appropriate direction. Usually the mark indicates the side of the piston that should face forward. The piston side loads on the major side tend to increase with the use of a longer stroke and with forced/boosted induction pressures. Again, assuming a clockwise-rotating crankshaft, the major thrust side will be at the exhaust side of the engine’s right bank and the intake side of the left bank.

Minor Thrust Side

The piston’s minor thrust side is directly opposite the major thrust side. The minor thrust side is forced to the opposite side of the cylinder wall as it moves up on the compression stroke by the resistance generated by meeting the air/fuel mixture. The role of the minor thrust side is basically to provide piston stability, with the major thrust side taking the brunt of the cylinder wall contact. Due to its “less force” role, the minor thrust side skirt can be narrower, saving weight without sacrificing strength.

To address, or “fine-tune,” these forces between the major and minor thrust sides, asymmetric pistons have been developed that feature two different-size skirts.

This style of piston is specifically designed with a larger (wider) skirt on the major thrust side and a smaller skirt on the minor thrust side. This provides a greater “footprint” for the major thrust side, where it’s needed the most to handle a higher degree of thrust loading, and allows the piston weight to be slightly reduced by featuring a small footprint on the opposite/minor thrust side, where the force is less. During the power stroke, when the piston changes direction at top dead center, combustion pressure pushes the piston down and at the same time pushes the thrust side of the skirt toward the cylinder wall.

Citing JE Pistons’ “asymmetric” design as an example, in its forged side relief (FSR) line, a wider skirt area is featured on the major thrust side, and the pin bosses are relieved at the outboard sides to allow the use of a shorter (and lighter) wrist pin.

The asymmetric design approach was initially developed for specific racing applications, but the concept has trickled down to street applications, with the LS platform as a good example.

Another benefit to the asymmetric approach is increased piston ring sealing and ring stability thanks to the skirt mass and profile. Basically, the dedicated major and minor thrust skirt design coupled with a slightly offset wrist pin directly addresses ring performance in addition to reduced wall friction.

Offset Pin

Asymmetric pistons also feature an offset wrist pin, with the pin centerline biased from zero toward the major thrust side by .020 inch. This slight offset tends to balance the piston to accommodate the difference in skirt mass and to compensate for and alter the effect of rod angle, transferring a bit of force away from the major thrust side.

This finite element analysis (FEA) view shows even stress forces at both major and minor thrust sides (note the dark dome areas), even though skirts differ in area. The offset location of the wrist pin aids in balancing out the pivot point. (Photo Courtesy JE Pistons)

FEA takes a snapshot of the piston’s stress levels at the worst-case scenario, which differs greatly from an engine that’s at part throttle. The image plots the stress level. The high-stress areas are shown in red in accordance with the chart on the right. This is a simulation of stress under firing. (Photo Courtesy JE Pistons)

Again citing JE’s development in this area, the asymmetric design allows the use of shorter, stiffer, and lighter wrist pins. According to JE, a typical weight savings is about 10 grams.

Note: The contact pressure FEA images here show contact pressure specifically between the skirt panel and the bore. It’s important to analyze both skirt profiles on an asymmetrical piston design even though the minor thrust experiences much less pressure. On a symmetrical design, typically only the major thrust is analyzed. Stress images show how the stress at the skirts affects the rest of the piston.

The stress of the piston under major stress. (Photo Courtesy JE Pistons)

Piston Coatings

A variety of coatings is available for both internal and external engine components, with applications varying from power enhancement, engine efficiency, and durability/longevity to corrosion protection and external appearance. Here we’ll focus on coatings that apply to pistons. Today’s coatings include those designed to reduce friction and enhance lubricity and thermal barrier coatings for increased engine efficiency and protection of components exposed to extreme heat levels.

Thermal Barrier Coatings
Thermal barrier coatings (which are intended to serve as a heat shield) feature a ceramic formulation designed to prevent excess heat from passing into and being absorbed into the piston domes. This theoretically increases combustion efficiency and reduces piston dimensional changes that may occur due to thermal expansion.

The specific formulas differ depending on the coating manufacturer and the application. Specialized thermal barrier coatings applied to piston tops aid in reflecting heat, reducing the amount of piston expansion (allowing the builder to maintain more consistent wall clearance), and protecting the piston from extreme temperatures encountered in forced-induction systems. For forced-induction and nitrous applications, highly specialized thermal barrier coatings are available not only to enhance combustion efficiency but also to protect the piston from potential heat-related damage.

Bear in mind that the quality of application of a ceramic coating (especially for internal engine components) is extremely critical. The last thing you want is for hard and potentially damaging ceramic to break loose and contaminate the engine. That’s why you need to use established coating services, such as those offered by Swain Tech Coatings, Polydyn, and others. When properly applied, the coating not only adheres to the applied surfaces but permanently bonds to the material, essentially becoming part of the base material. If substandard application practices are used, the coating may not be fully bonded and could flake off. In other words, don’t try this at home.

While building engines for 24-hour endurance racing, I’ve had many dozens of engine build components ceramic coated (heads, pistons, valves, exhaust) with absolute success, but I’ve seen a few devastating issues with components that were “coated” by inexperienced mom ’n’ pop shops that caused serious damage. Don’t be afraid of ceramic internal coatings; the top shops do an outstanding job, and they know what they’re doing. Don’t be tempted to go for bargain-basement services.

Anti-friction piston skirt coatings are available already installed on new pistons, as shown in this example, or by sending your pistons to a specialty coating service.

Anti-Friction Coatings

Anti-friction coatings are applicable to surfaces that make or potentially make contact, such as (but not limited to) main bearings, rod bearing, cam bearings, and piston skirts. Anti-friction coatings, also called lubricity coatings, provide a temporary lubricity when/if the surface is starved for oil, upon cold starts, and during piston rock when transitioning from top dead center to bottom dead center. This type of protective coating also serves to improve oil retention on the surface. While specific antifriction coating formulas vary among the coating services, the materials are generally composed of moly-, graphite-, or Teflon-based materials. The intent is to provide better oil retention and to provide a super-slippery surface.

Specialty coating options for pistons include a thermal barrier dome coating, anti-friction skirt coating, or both.

Piston skirt coatings are generally applied at an average thickness of about .0005 inch per side, which might provide about a .001-inch increase in piston overall skirt diameter. The moly has been applied in such a thin layer, no additional bore dimension changes are required to run “moly-coated” pistons. Unless otherwise instructed by the piston maker, do not compensate for the added moly coating when finishing your bores. A specialty coating service may apply anti-friction piston skirt coatings or, depending on the piston manufacturer, new pistons are often available with the coatings already applied. While you may or may not need a skirt coating, there’s no downside to this application. A skirt coating won’t hurt, and it may very well help to prevent skirt wear when called upon.

Written by Mike Mavrigian and republished with permission of CarTech Inc

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The camshaft is one of the major determining components of the effective operating range of the engine. Of course, cam timing must be combined with the proper intake manifold, head flow, and primary length on the exhaust for optimum operation over a given RPM range, but the right cam can almost determine the character or personality of the engine. Stock or ultra-mild aftermarket cams provide a dead-smooth idle, while more radical grinds can transform that mild-mannered LS engine into one radical ride.

The factory LS3 is a common upgrade for 4.8 and 5.3 applications because those lesser LS engines were equipped with the mildest factory cams ever offered. Of course, the high-performance LS7 was factory equipped with the most powerful cam ever offered on an LS, but the stock stuff is just begging to be replaced.

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The LS3 and LS7 are fantastic engines, offering an impressive combination of power, reliability, and even fuel mileage. Another area where they excel is how well they respond to performance upgrades, especially camshafts. These factory performance engines respond so well to wilder cam timing because they have everything else required to make power, including displacement, intake, and head flow. All that is lacking to dramatically improve the power output of a typical LS3 or LS7 is cam timing.

The single best (and most powerful) modification you can make to your LS3 or LS7 engine is a cam swap. These engines already feature a good intake and plenty of head flow, so all they need to make amazing power is more aggressive cam timing.

With the factory heads and intake already capable of supporting more than 600 hp (700 hp on the LS7), mild cams are definitely the limiting factor. Given this situation, cam upgrades for LS applications have become hot sellers. Plop just about any cam in an otherwise stock LS and watch the power soar. I have seen power gains of 65 to 70 hp from a simple cam swap on an otherwise stock LS application. The gains can be even greater higher in the rev range.

There is, of course, a limit to how wild you can go with cam timing on an otherwise stock LS (3 or 7) engine. Although the LS duo certainly responds to more aggressive cam timing, two limitations are inherent in the stock combinations. First, stock valve springs were designed for stock cams and are, therefore, insufficient for performance use. From available valve lift and RPM potential standpoints, spring swaps are not just a good idea, they should be considered mandatory for most cam upgrades on an LS3 or LS7.

The other limiting factor in terms of cam timing on a stock LS3 or LS7 application is available piston-to-valve clearance. Although lift plays a minor role, the real culprit in piston-to-valve clearance is duration (how long you hang that valve open). Each successive increase in duration (the intake hits before the exhaust) decreases the available clearance. Cams that exceed 230 degrees of intake duration should always be checked, especially if they were ground with a few extra degrees of advance.

One of the most common questions regarding camshafts is which one is right for your LS combination. The term “right” here obviously has different meanings for different people, so choosing the so-called right cam can be difficult for even cam experts. The difficulty comes not in the technical nature of cam timing or profiles, but in deciphering exactly what you want. This becomes even more difficult when you are unsure.

LS cams are available in single- and three-bolt configurations. If you are upgrading an LS3 and select a three-bolt cam, make sure you have the matching three-bolt cam gear

Asking enthusiasts what cam they want is a little like asking them how much power they want. The problem is that they want as much power as possible and they also want a factory-smooth idle, 50 mpg, and maintenance-free operation. It goes without saying that it is not possible to combine all of those elements. Obviously that is a lot to ask of any camshaft and, ultimately, trade-offs become necessary. The question then becomes how many of the trade-offs you are willing to accept in your quest for power. Luckily for LS3 and LS7 owners, it doesn’t require much in the way of cam timing to make a major difference in power.

Factory hydraulic roller lifters work well, but there are aftermarket performance units available, including retrofit, short travel, and even solid roller versions.

The factory LS3 and LS7 feature offset intake rockers. The LS3 shares the 1.7-ratio, exhaust rocker with cathedral-port LS applications, but the 1.8-ratio LS7 is specific to that cylinder head configuration.

Test 1: Stock LS3 vs Comp Cams 281LRR on a Modified LS3

Cam swaps are popular for the LS family, especially the LS3, for good reason. Nothing adds power to an LS3 like a cam swap. Short of power adders or a stroker engine, no modification equals the power gains offered by a cam swap. This is because an LS3 already has sufficient displacement, intake, and (especially) cylinder head flow to make serious power. The only thing missing from the combination is cam timing. Add the right cam to an LS3 and watch the power needle climb.

The power gains are even more impressive when you further increase the power potential with ported LS3 heads such as the ones from Chevy Performance run on this LS3 crate engine. Chapter 2 illustrated that an added ported head to a stock engine offers very little in the way of extra power, but adding a cam to a combination with ported heads shows big gains.

This test engine was a GM Performance LS3 crate engine from Gandrud Chevrolet upgraded with GM Performance CNC L92 heads, ARP head studs, and a manual throttle body. The LS3 also featured Lucas 5W-30 synthetic oil, long-tube headers, and a Holley Dominator EFI management system.

Run first with the stock LS3 cam, the LS3 produced 503 hp at 5,500 rpm and 497 ft-lbs of torque at 4,600 rpm. The GM Performance L92 heads were supplied with stock LS3 springs, so it was necessary to install a set of dual springs from BTR to test the cam. After replacing the stock LS3 cam (and springs) with the Comp 281LRR cam (.617/.624 lift split, 231/239 duration split, and 114 LSA), the peak numbers jumped to 569 hp at 6,500 rpm and 522 ft-lbs of torque at 5,200 rpm. There was little change in power below 4,000 rpm, but the gains increased thereafter with engine speed.

The stock LS3 cam featured a single-bolt for the cam gear, but the Comp cam was a three-bolt design (Comp offers single-bolt cams as well). The three-bolt cam requires a 4X, three-bolt cam gear.

Here are the stock, single-bolt, 4X LS3 cam gear; the 4X three-bolt (LS2) gear; and an early 0X truck gear (cam sensor in rear of cam).

Stock LS3 vs Comp Cams 281LRR on a Modified LS3 (Horsepower)

Stock LS3 Cam: 503 hp @ 5,500 rpm Comp 281LRR Cam: 569 hp @ 6,500 rpm Largest Gain: 70 hp @ 6,400 rpm

The most amazing thing about the power generated by the Comp cam swap was not that it added a ton of power (it did), but that the amazing gains came with no loss in power at 3,000 rpm. The gains from the cam swap increased with engine speed and peak with 70 hp at 6,400 rpm.

Stock LS3 vs Comp Cams 281LRR on a Modified LS3 (Torque)

Stock LS3 Cam: 497 ft-lbs @ 4,600 rpm Comp 281LRR Cam: 522 ft-lbs @ 5,200 rpm Largest Gain: 57 ft-lbs @ 6,100 rpm

What I like about adding a cam to an LS3 combination is that the amazing top-end power gains come with no penalty in low-speed torque. In fact, torque gains occurred just below 4,000 rpm and increased with engine speed. Larger cam profiles start to trade off low-speed torque for possible gains in peak power.

Test 2: Stock LS3 vs BTR Stage IV on an LS3

Adding just about any performance cam (and springs) to a stock LS3 is going to get you a lot of extra power, but adding the right cam can offer even more. To illustrate the gains possible with a cam-only upgrade on a stock LS3, I installed the Gandrud Chevy LS3 crate engine on the dyno and treated it to a Stage IV cam from BTR. Right at the limit of available piston-to-valve clearance, the Stage IV cam was the perfect candidate to work with the stock LS3 heads.

Remember, even in stock trim, an LS3 has an excess of cylinder head flow. The stock heads are capable of supporting nearly 700 hp on the right application (see Chapter 2), so the only thing missing in the combination is cam timing. It is also important to remember that not all cam-only upgrades are created equal and that most (like this one) must be combined with appropriate valve springs (these came from BTR as well).

The Gandrud crate LS3 was installed on the engine dyno and run with the stock cam using long-tube headers, a Holley HP management system, and Lucas oil. Also present was a FAST (manual) throttle body, Meziere electric water pump, and K&N oil filter. Run with the stock LS3 cam, the LS3 produced 496 hp at 5,800 rpm and 488 ft-lbs of torque at 4,700 rpm.

Replacing the stock cam with the Stage IV from BTR also required swapping out the single-bolt, 4X cam sprocket for a three-hole, 4X version. After installation of the new cam, the power output jumped to 570 hp at 6,500 rpm and 522 ft-lbs of torque at 5,300 rpm. Not only was peak power production way up, but the cam swap netted torque gains all the way down to 3,000 rpm. Below 4,500 rpm, the BTR cam offered an extra (and consistent) 20 to 25 ft-lbs of torque, but this number increased substantially above 4,500 rpm.

The stock LS3 (flat-top) piston features no intake or exhaust valve reliefs. Available piston-to-valve clearance is the limiting factor in terms of cam timing on a stock LS3.

Although BTR offers cam profiles that require no spring upgrades, this Stage VI cam was combined with a dual-spring upgrade.

Stock LS3 vs BTR Stage IV on an LS3 (Horsepower)

Stock LS3 Cam: 496 hp @ 5,800 rpm BTR Stage IV Cam: 570 hp @ 6,500 rpm Largest Gain: 90 hp @ 6,600 rpm

As much as I liked the huge peak power gains (74 hp) and extra 90 hp at 6,600 rpm, I also liked the fact that the BTR Stage IV cam offered gains down low. You know you have made the right cam choice for your application when you get huge power gains with no trade-offs in low-speed torque.

Stock LS3 vs BTR Stage IV on an LS3 (Torque)

Stock LS3 Cam: 491 ft-lbs @ 4,700 rpm BTR Stage IV Cam: 522 ft-lbs @ 5,300 rpm Largest Gain: 67 ft-lbs @ 6,400 rpm

Adding as much as 90 hp to an LS3 is an amazing thing, but it is the extra 25 ft-lbs down at 3,700 rpm that will be used more often in daily street driving. The extra 20–25 ft-lbs of torque up to 4,500 rpm will be most helpful in getting this LS3 up on the cam.

Test 3: LS9 vs LJMS Stage 2 Turbo on a Short-Stroke LS3

As you saw in the first two tests, cam timing is important for LS3 applications, but nowhere is it more important than on a turbo application. Spool-up of the turbo is a function of the power output of the engine at the desired spool RPM. The greater the power output, the quicker the spool-up. Things that can alter spool-up include displacement, cam timing, and intake design.

Small-displacement engines have more difficulty spooling up turbos (especially large ones), as do wilder cam timings (that may sacrifice low-speed torque) and short-runner intakes (that also reduce torque production lower in the rev range). This test illustrated the power gains offered by a cam swap on a short-stroke, LS3 turbo combination equipped with a short-runner, Holley Hi-Ram intake and large, 76-mm Precision turbo (meaning it had three strikes against it in terms of spooling). Thus, cam selection was even more critical on this engine than a stock LS3.

The short-stroke turbo engine featured an aluminum LS3 block equipped with a 4.8 crank, 6.30-inch forged Lunati rods, and custom JE pistons. The combination also included Total Seal rings, TFS Gen X 255 heads, and a Holley Hi-Ram intake (with 102-mm FAST throttle body). Rounding out the package was a Moroso oiling system, ATI dampener, and custom DNA turbo manifolds. The manifolds fed a single Precision 76-mm turbo, a CX Racing ATW intercooler, and TurboSmart 45-mm Hypergate waste gates.

The engine was first run (on the waste-gate springs) with an LS9 cam. So equipped, the turbo combination produced 697 hp and 598 ft-lbs of torque. After installation of the LJMS Stage 2 Turbo cam, the power numbers jumped to 733 hp and 621 ft-lbs of torque, but the real story is how much extra low-speed torque the cam swap offered. The gains would be even greater had I elected to equalize the boost pressure because the boost dropped by .5 psi after the cam swap (I ran it on the spring).

The test engine was an LS3 block equipped with a 4.8 crank, 6.3-inch Lunati forged rods, and JE pistons. The short-stroke LS3 was topped with a set of TFS Gen X 255s, a Holley Hi-Ram intake, and a custom turbo kit that featured stainless manifolds from DNA feeding a Precision 76-mm turbo.

Adding a turbo to the max-performance build makes cam selection even more important.

LS9 vs LJMS Stage 2 Turbo on a Short-Stroke LS3 (Horsepower)

LS9 Cam: 697 hp @ 6,900 rpm LJMS Stage 2 Turbo Cam: 733 hp @ 6,600 rpm Largest Gain: 34 hp @ 6,600 rpm

In some instances, a cam swap offers substantial power gains at higher engine speeds. Replacing the most powerful factory cam available (the LS9) with an LJMS Stage 2 turbo cam resulted in peak power gains as well as gains through the entire rev range. The cam swap also dropped the boost pressure by as much as .5 psi. More power with less boost is always a good thing.

LS9 vs LJMS Stage 2 Turbo on a Short-Stroke LS3 (Torque)

LS9 Cam: 598 ft-lbs @ 5,600 rpm LJMS Stage 2 Turbo Cam: 621 ft-lbs @ 5,500 rpm Largest Gain: 61 ft-lbs @ 3,800 rpm

The extra 34 hp was obviously welcome, but it was the extra 60 ft-lbs down low that really made this LJMS cam a success. By increasing the low-speed torque production (actually through the entire curve), the LJMS cam would certainly offer increased boost response on the street (or strip). The artificial load from the engine dyno negates some of the gain, but boost response with the turbo cam would be greatly enhanced with an extra 60 ft-lbs on tap.

Test 4: NA vs BTR Stage IV Blower Cam on an SC LSX

Building a performance LS engine means using the right tool for the right job. This applies to more than just a torque wrench or feeler gauges; it applies to running the proper cam grind for the intended use and application. In this case, the application was a supercharged B15 LSX engine. Built by General Motors to withstand 15 psi (the 15 in B15), the forced-induction combo (not just blowers) featured a forged crank and pistons (powdered metal rods) along with six-bolt LSX LS3 heads.

Add a boost-friendly static compression ratio of 9.0:1 and you have the makings of the perfect combination for a supercharged cam test. All I did was add a Whipple supercharger, 150-pound injectors, and a Holley HP management system and I was ready to roll.

This test was designed to compare an NA cam to a dedicated blower grind on a supercharged LS3 application. In addition to being designed for a positive-displacement blower application, the Stage IV cam from BTR was also slightly more aggressive. The NA cam (from Comp Cams) spec’d out with a .617/.624-inch lift split, 231/247-degree duration split, and 113-degree LSA. By comparison, the BTR blower cam offered the same lift split, an extra 8 degrees of intake duration, 11 degrees of exhaust duration, and 6 degrees of LSA (113 vs 119).

Equipped with the NA cam, the Whipple Supercharged LSX produced 855 hp at 6,700 rpm and 713 ft-lbs of torque. After installation of the BTR Stage IV cam, the peak numbers jumped to 880 hp and 714 ft-lbs. The milder NA cam actually offered more power up to 4,800 rpm, but the BTR cam pulled away thereafter. Interestingly, the boost was higher with the BTR cam than with the NA cam, despite no change in pulley size.

This test was run on a B15 crate engine equipped with a 3.3 Whipple supercharger.

Even with the blower, the cam swap was easy. Run with the (plenty powerful) NA cam, the supercharged combo produced 855 hp and 713 ft-lbs of torque.

NA vs BTR Stage IV Blower Cam on an SC LSX (Horsepower)

NA Cam: 855 hp @ 6,700 rpm BTR Blower Cam: 880 hp @ 6,700 rpm Largest Gain: 29 hp @ 6,500 rpm

Although designed for an NA (rectangular-port) application, the Comp cam offered plenty of power on the supercharged LSX. The tighter LSA and shorter duration on the NA cam offered more power down low on the supercharged applications, but the wilder Stage IV BTR cam came on strong at the top of the rev range.

NA vs BTR Stage IV Blower Cam on an SC LSX (Torque)

NA Cam: 713 ft-lbs @ 4,600 rpm BTR Blower Cam: 714 ft-lbs @ 5,300 rpm Largest Gain: 23 ft-lbs @ 3,600 rpm

The torque curve shows what might be considered typical of a sizable change in cam duration. Stepping up 8 degrees in cam duration and increasing the LSA by 6 degrees (from 113 to 119) enhanced top-end power production. There was a trade-off in torque production below 4,700 rpm because the milder (NA) cam offered better power down low.

Test 5: Effect of LSA on a Supercharged LSX

Does the LSA affect the boost and power curves of a supercharged LS3? Obviously the answer is yes or this would be one very short test. Typically blower cams (such as the factory LSA and LS9 cams) are ground with very wide LSAs. Those factory cams featured 121.5-degree LSAs, but it is not uncommon for blower cams to be in the 119- to 120-degree area. In comparison, typical performance cams for LS3 applications are slightly tighter, in the 112- to 114-degree range.

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This begs two questions: What happens if you just run your blower engine with a cam that you know works well on an NA LS3? Will a powerful LS3 cam work well once you add a supercharger?

To answer these questions, I set up a test on the GM B15 LSX engine. Equipped with a Whipple supercharger, I wanted to test two cams with distinctly different LSAs. The two cams (one from Comp and the other from BTR) were as close as possible in specs other than the LSA. The tight LSA cam from Comp had a .617/.624-inch lift split, 231/247-degree duration split, and 113-degree LSA. The BTR blower cam had a .617/.596-inch lift split, 231/248-degree duration split, and wider, 120-degree LSA.

Equipped with the 113-degree cam, the blower engine produced 758 hp and 681 ft-lbs of torque. Replacing the 113-degree cam with the BTR cam increased the power output slightly to 768 hp and 679 ft-lbs of torque. The blower cam offered slightly more power at the top of the rev range, but the tighter LSA cam offered more low-speed torque.

An average (tight LSA) street cam works well, even on a blower engine, but is there extra power to be had from a wider LSA?

Tuning is critical on any LS3 application, but it’s super critical on a supercharged combination. I employed a FAST management system to dial in each combination, but the air/fuel and timing curves were identical for each cam.

Effect of LSA on a Supercharged LSX (Horsepower)

113-Degree LSA Cam: 758 hp @ 6,200 rpm 120-Degree LSA Cam: 768 hp @ 6,200 rpm Largest Gain: 10 hp @ 6,200 rpm

The wider LSA (typical of a positive displacement blower cam) improved power production above 5,400 rpm but offered a slight bump at 5,000 rpm as well. Down low, the wide LSA lost power to the tighter LSA, but low-speed power is usually not a problem with the immediate boost response offered by a positive displacement supercharger.

Effect of LSA on a Supercharged LSX (Torque)

113-Degree LSA Cam: 681 ft-lbs @ 4,200 rpm 120-Degree LSA Cam: 683 ft-lbs @ 4,300 rpm Largest Gain: 19 ft-lbs @ 3,200 rpm

The 113-degree LSA cam increased torque very low in the rev range. From 3,200 to 3,900 rpm, the narrow LSA offered more low-speed torque, but lost out in terms of peak power to the wide LSA. The question now is, Where do you want your extra power?

Test 6: Effect of LSA on a Stroker LS3

To find out how LSA affects the power curve on NA engines, I had Crane grind me a pair of cams with identical lift and duration values but altered LSAs. Both cams featured .624-inch lift (intake and exhaust) and 232/242-degree duration splits, but one cam featured a tight 108-degree LSA, while the other came in at 120 degrees. This obviously altered the cam timing events, but the test on LSA is interesting nonetheless.

The 402 stroker test engine was actually a hybrid of sorts, featuring an LS2 block and LS3 heads. The aluminum block was treated to a 4.0-inch stroker crank from Speedmaster, along with Carrillo rods and CP (flat-top) pistons. Topping off the hybrid stroker was a set of CNC-ported Chevy Performance L92 heads. Offering a tad more than 350 cfm, the heads flowed more than enough to support the intended power level for the test. The stroker also featured a FAST LSXR LS3 intake, Big Mouth throttle body, and 17 ⁄8-inch Kooks headers.

The idea was to run the pair of cams to illustrate the power differences (if any) offered by the change in LSA. Equipped with the 120-degree cam, the stroker produced 570 hp at 6,200 rpm and 535 ft-lbs of torque at 5,200 rpm. After swapping to the 108-degree cam, the peak numbers stood at 572 hp at 6,300 rpm and 543 ft-lbs of torque at 5,100 rpm. A difference in 2 hp is not significant, but the real change came elsewhere in the curve.

The tighter LSA dramatically increased power production lower in the rev range. The cam swap netted an additional 36 ft-lbs of torque down low, but the additional torque gains continued through the rev range. Only for a short 150-rpm stint did the two cams produce the same power. The one downside to the tight (108-degree) LSA cam was idle quality because the idle vacuum was down significantly compared to the 120-degree cam.

JE supplied a set of asymmetrical, flat-top pistons for the 402 stroker.

The test engine was a 402-inch stroker LS2 block equipped with LS3 heads. The short-block included a 4340 Scat crank and 6.125-inch rods.

Effect of LSA on a Stroker LS3 (Horsepower)

120-Degree LSA Cam: 570 hp @ 6,200 rpm 108-Degree LSA Cam: 572 hp @ 6,300 rpm Largest Gain: 10 hp @ 6,500 rpm

In terms of horsepower production, the tight 108-degree LSA cam offered more power everywhere. The 120-degree cam was able to equal the 108-degree cam for 200 rpm (from 6,000 to 6,200), but lost out everywhere else. I expected the 120-degree cam to come on strong at the top of the rev range, but on this stroker, the 108-degree was the better choice.

Effect of LSA on a Stroker LS3 (Torque)

120-Degree LSA Cam: 535 ft-lbs @ 5,200 rpm 108-Degree LSA Cam: 543 ft-lbs @ 5,100 rpm Largest Gain: 36 ft-lbs @ 3,700 rpm

It is obvious from the torque curves that the tighter 108-degree LSA cam offered considerably more torque down low and through most of the curve. I saw this same scenario on the test with the supercharged application, although I did not try a 108-degree LSA on that engine. The downside to the tight LSA cam was idle quality, but if you are after power, the 108 was definitely the way to go.

Test 7: Carb vs EFI Cam on a 417 LS3 Stroker

This test was interesting in that it came about after hearing a heated discussion online about the merits of cams designed specifically for carbureted LS applications. In truth, the carbureted cam design has less to do with what supplies the fuel than does the intake manifold design. Carbureted cams are generally designed for short-runner, single-plane manifolds rather than a specific carburetor. That enthusiasts even use the terms “carbureted” and “EFI cams” is reason enough to schedule a test.

As this and other tests revealed, the optimum cam for a given application works well with both forms of induction. By this I mean that if a cam offers more power in carbureted form, it will do so if you switch over to fuel injection.

The test engine for this comparison was a 417 stroker built from an LS3 aluminum block. Included were a 4.0-inch Scat crank, K1 6.125-inch rods, and JE forged pistons (with Total Seal rings). Topping the stroker was a set of GM Performance CNC L92 heads from Gandrud Chevrolet. The carbureted combination was run with an Edelbrock Victor Jr. intake and Holley 950 Ultra XP carburetor; the EFI combo included a FAST LSXR intake, Big Mouth throttle body, and 75-pound injectors.

In the carbureted corner was a Comp grind that offered a .623/.596-inch lift split, 247/258-degree duration split, and 110-degree LSA. The EFI cam was slightly milder with a .620/.596-inch lift split, 239/250-degree duration split, and wider 113-degree LSA. The graphs reveal that when tested with both the carbureted and EFI induction systems, the “carb” cam consistently offered more power. Neither the carb nor the EFI favored one of the cams; the carb cam was right for the application regardless of the induction system.

What cam works better, one designed for a carbureted application or one for an EFI application?

In addition to running both cams with an Edelbrock (carbureted) intake and Holley carb, I also ran the two cams with a long-runner, FAST EFI intake.

Carb vs EFI Cam on a 417 LS3 Stroker (Horsepower)

Carb Cam (carb combo): 628 hp @ 6,700 rpm EFI Cam (carb combo): 611 hp @ 6,700 rpm Largest Gain: 17 hp @ 6,700 rpm

In this comparison, I tested a pair of cams designed for carbureted and EFI applications. The carb cam was slightly wilder in specs and offered a tighter LSA. Run on a carbureted stroker, the carb cam offered more power above 5,300 rpm and below 4,700 rpm, but the two combinations yielded near identical torque values. Given the longer duration, I expected the carb cam to trade power down low to the EFI cam, but such was not the case.

Carb vs EFI Cam on a 417 LS3 Stroker (Torque)

Carb Cam (EFI combo): 604 hp @ 6,400 rpm EFI Cam (EFI combo): 585 hp @ 6,400 rpm Largest Gain: 20 hp @ 6,500 rpm

I tested the carb versus EFI cam once again on the same stroker combination, but this time it was equipped with a long-runner, EFI manifold. Once again, the carb cam offered more power above 5,300 rpm. The two produced identical low-speed power (which surprised me), but the EFI cam offered slightly more power from 4,500 to 4,900 rpm.

Written by Richard Holdener and republished with permission of CarTech Inc

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Full-Race Screamer LSX

Simply put, this 429-ci race engine is one of the meanest LS small-blocks ever built. Without the assistance of nitrous or forced induction, it kicks out a staggering 1,002 hp and 663 ft-lbs of torque. At 2.34 hp per ci, its specific output is right on par with that of a NASCAR Sprint Cup motor.

Built by the School of Automotive Machinists, the 429 powers a 3,500- pound 1999 Camaro down the quarter mile in 8.52 seconds at 158 mph. Producing that kind of power in naturally aspirated trim requires a meticulously thought-out and executed combination that’s extreme in every regard. Based on a GMPP LSX block, the engine not only boasts a healthy dose of displacement, but it turns 9,600 rpm and breathes through a set of heavily massaged C5R cylinder heads that flow 430 cfm. To get the most out of those cfm, the 429 utilizes a Comp Cams 285/302- at-.050 solid roller cam that boasts nearly a full inch of valve lift. Obviously, this isn’t the kind of engine that will ever see street duty, as evidenced by its 7,500- to 9,600-rpm powerband. Because the Camaro that it powers is equipped with a Liberty’s clutchless 5-speed manual transmission, and the motor never turns less than 8,000 rpm at the track, the 429’s tight RPM band suits it perfectly.

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Interestingly, SAM students and instructors built this monster small-block as part of a class project. The school operates one of the most respected vocational programs in the performance industry. Its unique curriculum covers short-block machining and assembly, cylinder head porting, CNC programming and operation, and race engine design. Between classes, SAM students work on the school’s various race cars, and they are actively involved in the design and testing process of engines, such as this 1,002- hp small-block. Many of SAM’s graduates go on to build race motors for some of the top names in racing, such as Hendrick Motorsports, John Force Racing, Warren Johnson Enterprises, Roush-Fenway Racing, McLaren Engines, and Honda IRL. For more information on SAM’s unique program, visit www.samracing.com.

Wee Beast LS1

In the realm of stroker Gen III/IV small-blocks, a 383-ci motor is a bit on the small side. Proving that a well-executed parts combination can sometimes overcome a displacement handicap, this 383 produces horsepower and torque figures on par with those of many 408- and 427-ci stroker motors, all while burning 91- octane pump gas. It churns out 535 hp and 508 ft-lbs of torque on a Dynojet chassis dyno. Using the accepted figure of 15-percent drivetrain power loss, that equates to roughly 629 hp and 598 ft-lbs of torque. Trying to extrapolate horsepower at the crankshaft from rear-wheel horsepower isn’t an exact science, but, needless to say, this 383 is one stout package (designed and built by Tony Mamo of Airflow Research).

It’s equipped with an Eagle crank and rods, Wiseco 11.35:1 pistons, lightly ported Air Flow Research 205-cc cylinder heads, and a mild Comp Cams 234/238- at-.050 hydraulic roller camshaft. Peak horsepower figures aside, what makes this 383 truly impressive is its broad powerband. It produces nearly 550 ft-lbs of torque at 4,000 rpm, and it carries most of it through the entire RPM range. This kind of low- and mid-range torque makes for an outstanding street/strip motor with explosive throttle response at all RPM.

For enthusiasts on a tight budget, a 383 makes a lot of sense, as the largest bore a 5.7L aluminum or 5.3L iron block can handle is approximately 3.905 inches. When you match that bore size with a 4.000-inch crank that requires minimal clearancing, the end product is an easy-to-build and affordable 383-ci short-block. As this wee beast illustrates, big performance can come in small packages.

Big Daddy LS2

There was a time when even big-blocks couldn’t crack 500 ci, but now small-blocks can reach that massive displacement total. Add that to the astounding airflow potential of the Gen III/IV cylinder heads, and it’s a match made in horsepower heaven. Few engine combinations illustrate the benefits of massive cubic inches better than the SAM 500-ci LS2 small-block.

It’s based on an ERL Super Deck II block that boasts a towering 10.200-inch deck height, and it can accommodate a 4.500-inch Callies stroker crank. When that’s combined with Darton sleeves that allow for a 4.202-inch bore, the result is a 500-ci monster. To feed all those hungry cubes, the LS2 relies on a stock LS7 intake manifold and a set of ported LS7 cylinder heads that flow 390 cfm. Actuating the valves is a modestly sized Comp Cams 248/254-at-.050 hydraulic roller cam that many people would consider conservative in an engine 50 ci smaller.

As no surprise, SAM’s 500-ci LS2 produces an incredibly stout torque curve that peaks at 630 ft-lbs at 5,100 rpm. Even more impressive is the fact that the 500 dishes out 600-or-more ft-lbs of torque from 4,500 to 6,200 rpm. Of course, torque is useless without horsepower, and the 500 doesn’t disappoint in that department, either. The motor posts a total of 717 hp, thanks to its generous displacement, and peak power arrives at a leisurely 6,500 rpm. If the same heads and camshaft were installed on a 427, it would have to turn at least 500 rpm more to produce the same peak horsepower.

As with all of its project engines, the SAM 500 was put through its paces at the race track. Installed in the school’s 1998 Camaro project car, the motor was good for a 9.96-at-135-mph pass in the quarter-mile. That’s not too shabby at all for a 3,700-pound car running on 93-octane pump gas.

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Welterweight Brawler LS3

Measuring right in the middle of the displacement spectrum, this 417-ci small-block strikes a sweet balance of affordability and size. By pairing a factory 6.2L aluminum block’s 4.065-inch bore— cleanup-honed to 4.071 inches—with a 4.000-inch stroke, the result is a healthy dose of displacement without the need for a costly re-sleeving procedure. For hot rodders looking to build a budget-priced aluminum Gen III/IV motor without breaking the bank, the 417 is an extremely popular displacement combination. This particular 417 (designed and built by Tony Mamo of Airflow Research) ups the ante with a Compstar rotating assembly and a heavy-breathing top end comprised of AFR 230-cc cylinder heads and a FAST 102-mm intake manifold and throttle body. When they’re all matched with a Comp Cams 243/247-at-.050 hydraulic roller cam, the result is 663 hp and 579 ft-lbs of torque on 91-octane pump gas.

Peak output, however, is the least impressive aspect of this potent combination. The 417 produces more than 500 ft-lbs of torque from 3,400 to 6,900 rpm. Furthermore, the 417 kicks out more than 570 ft-lbs from 4,600 to 5,900 rpm, and it peaks 579 ft-lbs at 5,500 rpm. The beauty of a highly optimized combination like this is that it combines outstanding peak horsepower with an incredibly flexible torque curve. That means it will pull all the way to 7,000 rpm when called upon, yet easily roast the hides anywhere in the low- and mid-RPM range. Further enhancing its street credentials, the 417 idles smoothly at 900 rpm, and it also boasts a hydraulic roller cam valvetrain that requires minimal maintenance. With its blend of power and streetability, the 417 treads on turf that only big-blocks could touch just a few years ago.

Brazilian Stock Car LS3 Motor

A budget race engine is somewhat of an oxymoron, but Mast Motorsports’ 416-ci LS3 is the exception to the rule. Because the engine had to be designed and built as a spec motor for Brazil’s Copa Nextel stock car road racing series, Mast had to stick with a strict $15,000 budget. Making the task that much more difficult, the motor has to last for a full 12-race season, which equates to roughly 2,800 hard racing miles. Nonetheless, the 416 produces 617 hp and 568 ft-lbs of torque.

To pull off this impressive feat of dollar-stretching performance and durability, Mast relies heavily upon production GM components. A testament to the race-bred nature of modern LS-series small-blocks, the Mast 416 utilizes a factory 6.2L aluminum block, finish-honed to 4.070 inches, and a stock LS3 intake manifold, throttle body, and cylinder heads. With a mild 240/256-at-.050 hydraulic roller cam actuating the valves, the 416 produces peak power at a modest 6,300 rpm, which greatly improves overall engine durability. Holding the short-block together are a Compstar 4.000-inch forged crank and rods and Mahle 11.3:1 pistons.

In contrast to the high-octane gasoline used in NASCAR Sprint Cup engines, the Brazilian Copa Nextel stock car series mandates an E98 fuel that’s 98 percent ethanol. Interestingly, the Mast 416 produced an additional 26 hp (617 vs. 591) on ethanol compared to 93- octane gasoline. Although ethanol has less BTUs of energy than gasoline, Mast says that its greater heat of vaporization enables it to pack a denser air/fuel mixture into the cylinders for an increase in power. The downside is that it takes 1.5 times as much ethanol to make similar power as gasoline, dramatically reducing fuel mileage. Nonetheless, considering that this 416 is a bonafide race engine, the fact that it’s based heavily on production GM components is simply amazing.

Written by Stephan Kim and Posted with Permission of CarTechBooks

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For instance, GM used several different types of factory powertrain control modules, which must be matched with specific engines and wiring harnesses. Furthermore, the stock PCM can be tuned by using specialized programs and a laptop computer, or a simpler hand-held device. Stand-alone aftermarket EFI systems eliminate much of the applicationspecific minutia associated with running a factory PCM, but they’re generally more expensive; however, there are several different manufacturers to choose from.

Options abound when it comes to feeding that freshly built LS stroker combo fuel and spark. Choices include running a stock PCM, using a stand-alone aftermarket EFI system, or even bolting on a carburetor. Stand-alone aftermarket systems represent the pinnacle of EFI tuning flexibility and power potential. (Photo courtesy of Comp Cams)

If simplicity is the primary objective, a carbureted induction system is the easiest to install and tune. Thanks to standalone ignition systems from companies such as MSD and GMPP, LS-series smallblocks can have their timing maps tuned with a laptop while still relying on a venerable carburetor for fuel delivery. Although space limitations here prevent breaking down every single option on the market, here’s a run-down of some of the most popular engine management systems available.

Electronic Fuel Injection

Part of the appeal of building an LS-series small-block for any project car is its high-tech credentials. Electronic fuel injection (EFI) adds to this mystique, and it offers irrefutable advantages over a carburetor in cars that see an appreciable amount of street duty. EFI-equipped motors start up reliably—even in cold weather—and offer improved fuel mileage, cleaner emissions, and superior drivability.

EFI has the edge over a carburetor in every single category except power and cost. Although fuel injection offers unparalleled benefits in streetability, fuel mileage, emissions output, and cold startup ability, the venerable carburetor almost always makes more horsepower. Additionally, the cost of a carburetor is far less than the total tally of eight injectors, a computer, and tuning hardware and software.

Factory 1999-and-later PCMs used with 24-tooth reluctor wheels are the most popular units with engine swappers. They can be used for both cable-actuated and drive-by-wire throttle bodies, and they are very inexpensive at $50 to $100.

Early Gen III PCMs that came equipped on 1997 and 1998 F-bodies and Corvettes have a different pin-out configuration and require an application-specific wiring harness. Because of that, they’re not very popular in retrofit applications.

On the other hand, EFI’s impact on horsepower output is a topic of much debate. In numerous back-to-back, EFI vs. carburetor dyno tests conducted by several highly respected engine builders, a fuel-injection system rarely produces more power than a carburetor. Usually, it’s the other way around, with the carb taking top power honors. That’s because carburetors atomize fuel very high in the intake manifold, which increases charge density and the inertial ram effect of the incoming air/fuel charge.

In fact, this is why Formula One engines have their fuel injectors positioned at the top of the intake manifold runners. In contrast, production engines, including the Gen III/IV small-block, have injectors placed at the very end of the intake runners. Additionally, EFI systems tend to be more temperamental and difficult to tune. These inconveniences aside, no carb can touch the versatility and streetability of a well-tuned EFI system. So although it may take more initial effort to dial in, a properly tuned fuel injection system offers the best balance of all-around performance and economy.

Stock Powertrain Control Modules

The LS-series small-block has only been in production since the late 1990s, but GM has matched it with a dizzying array of powertrain control modules (PCM). Nevertheless, when matched with software programs, such as those from HP Tuners or EFI Live, the factory computer is an incredibly powerful tuning device that can tame even the most radical engine combinations. These software programs offer levels of tuning flexibility similar to that of an aftermarket stand-alone system, and at a fraction of the cost. Just a few years ago, this type of technology was unheard of.

If you are opting for a used-core engine as the basis of a stroker LS project, know that they’re often sold with a matching PCM and wiring harness. This ensures that the engine and PCM are compatible with each other. However, with the increasing availability of aftermarket blocks, not all engine builds start with a used-core engine. If you are electing to run a stock PCM-based engine management system, it’s imperative to choose the right computer. Factory GM computers can be broken down into two basic groups: those designed for 24-tooth reluctor wheels and those designed for 58-tooth reluctor wheels.

Engines equipped with 24-tooth wheels include 1997–2004 LS1s, 2001–2004 LS6s, 2005 and 2006 LS2s, and 1999–2006 Vortec truck motors. Gen IV small-blocks—including the LS3, LS7, LS9, LSA, L76, L99, and 2007-andlater Vortec truck engines—come equipped with 58-tooth reluctor wheels. Generally, either style PCM can be used to operate any LS-series small-block, as long as it’s matched with the correct reluctor wheel and wiring harness. For instance, if you’re using an LS3 core as the basis of a stroker built, it can be paired with a PCM designed to work with a 24-tooth reluctor wheel, as long as a 24-tooth reluctor wheel is installed on the crankshaft.

A major difference between the two is that 24-tooth reluctor wheel PCMs can be programmed to operate both drive-by-cable and drive-by-wire throttle bodies, and 58-tooth reluctor wheel PCMs are only compatible with drive-by-wire throttle bodies. Because PCMs designed for 58-tooth reluctor wheels have been produced in much lower quantities thus far, they tend to be harder to find and more expensive. Consequently, the 24-tooth reluctor wheel PCMs are far more popular with LS enthusiasts. They’re plentiful and can be purchased for as little as $50 to $100. One caveat is that the PCMs used on 1997 and 1998 LS1-powered F-bodies and Y-bodies have a different pin-out configuration, which requires a different wiring harness, so they are less common for engine swap applications.

Powerful software programs, such as those from EFI Live and HP Tuners, enable hot rodders to manipulate all of the factory tuning parameters. In addition to modifying the fuel and spark maps, these programs can adjust an automatic transmission’s shift points and firmness, and they can also disable the factory Vehicle Anti-Theft System, which is very handy in engine swap applications.

A wideband oxygen sensor is a critical tool for properly tuning the air/fuel mixture. Naturally aspirated engines typically produce the most power with the air/fuel ratio locked in at 11.5 to 12.5:1. Innovate Motorsports offers several affordable and easy-to-use wideband sensors, which feature data logging for enhanced flexibility and control.

The race track is the ultimate tuning venue, but a chassis dyno provides an excellent test bed for getting an engine’s baseline fuel and spark curves dialed-in. Mustang and Superflow dynos can load the drive wheels to simulate wind resistance and the vehicle’s weight. One great benefit of EFI is that it can automatically account for differences in air quality and density. By comparison, carburetors must be re-jetted constantly to account for changing ambient air conditions.

Perhaps the biggest drawback of both stock and aftermarket tuning platforms is that the dizzying array of tuning parameters makes it easy to get lost. Fortunately, there are myriad online forums where enthusiasts can seek help and even specialty training programs that you can enroll in to seek additional assistance. If that’s still too intimidating, many speed shops offer their own tuning services. (Photo courtesy of Comp Cams)

The FAST XIM ignition controller was originally designed to work in conjunction with the FAST EFI computer. With the growing popularity of carbureted LS engines, hot rodders are now using the XIM to take over the factory ignition control functions. The XIM is compatible with both 24- and 58-tooth reluctor wheels. (Photo courtesy of Comp Cams)

A standalone aftermarket computer box is built from a durable aluminum housing and resembles a heat sink. Because low impedance injectors force the computer to endure high electrical loads, the box must be able to efficiently dissipate heat to prevent its internals from cooking. The lack of heat capacity is why low-impedance injectors can’t be used with stock PCMs. This is one major reason why extremely high-horsepower applications almost always run aftermarket computers. (Photo courtesy of Comp Cams)

If you scored a Gen III/IV small-block to drop into your project car but don’t have a computer, Mast Motorsports’ M-90 PCM is the perfect solution. It offers all of the tuning power of a typical stand-alone EFI system, but it includes an idiot-proof wiring harness. After plugging all of the connectors onto the engine, all you have to do is hook up the power wire, ground wire, and fuel pump wire before firing up the motor.

PCM Tuning

For decades, the only way to comprehensively reprogram a fuel-injected engine was to bypass the stock PCM entirely with a stand-alone aftermarket computer. That all changed in the late 1990s, when computer-savvy hot rodders figured out how to hack into the factory computer codes and fully unlock the tuning potential of stock PCMs. In the LS camp, tuning software from HP Tuners and EFI Live has set the standard for flexibility and ease of use. These systems are essentially software programs that are downloaded to a laptop computer, and they interface with the factory PCM through the diagnostic port. The versatility of these programs is truly impressive, as they allow modifications of the fuel injector pulse widths, ignition timing, rev limits, knock retard, transmission shift points, cooling fan thresholds, speed limiters, final drive ratios, idle speed, and data logging. Additionally, the compatibility with two- and three-bar MAP sensors allows control over extreme-horsepower, forced induction applications.

Self-learning aftermarket EFI systems, such as FAST’s EZ-EFI, have just recently hit the market. Using feedback from the oxygen sensors at wide open throttle, EZ-EFI can create a custom fuel map within two dyno pulls. (Photo courtesy of Comp Cams)

Compared to a carbureted fuel system, EFI operates at much higher fuel pressures of 40 to 60 psi. For any given horsepower output, EFI requires a larger-capacity fuel pump. Aeromotive and FAST sell complete fuel systems that include an external pump, filters, a pressure regulator, and hoses. (Photo courtesy of Comp Cams)

Because reprogramming the stock PCM can be intimidating, both HP Tuners and EFI Live offer comprehensive support forums on their websites, as well as a stockpile of various tunes users can download to use as a solid baseline program instead of trying to create one from scratch. Prices range from $400 to $800. The factory PCM can also be tuned using hand-held devices from companies such as Superchips, Granatelli, Jet, and Diablosport. These hand-held units plug into the PCM’s diagnostic port, but they do not require a laptop to operate. Most offer generic tunes that alter the fuel and spark maps based on fuel octane ratings, and some allow alterations of shift points, rev limits, and shift firmness. Due to their limited range of flexibility, handheld tuners are best suited for stock or near-stock engines, but not for a heavily modified stroker combination.

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Aftermarket EFI Systems

Without question, tuning software, like that from HP Tuners and EFI Live, has pushed the stock PCM beyond what anyone could have imagined just a decade ago. In fact, it’s not unheard of anymore for a 1,000-hp forced-induction engine to make do with a factory computer. Even so, there’s a limit to the stock PCM’s capabilities, which is why you won’t find a single Outlaw drag car running anything but a standalone aftermarket EFI system.

Once eclipsing the 1,000-hp mark, where heavy doses of boost and multiple stages of nitrous are the norm, an aftermarket EFI system offers a level of precision and versatility that a stock computer just can’t match. Obviously, these systems are geared more toward race cars and extremely high-end street cars, but with the tremendous horsepower potential of the LS platform, street cars equipped with aftermarket EFI systems are becoming more common.

Holley is getting in the self-learning EFI segment, too, with its new Dominator standalone system. It features dual channel wideband oxygen sensors, 12-channel distributorless ignition system outputs, and control of up to four stages of nitrous. As with competing systems, the Holley Dominator EFI unit creates a custom fuel curve within a couple of wide open throttle dyno pulls.

Fuel injectors may look similar from afar, but they come in various shapes, sizes, and spray patterns. Injectors must fit snugly into the intake manifold and fuel rail to prevent leaks. (Photo courtesy of Comp Cams)

There are literally dozens of standalone EFI systems on the market, but the most popular units with LS enthusiasts are offered by FAST, BigStuff3, and Accel. Compared to their stock-based counterparts, aftermarket systems offer more durable injector drivers, traction control, individual cylinder tuning, wideband oxygen sensor compatibility, multiple stage boost and nitrous controls, and data-loggers featuring accelerometers and blazing sampling rates. The biggest downside to these systems is cost, as they ring up a bill between $2,000 and $3,000. That price includes an aftermarket computer, wiring harness, engine sensors, and computer software. For the ultimate in tuning flexibility and horsepower potential, an aftermarket EFI system offers limitless possibilities.

Self-Learning EFI Systems

As computer technology continues to evolve at an alarming rate, it has enabled the automotive aftermarket to develop EFI systems that can now program themselves. Holley has recently launched its Dominator EFI system for LS-series small-blocks, and it offers many of the same flexible tuning features as competing systems. The ace up its sleeve, however, is a self-tuning fuel table that greatly simplifies the tuning process. By utilizing dual-channel wideband oxygen sensors, the Dominator EFI system can precisely create a fuel map based on an engine’s fueling needs within a couple of wide-open-throttle (WOT) dyno pulls. From there, the end user can fine-tune the programming. It’s very impressive technology, for sure, and it offers a glimpse into the future of aftermarket EFI systems.

Fuel Pump and Injector Sizing

Of the multitude of decisions that go into properly planning and building a stroker LS small-block, choosing an appropriately sized fuel pump and injectors is relatively easy. For naturally aspirated combinations, a good rule of thumb to follow is to use a fuel pump that flows .5 lb/hr of fuel for every 1 hp. For example, a 1,000-hp naturally aspirated combination needs a pump that can flow 500 lb/hr of fuel, and a 500-hp engine requires a pump capable of flowing 250 lb/hr of fuel. Because maxing out a fuel system leaves no margin for error, it’s not a bad idea to add another 10 percent of flow as a safety factor. Forced induction applications tend to be less efficient, so fuel pump flow rates of .60 to .65 lb/hr per hp are ideal.

Once proper fuel pump size has been determined, selecting the right injectors can be calculated in a similar fashion. A 1,000-hp naturally aspirated engine that requires a 500-lb/hr fuel pump would need 62.5 lb/hr injectors. That’s because 500 pounds of fuel divided by eight injectors yields 62.5 lb/hr per injector. That said, fuel injectors are all rated at a certain fuel pressure, and increasing fuel pressure can bump up the flow rate of an injector. Therefore, it’s important to compare the pressure an injector is rated at to the fuel pressure you will be running in your engine combo when selecting fuel injectors.

Certain fuel injectors are taller than others, but that’s not too big of a deal. Spacers can be placed between the fuel rail stands and the intake manifold to account for taller injectors.

When installing a wideband oxygen sensor in a car, it must be positioned in front of the catalytic converters. Placing it behind the converters will skew the air/fuel ratio readings.

Carburetors

Although the Gen I small-block Chevy was built in both carbureted and fuel-injected configurations from the factory, all LS-series motors came equipped with EFI. This presented a problem for traditionalists, as they recognized the horsepower potential of the LS platform, but they didn’t care for the complexity of EFI.

Edelbrock and MSD teamed up to create an ingenious solution in 2003 that allowed a person to bolt a carburetor to the Gen III small-block. Edelbrock got things rolling by creating a series of carbureted intake manifolds for LS-series small-blocks that could accommodate a MAP sensor. MSD then developed a revolutionary new ignition controller that took care of the spark side of the equation. The MSD 6LS controller looks like any other MSD ignition box, but it features a wiring harness that plugs into the coil packs and crankshaft and camshaft position sensors of the LS small-block. That and the MAP sensor integrated into the Edelbrock intake manifold are all that’s needed to manipulate the factory ignition system. The MSD box allows creating custom timing advance maps with a laptop computer and has controls for vacuum advance, rev limits, and a step retard for nitrous use. Furthermore, MSD has two versions of its LS ignition box that will work with both 24- and 58-tooth reluctor wheels. With a carburetor administering fuel, and the MSD box controlling the electronics, for traditionalists, this combo is the best of both worlds.

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The coil-on-plug ignition system used on all LS-series small-blocks features one coil pack for each cylinder. This yields an extremely powerful spark that’s capable of supporting well over 1,000 hp without any modifications. GM used several different styles of coils and brackets over the years, but, performance-wise, they are all nearly identical. (© GM Corp.)

Ignition

For good reason, when the topic at hand is the phenomenal performance of the Gen III/IV small-block, most of the talk revolves around the cylinder heads. However, the cast of supporting components, such as the ignition system, shouldn’t be overlooked. As with many modern cars, the LS-series small-block utilizes crankshaft and camshaft position sensors to precisely measure the location of each piston. This allows the ignition system to fire each cylinder at exactly the right moment to maximize power and reduce emissions. Another benefit is that the system eliminates the need for a conventional distributor. In its place are eight coil packs, one for each cylinder, that bolt on top of the valve covers.

Another benefit of the Gen III/IV small-block’s coil-on-plug arrangement is that it significantly cuts down on the length of the plug wires. Not only does this make it easier to route them away from the headers, it also reduces the potential for electrical interference. With the LS small-block, burning spark-plug wires is a thing of the past.

Functionally, the factory coil-on-plug system offers outstanding performance. Unfortunately, it’s not very aesthetic. Mast Motorsports offers coil brackets that relocate the coils to the side of the valve covers. This cleans up the engine compartment and allows accessing the valve cover bolts without removing the coil brackets.

With one coil pack dedicated to each cylinder, this coil-on-plug arrangement results in a tremendous amount of spark energy. In fact, the stock ignition system performs reliably in engines producing well in excess of 1,000 hp. The only reason to replace a factory coil pack is if it’s stopped working, due to age. Otherwise, the stock coils can handle just about anything an engine can throw at them. Over the years, GM has produced five different types of coils. Although they look different externally, due to the fact that they’re made by different suppliers, GM says that the performance among them is identical. MSD also offers stock replacement coil packs, which are said to produce three times the spark energy.

Written by Stephan Kim and Posted with Permission of CarTechBooks

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Intake manifold design is always a compromise. That’s because a manifold designed with airflow as the top priority would never fit under the hood of a car. Underhood installation constraints are the biggest obstacles to overcome during the R&D process of intake manifold design.

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Intake Dynamics

At its core, the purpose of the intake manifold is to distribute air evenly to all eight intake ports. Unlike cylinder head and camshaft dynamics, which most enthusiasts find quite fascinating, intake manifold design is very boring in comparison. That’s because the most important, yet least interesting, aspect of manifold design is hood clearance.

As with the location of the cylinder head’s intake ports in relation to the valves, a runner design with the highest, most direct approach into the intake ports will usually produce the most horsepower. In essence, the intake manifold’s runners are an extension of the cylinder head’s intake ports, and the cross-sectional area of the manifold’s runners determines the power potential of an intake. Establishing the proper cross-sectional area of the runners is a balance between airflow and air velocity.

Unlike a cylinder head, whose intake port lengths can’t be changed, the length of the runners on an intake manifold can easily be tweaked, provided there’s adequate underhood space. Changing runner length affects the RPM at which pressure waves inside the manifold best promote cylinder filling. Longer runners increase low- and midrange torque, and shorter runners promote improved high-RPM horsepower. That’s because longer runners increase the distance the air must travel, which promotes increased air velocity at low RPM. However, longer runners become restrictive at high RPM.

Plenum size and shape are other factors that impact intake manifold design. The plenum is the central chamber inside the manifold positioned directly ahead of the intake runners. A larger plenum promotes top-end power production, and a smaller plenum has the opposite effect. The shape of the plenum, incidentally, greatly affects cylinder-tocylinder air distribution.

Because EFI intake manifolds are designed to fit under very low hood profiles, most of the important elements of an intake’s design—such as runner length, cross-sectional area, and plenum volume—can’t be measured without cutting the manifold in half. Consequently, the only way to gauge an intake manifold’s airflow potential is through reading the manufacturer’s published specs or by studying the performance of various intakes on combinations similar to your own. It’s certainly not the most scientific approach, but it’s by far the most effective.

The stock LS7 intake manifold is quite possibly the best-flowing small-block manifold GM has ever built. It can easily support 650 to 700 hp, and it provides more than enough airflow for the vast majority of street engines.

Unlike those of a carbureted intake manifold, the inner workings of an EFI manifold can’t be seen without cutting it in half. To maximize runner length under a steeply sloped hood, the driver-side intake ports draw air from the passenger side of the plenum, and vice-versa.

Early Stock Intakes

The early days of modifying the Gen III small-block’s induction system were simpler times, as the LS1, LS6, and truck intake manifolds were the only choices available. With the high-flow potential of the LS cylinder heads, enthusiasts quickly reached the limit of these factory manifolds. Nonetheless, certain situations still warrant their use, so it certainly doesn’t hurt to examine their virtues and drawbacks.

All factory GM Gen III/IV intake manifolds are built from nylon, not of aluminum or iron as in small-blocks of generations past. This not only reduces weight, but it helps minimize heat absorption from the lifter valley into the intake manifold. Unfortunately, the nylon material is more difficult to port than metal, especially because it’s just 3 mm thick. With the assortment of factory and aftermarket intake manifolds on the market, it often makes much more sense to upgrade to a higher-flowing unit rather than to port a stock intake.

Dollar for dollar, it’s tough to beat the performance of the GM LS3 and L92/L76 intake manifolds. Both designs are nearly identical, and each manifold can feed plenty of air for a 600- to 650-hp stroker combo. The cost for that caliber of performance is just $250 through GMPP.

The production LS6 intake manifold is an excellent choice for an engine with cathedral-port heads producing up to 600 hp. GM installed it on all 5.7L LS1 and LS6 engines from 2001 onward, so they’re plentiful on the used market.

Fuel Air Spark Technology was one of the first companies to come to market with an aftermarket LS-style intake manifold. The design has since been superseded by FAST’s LSXR line of intakes, but there are a fair number of the old-school units floating around on the used market. Gains of 20 to 30 hp over the stock LS6 intake manifold are common. (Photo courtesy of Comp Cams)

The intake manifold found on the original 5.7L LS1 is best suited for mild, stock-displacement engine combinations. Although the manifold performs well in the 350-hp applications it was designed to support, simply porting the heads and installing a mild hydraulic roller camshaft in an otherwise stock 346 can leave it gasping for breath. The manifold’s biggest flaw is its lack of plenum volume that can’t provide enough airflow when matched with a set of quality cylinder heads. In fact, very early aftermarket LS1 camshafts relied on reverse-split patterns—with more intake duration than exhaust duration—to make up the deficiencies of the stock LS1 intake. At anything beyond 500 hp, the LS1 intake is a poor choice. Throw extra cubic inches and cylinder head airflow into the mix, and the LS1 intake is an even less appealing option.

Realizing the limitations of the LS1 intake manifold, GM improved upon it tremendously while designing the LS6. The main difference between the LS1 and LS6 intake manifolds is that the LS6 unit incorporates a dropped-floor design. This adds much-needed plenum volume and boosts airflow significantly. Additionally, the throttle body opening on the LS6 intake is enlarged from 75 to 80 mm. These changes enable the stock LS6 intake to easily support 600 hp, and at roughly $500, it’s a much more reasonably priced alternative to many aftermarket units. From 2001 onward, the LS6 intake was used on all LS1 engines, as well as the LS6, which means finding a good deal on a used unit isn’t that difficult.

The Weiand Air Ram intake manifold features a removable bottom panel for access to the runners and extra-thick walls for porting. According to the manufacturer, it’s good for 25 hp over a stock intake.

Edelbrock’s Pro-Flow XT intake manifold for cathedral-port heads does away with the factory-style, cross-ram setup for a more conventional runner design. The result is a manifold with copious plenum volume and long tapered runners for a broad powerband. Provided enough hood clearance, Edelbrock advertises a 30-hp gain over a stock LS6 intake. (Photo courtesy of Edelbrock)

GM originally planned on using a common intake manifold on all early LS-series small-blocks, but installation constraints forced engineers to raise the throttle body inlet 3 inches in order to clear the cooling fan in truck applications. This led to the development of a truck-specific intake manifold, used on 4.8L, 5.3L, and 6.0L Vortec truck engines. The truck manifold’s long runners should hint at poor high-RPM performance, but that simply isn’t the case. In back-to-back dyno testing, the truck manifold often produces even more power than the LS6 intake while boosting low-end torque. The downside is that its taller design requires more hood clearance.

The black sheep of the factory cathedral-port intake manifolds is the LS2 unit. Because the LS2 small-block uses LS6 cylinder heads, the LS2 intake manifold is very similar in design to the LS6 unit. The LS2 manifold features a larger 90-mm throttle body opening, re-contoured runners, and slightly more plenum volume than the LS6 unit, but it actually flows less air. So, unless you can find a very good deal on a used LS2 intake manifold, in the wake of stock intakes, the LS6 unit is a better option.

Stock Rectangle-Port Intakes

Perhaps the greatest asset of the LS-series small-block is the outstanding performance capabilities of its stock components. GM added to this mystique by creating the rectangle-port cylinder heads that made their debut in the LS7 and L92. Aware that these new head castings would flow substantially more air than their cathedral-port forebears, GM went to work and created intake manifolds that could keep pace. The result is a family of state-of-the-art intake manifolds that pack tons of performance for the dollar. These intake manifolds share the same nylon construction as previous LS-style units, but they have larger 90-mm throttle body openings that hint at their high-flow potential.

Professional Products’ Power Plus Typhoon intake manifold for cathedral port heads is available with 85- and 96-mm throttle body openings. It boasts a removable bottom cover for easy runner access and includes a set of fuel rails. In forced-induction and nitrous applications, aftermarket intakes are less prone to blowing apart compared to their factory GM counterparts.

If the stock LS7 intake manifold isn’t enough, FAST offers its LSXR unit as an upgrade. Independent dyno testing has proven that the FAST LS7 intake is good for an additional 25 to 30 hp over the stock unit.

A unique feature of the FAST LSXR intake is that the top cover and runners can be disassembled from the base of the intake manifold. This provides tremendous flexibility for porting. (Photo courtesy of Comp Cams)

FAST’s new LSXR intake manifolds feature several improvements over the models they replace, such as longer runners, increased plenum volume, and larger throttle body openings. They’re available for both cathedral- and rectangle-port cylinder heads. In applications that have enough hood clearance for an LSXRT manifold, the unit’s additional plenum volume and runner length have the potential to produce an extremely flexible powerband throughout the RPM range. (Photo courtesy of Comp Cams)

The production LS7 intake manifold was the first of the factory rectangle-port units, and considering that it was designed to feed a 427-ci engine that turns 7,000 rpm, it’s a very impressive piece of engineering. The LS7 intake has proven to be extremely effective in stroker combos putting out 650 to 700 hp. Just as impressive is its ability to deliver a very broad torque curve from idle to peak power. Available for less than $400 through GMPP, the LS7 intake manifold is a raging bargain. In reality, very few stroker combos need more airflow than the LS7 intake can provide.

Thanks to Edelbrock and MSD, it’s now possible to run a carburetor on an LS-series small-block. Edelbrock’s carbureted LS intake manifolds feature an integrated MAP sensor. When a manifold is combined with the MSD ignition box, which has a harness that plugs into the factory coil packs and camshaft and crankshaft position sensors, the result is an LS small-block that doesn’t need a factory computer. (Photo courtesy of Edelbrock)

Like Edelbrock, GMPP offers a full line of carbureted LS-style intake manifolds. The LSX-DR intake loudly broadcasts its high-winding intentions with enormous runners and a mounting pad for 4500-series Holley carburetors.

Just because you bolt a carbureted intake to an LS small-block doesn’t mean you have to run a carburetor. Mast Motorsports’ Retro LS intake manifold includes an adapter that allows for the mounting of a throttle body onto the carburetor pad. The single plane intake comes with runners drilled for fuel injectors. Mast offers the complete kit with the manifold, adapter, injectors, and fuel rails.

Raising the carburetor allows for much more flexibility when designing a single plane intake manifold. The runners can be made very large for high-RPM breathing, but they can also be made very long to promote low-RPM torque production. Tapering down the cross-sectional area of the runner also helps enhance this dual purpose effect.

When converting a single-plane intake for EFI duty, there are two options for mounting a throttle body. Companies, such as FAST and Wilson Manifolds, offer carb-style throttle bodies that mount directly to the carb pad. Wilson also offers 90- and 100-degree elbow adapters that mount to the carb pad. This allows for the attachment of a conventional forward facing EFI throttle body, which is often necessary for hood clearance concerns.

Just as the LS7 cylinder heads inspired the design of the L92 heads, Gen IV small-blocks equipped with the L92 castings feature an intake manifold that doesn’t flow quite as well as the LS7 unit, but it is an exceptional performer in its own right. The LS3, L92, L99, and L76 all share a common intake manifold. Although there are very subtle variations between the LS3 and L92/L76 units, such as the use of noise-reduction covers on the LS3 intake and the mounting of the MAP sensor, the basic design and performance of both manifolds are virtually identical. Compared to the LS7 intake, the LS3 and L92 manifolds have runner and plenum designs better suited for the smaller-displacement short-blocks they’re bolted to. Providing enough flow to support 600 to 650 hp, at just $250 through GMPP, the LS3 and L92 intake manifolds are great values.

Designed for factory hydraulic roller small-blocks, it’s not surprising that both the LS7 and LS3 intake manifolds are very well matched to the RPM range associated with the hydraulic roller stroker small-block. The LS7 intake provides enough air for 427- to 454-ci motors to pull up to 6,500 to 7,000 rpm, and the LS3 intake works well in 396- to 416-ci motors in the same RPM range. Because the ports on the LS7 head and LS3/L92 heads are shaped differently, intake manifolds must be matched up accordingly. For instance, the LS3 intake manifold isn’t compatible with LS7 heads, and the LS7 intake won’t work on LS3 heads. Furthermore, even though the L92/L76 intake manifold is used in truck applications, it has a low-profile design that fits under steeply sloped car hoods.

LS6-Style Aftermarket Intakes

Whether it’s a solid roller screamer that turns 8,000 rpm or a 500-ci hydraulic roller behemoth that makes more than 725 hp, there comes a point where even the high-flow factory intake manifolds run out of breath. Fortunately, companies, such as FAST, Holley, Wilson, Weiand, BBK, Professional Products, and Edelbrock, offer an assortment of intake manifolds for both cathedral- and rectangle-port cylinder heads. Early aftermarket intake manifolds were based on the stock LS6 design. Because of that, they offer slight improvements in airflow and horsepower, but exactly how much is debatable. Holley, Weiand, BBK, and Professional Products all offer aluminum intake manifolds that fall within this design category, and independent dyno testing results on both 5.7L and stroker motor combos are all over the map. Performance gains over the stock LS6 range anywhere from 10 to 25 hp.

By raising the carburetor or throttle body, air can travel in a direct path into the intake ports. Air doesn’t like to bend or change directions, so this arrangement yields dividends in airflow and power.

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A dual-plane intake manifold produces more low-end torque than a single-plane intake, at the expense of high-RPM power. They’re best suited for heavy street cars that will rarely exceed 6,500 rpm.

If readily available factory or aftermarket intake manifolds are deemed unsuitable for an application, there’s always the option of stepping up to a custom sheetmetal intake. Each one is custom fabricated for a specific application, so although a custom intake is extremely expensive at $1,500 to $2,000, it’s capable of performing extremely well.

Common features among these LS6-style intake manifolds include aluminum construction, thicker runner walls, and larger throttle body openings. Compared to the plastic factory manifolds, plumbing in nitrous injection nozzles and porting the runner is much easier on the cast-aluminum units. Furthermore, they’re more durable under high-boost, forced-induction applications. Considering that these manifolds were designed before aftermarket blocks enabled the huge displacement figures that are common today, they’re best suited for smaller engines between 346 and 396 ci. Even so, LS6-style aftermarket intakes only cost $100 to $200 more than a stock LS6 intake, so they can work quite well on a smaller displacement stroker combination.

Without question, Fuel Air Spark Technology’s (FAST) cathedral-port intake manifolds have set the performance benchmark for LS6-style intakes. FAST was the first to release an aftermarket LS intake, and its manifolds have a consistent track record of proven performance in a diverse range of applications. They feature high-strength polymer construction that’s 30 percent stronger than stock and a unique three-piece design that allows engine builders to disassemble the intake for easy access to the runners should porting be necessary. The runners on the FAST manifold have a large cross-sectional area that tapers as the runners approach the cylinder heads. This affords excellent top-end power while retaining air velocity to preserve low-end torque. Gains of 20 to 30 hp are common over a stock LS6 intake in hydraulic roller stroker combinations. Furthermore, the FAST intake is offered with 78-, 90-, 92-, and 102-mm throttle body openings.

FAST’s 102-mm LSXR intakes are the latest units in the company’s original LS manifold design evolution. It would be somewhat inaccurate to call them LS6-style intakes, because their design has several significant improvements over the standard LS6 architecture. In addition to their larger throttle body openings, they feature an increase in plenum volume and longer runners. Additionally, all eight runners can be removed individually for greater porting flexibility. Building upon these improvements, FAST’s 102-mm LSXRT intake offers the same basic improvements as the LSXR, but with even more plenum volume and runner length. Given enough hood clearance, the LSXRT is tough to beat in high-end street/strip or race applications that require a broad powerband with outstanding high-RPM airflow.

Aftermarket Rectangle-Port Intakes

The factory LS7 intake is an exceptional piece of engineering, but it was never intended to be pushed beyond the 700-hp mark that’s becoming more common with 450-plus-ci short-blocks and serious cylinder heads that now define stroker Gen III/IV small-blocks. Furthermore, because the stock LS7 intake isn’t compatible with L92/LS3 cylinder heads, enthusiasts opting for these castings needed a quality aftermarket intake manifold. FAST responded again with its 102-mm LSXR intakes, which are designed specifically for rectangle-port cylinder heads. Two versions are offered, one for LS7 cylinder heads and another for L92/LS3 heads. Independent third party dyno testing of the LS7 LSXR intake on a 500-ci hydraulic roller small-block showed an increased output from 720 to 750 hp over a factory LS7 manifold.

As a company famous for its diverse range of intake manifolds, it’s hardly surprising that Edelbrock offers a full line of dual- and single-plane LS-style intake manifolds, as well as EFI intakes. Edelbrock’s Super Victor intake manifolds are famous for their high-RPM breathing capabilities. (Photo courtesy of Edelbrock)

Since cylinder heads are only part of the induction equation, GMPP released a new line of LSX intake manifolds to go along with its LSX-LS3, LSX-LS7, LSX-CT, and LSX-DR heads. As with most carbureted LS-style intakes, they can also be drilled for fuel injectors. (© GM Corp.)

Porting the stock throttle body is a common trick that yields noticeable improvements in airflow. Katech’s LS1 units feature a smoothed throttle body radius, and the company claims that they flow an additional 5 to 10 percent of air over stock units.

Factory throttle bodies are either actuated by a throttle cable or an electric motor. A drive-by-wire throttle body can be used in an engine swap application, but it must interface with a matching computer and a stock gas pedal.

Carbureted Intakes

Edelbrock stunned the hot rodding community when it released a dual plane carbureted intake manifold for the LS1 in 2003. The Performer RPM LS1 intake manifold, designed for cathedral port heads, works in conjunction with an MSD ignition control module that replaces the stock computer; the manifold also allows replacing the factory EFI system with a carburetor. As with most dual-plane intakes, the Performer RPM places a priority on low- and mid-range torque over top-end power, with an operating range from idle to 6,500 rpm. More importantly, however, the success of the Performer RPM and its overwhelmingly positive reception by the hot rodding public set the quintessential paradigm shift in motion.

Almost all aftermarket throttle bodies are cable-actuated, which explains their popularity in engine swap applications. FAST’s billet Big Mouth units are available in 92- and 102-mm diameters. (Photo courtesy of Comp Cams)

Professional Products’ 80- and 85-mm throttle bodies for early Gen III engines are budget priced at $190. Each one includes an adapter plate that fits between the intake manifold and throttle body. This is said to smooth the throttle-body-to-manifold transition for improved airflow.

Ideally, the diameter of the throttle body and the intake manifold opening should be the same. Not surprisingly, FAST offers 102-mm throttle bodies to go along with its 102-mm LSXR intake manifolds. (Photo courtesy of Comp Cams)

Shortly after the launch of Edelbrock’s ground-breaking carbureted intake, companies, such as GM Performance Parts and Performance Induction, got in on the action with carbureted manifolds of their own. From these companies, there are now dozens of carbureted intakes in single-plane and dual-plane configurations for everything from stock-displacement motors all the way up to 10,000-rpm full race applications.

The significance of these intakes is two-fold. Not only do they enable hot rodders to enjoy the simplicity of carburetors on late-model LS engines, but they also allow EFI to be retrofitted onto these carb-style intakes. This is easily accomplished by drilling holes into the intake runners for fuel injectors and bolting a throttle body onto the carburetor pad. A conventional forward-facing throttle body can also be used by holding a 90-degree elbow adapter to the carburetor pad.

For high-end street or race applications where hood clearance isn’t an issue, a single-plane intake offers irrefutable advantages in horsepower. They typically have very large plenum volumes and generous cross-sectional area to assist with high-RPM breathing, as well as long runners to help minimize low-RPM power loss. In the average 440- to 460-ci hydraulic roller stroker combo with 250 to 260 degrees of duration at .050 inch, a single-plane intake usually sacrifices 20 to 30 ft-lbs of torque in the mid-range, compared to an EFI intake, but it makes up for it with an additional 50 to 60 hp beyond 6,500 rpm. Furthermore, single plane intakes extend the useable powerband by as much as 500 rpm. So, in applications with lots of hood clearance where giving up some low- and mid-range torque isn’t a big deal, a single plane intake manifold offers huge dividends in horsepower.

GM stepped up to a larger 90-mm throttle body in the 6.0L, 6.2L, and 7.0L Gen IV small-blocks. These units are capable of moving lots of air, and they can support more than 700 hp.

If using a 90-degree adapter on a single plane intake, it must be matched with a throttle body recommended by the manufacturer. Wilson Manifolds’ square flange throttle bodies bolt to the company’s elbows, but they aren’t necessarily LS-specific parts.

Throttle Bodies

As GM continually upped the horsepower ante with successive iterations of the LS-series small-block, throttle body diameters increased accordingly. The original 5.7L LS1 came equipped with a 75-mm throttle body, and the LS6, LQ4, and LQ9 received 80-mm units. By the time the 6.0L, 6.2L, and 7.0L Gen IV small-blocks entered production, the factory throttle bodies had grown to 90 mm.Additionally, aftermarket companies, such as Edelbrock, BBK, Wilson, FAST, Holley, and Summit, offer aftermarket units ranging from 80 to 102 mm.

FAST and Wilson Manifolds both offer throttle bodies that bolt directly to the carb pad on a single-plane intake. Naturally, they resemble a carburetor without a base plate. (Photo courtesy of Comp Cams)

Unlike a carburetor, the throttle body in an EFI engine doesn’t directly impact the amount of fuel that’s metered into the air intake charge. Consequently, EFI motors aren’t nearly as sensitive to changes in throttle plate diameter. A good rule of thumb to follow is that the throttle body diameter should match the inlet opening of the intake manifold. For example, if you are installing a factory LS3 intake manifold on a stroker engine buildup, its 90-mm inlet opening should be paired with a 90-mm throttle body. There aren’t any adverse affects of installing a throttle body that’s slightly larger than the inlet opening of the intake manifold, but it won’t improve performance, either.

Almost all aftermarket throttle bodies are cable-driven, whereas GM installed both cable-actuated and driveby-wire units in production cars. For stroker engine builds utilizing a cableactuated throttle body, switching to an aftermarket unit is a bolt-in affair. Alternately, a factory GM drive-by-wire unit can be retrofitted by installing a GM gas pedal that interfaces with the throttle body. For stroker LS buildups destined to power GM vehicles originally equipped with drive-by-wire throttle bodies, the stock 90-mm unit is the best option. This factory throttle body provides plenty of airflow, and it has proven to be effective beyond 700 hp. Other alternatives include converting to a cable-actuated throttle body or porting the stock unit. Smoothing out the throttle body radius and putting a bullnose finish on the leading edge of the throttle blades can boost airflow by 5 to 10 percent.

Written by Stephan Kim and Posted with Permission of CarTech Books

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As is often the case, the push to improve valvetrain durability starts at the highest levels of professional motorsports. Most sanctioning bodies limit maximum displacement as a means of trying to regulate horsepower levels, and teams inevitably reach a point where maximum RPM, rather than cylinder head airflow, is the limiting factor in power output. Consequently, he who turns the most RPM stands the best chance of winning the race, which explains why NHRA Pro Stock motors are now turning more than 11,000 rpm. Perhaps even more impressive are NASCAR Sprint Cup engines, which turn 9,500-plus rpm reliably for 500 to 600 miles each race. As the lessons learned on track have trickled down into the hot rod market, RPM is now limited more by the size of an enthusiast’s bank account than by the durability of the valvetrain components. The good news is that valvetrain hardware is more durable and affordable than ever.

Just a few decades ago, a pushrod V-8 capable of turning 7,000-plus rpm required a full-race valvetrain that would never survive the rigors of daily driving duty. Thanks to dramatic improvements in valvetrain technology, the LS7 small-block offers peak engine speeds on par with DOHC motors and is backed by a 100,000-mile warranty. (© GM Corp.)

Valvetrain Dynamics

In essence, the valvetrain is the link between the camshaft and the cylinder heads. Without the valvetrain, there is no valve actuation, and without valve actuation, there is no horsepower. This simple truth helps put the importance of the valvetrain into perspective. The more precisely the valvetrain translates the motion of the camshaft to the valves, the more horsepower it produces. In addition to precision, an optimized valvetrain must perform reliably over extended periods of time. This is far easier said than done, as the valve actuation process is nothing short of violent.

It all starts at the crankshaft, which transfers rotational motion to the camshaft through a crank gear, cam gear, and timing chain. As the camshaft turns, its eccentric lobes push up on the lifters. The reciprocating motion of the lifters then pushes upward on the pushrods and rocker arms, which then pivot like a see-saw to push open the valves. All the while, the entire valvetrain is working against the force of the valve springs, which attach to the valves with spring retainers and locks.

All of these moving parts make precise and reliable valve actuation extremely challenging, especially when the cumulative mass and deflection of all the components are taken into account. The more the valvetrain deflects, the smaller the cam appears to the engine, as the motion of the cam lobe isn’t precisely transferred to the valve. Consequently, camshaft manufacturers must consider the valvetrain mass and inertia as a whole when designing cam lobe profiles, as the entire valvetrain must work as a single system.

The goal is to get the valve motion to match the designed cam motion. During the initial opening of the valve, the lifter, pushrod, rocker arm, and valve spring are compressed into action. The lobe design has to be quick yet smooth to prevent transferring bad harmonics into the system, which causes springs to surge and potentially destroy them. As the lifter runs up the ramp to the peak of the lobe, the valve is opening farther, the parts are compressing more, and the dynamic loads are getting higher, placing tremendous stress on the system. At maximum lift, the valvetrain is fully compressed and struggling to rebound against the force imparted on it.

All LS-series small-blocks use beehive-style valve springs, units in which the upper coils are wound in a smaller diameter than the bottom coils. This arrangement removes weight from the area of the valve spring that experiences the fastest acceleration and moves the farthest distance. The result is a substantial reduction in active mass and superior high-RPM valvetrain control while using lower pressure. Beehive springs are naturally progressive in rate, and each coil vibrates at a slightly different frequency. This prevents the springs from going into resonance and provides a self-dampening effect. (Photo courtesy of Comp Cams)

The key to eliminating valve float is to reduce valvetrain mass and deflection. Durable springs, lightweight valves, and lightweight retainers go a long way in promoting valvetrain stability and extending the RPM range of an engine.

A hydraulic roller lifter has an internal piston assembly that pushes up against a cushion of oil stored inside the lifter body. This deflection allows a hydraulic lifter to absorb the expansion of the valvetrain components as they heat, but it also increases mass and the lifter’s tendency to pump up at high RPM.

Not only are solid roller lifters lighter than their hydraulic counterparts, they can endure far steeper cam lobe profiles to open and close the valves more quickly. Furthermore, they can handle higher spring pressures for improved high-RPM stability. For sustained 7,500-plus-rpm operation, using a solid lifter valvetrain is a must. Their only downsides are cost, noise, and routine lash adjustments.

At high RPM, the inertia of the entire valvetrain going over the nose of the lobe resists the return spring force, and the lifter tries to hang in the air instead of following the cam profile. To prevent this, the mass of the valvetrain and spring pressure must be sufficient enough to allow the lifter to follow the cam all the way down the ramp. If not, the lifter bounces, which damages the valve seat and sends harmonics through the system that destroy the springs as well as the needles in the roller lifters. This reduces the volumetric efficiency of the engine, as the valve doesn’t seat properly and robs horsepower.

As daunting as all that may seem, selecting the right valvetrain components for a stroker motor is rather straightforward, because camshaft manufacturers have already done most of the homework for you. A cam’s duration and lift specs determine the type of valve springs that will need to be used. After spring pressure and max valve lift have been established, the balance of the valvetrain components can be selected based on durability and the target RPM range of the engine combo. If that’s not easy enough, cam manufacturers often publish a list of matching valve springs, retainers, locks, rocker arms, pushrods, lifters, and timing sets to go along with their off-the-shelf camshafts.

Fighting Float

The inherent challenge of designing valvetrain components is trying to make them as light as possible to reduce valvetrain inertia while also making them as stiff as possible to reduce deflection. Unlike an overhead cam engine that positions the camshafts on top of the cylinder heads for a more direct actuation of the valvetrain, an OHV motor must transfer the reciprocating motion of the cam lobes from the center of the block all the way up to the cylinder heads by using lifters, pushrods, and rocker arms. More moving parts means more weight, which is a big problem when the entire valvetrain must reverse its direction of travel every time the valves open and close.

Early solid roller lifters were plagued by reliability issues in street applications. That’s because they were used mainly in race engines where lifters relied solely on oil thrown up by the crankshaft for lubrication. That works fine in high-RPM race conditions, but not so much for street cars that spend lots of time at idle. To solve this problem, valvetrain manufacturers, such as Comp Cams and Isky, have developed street solid roller lifters that feature oil passages to the needle bearings. This provides a steady supply of oil, regardless of engine speed. (Photo courtesy of Comp Cams)

By nature, aggressive cam lobe profiles require stiffer spring pressure to keep the lifter seated, but this also increases stress on the valvetrain. That, in turn, increases the potential for deflection, and the stiffer valvetrain hardware required to resist this deflection can increase mass. It’s an ugly cycle, but having too much valvetrain deflection and mass leads to valve float, a condition where the valvetrain can no longer control the motion of the valves. When this happens, the valves open and close erratically, crash into the valve seats, and limit how many RPM an engine can turn. In extreme cases, the valves can slam into the pistons and destroy an entire engine.

In order to prevent valve floating, taking a single-system approach to valvetrain setup works best. For example, because installing stiffer valvesprings increases the loads placed on the rocker arms, pushrods, and lifters, it’s imperative to make sure the rest of the valvetrain is up to par when upgrading just one of the components in the entire system.

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Lifters

The cylindrical slugs of metal that ride the surface of the cam lobes are called lifters, tappets, or followers. They’re retained inside recesses, known as the lifter bores, in the block. Regardless of what you call them, lifters can be classified into four groups: hydraulic flat tappets, hydraulic rollers, mechanical flat tappets, and mechanical rollers. Unlike their flat-tappet counterparts, roller lifters have a roller wheel assembly integrated into the base of their bodies. This substantially reduces friction and allows for much steeper cam lobe profiles. In other words, for any given amount of duration, a roller lifter can handle much more lobe lift. This allows camshaft designers to lift the valves high enough to take advantage of the high-lift flow potential of modern cylinder heads while keeping duration short enough to retain excellent drivability.

All LS-series small-blocks are equipped with hydraulic roller lifters from the factory, and in fact, GM hasn’t installed flat-tappet cams in a production small-block in decades. In the distant past, roller lifters were more prone to failure, as the allocated valvespring pressure loads onto a smaller surface area. Additionally, the roller wheels ride on needle bearings, which present another area of potential failure. Nonetheless, roller lifter technology has improved to the point that these drawbacks have been mostly eliminated. For proof, look no further than any production GM small-block built today, whose roller lifters often last for 200,000 miles. Unless an engine is being built for an obscure racing class that bans roller lifters, LS-series small-blocks are rarely built with flat-tappet lifters.

That said, both mechanical and hydraulic roller lifters are used in the vast majority of stroker Gen III/IV engine combos. Mechanical lifters, or solid lifters, as they are sometimes called, are solid pieces of metal. Because the valvetrain components expand as they heat up, a valvetrain utilizing mechanical lifters must be set up with some slack to accommodate this growth. This clearance, or lash, is measured between the rocker arm and valve stem tip using a feeler gauge. Naturally, this slack makes for noisier valvetrain operation. Conversely, hydraulic lifters incorporate an internal cavity filled with oil and a piston. This hydraulic assembly enables much of the lash to be removed from the valvetrain, because the piston inside the lifter compresses as the valvetrain expands, which eliminates the clatter associated with mechanical lifters. In addition to quieter operation, hydraulic lifters eliminate the need for periodic lash adjustments that mechanical lifters require.

The reduced maintenance and quieter operation offered by hydraulic lifters are the primary reasons why GM uses them in all LS-series small-blocks. Those benefits aside, solid roller lifters offer clear performance advantages, particularly at high RPM. On a typical street/ strip engine, a hydraulic roller system often experiences valve float between 6,500 and 7,000 rpm. Hydraulic lifters are more prone to valve float, due to their greater mass and tendency to pump up at high-RPM. Furthermore, higher valvespring pressure goes a long way in reducing valve float, but there is only so much pressure the piston assembly of a hydraulic lifter can handle.

When assembling a set of cylinder heads, it’s always a good idea to pressure-test the valve springs. This ensures that pressure is uniform across all 16 valves. Pressure-testing used springs is the best way to determine whether or not a set of used springs needs to be replaced.

Valve spring pressure is dependent upon how much the spring is compressed, so a spring’s installed height must be adjusted to make sure that the spring is neither too soft nor too stiff. This is accomplished by placing the necessary number of shims beneath the spring, then slipping a height mic over the spring to measure the installed height.

By comparison, a solid roller lifter’s lower mass and ability to manage greater valve spring pressure make it the clear victor in applications exceeding 6,500 rpm. Although the performance difference between a hydraulic roller system and a solid roller application with similar camshaft specs might be marginal up to about 6,000 rpm, beyond that point the horsepower advantages of a solid roller lifter can easily exceed 50 hp.

When it comes to valvetrain hardware, RPM usually costs money, and that’s definitely the case with a mechanical roller valvetrain. Not only are the stiffer valve springs necessary to run a solid roller valvetrain more expensive, the rocker arms and the lifters themselves also cost more. Additionally a solid roller valvetrain’s steeper cam lobes and stiffer valve springs increase the stress on the entire valvetrain, so stronger rocker arms are required, which adds cost.

For example, a set of mild beehive valve springs for a hydraulic roller cam costs about $200, and super-stiff dual springs for a solid roller cam cost $400. Likewise, a set of $150 stock GM rocker arms will work fine in a hydraulic cam application, but a high-RPM solid roller combo often requires a $1,500 shaft mount rocker arm setup. At the end of the day, solid roller lifters offer irrefutable advantages over their hydraulic counterparts, but they require much more expensive valvetrain hardware.

Although flat-tappet lifters aren’t common in Gen III/IV engine builds, it’s worth noting that a solid flat-tappet lifter system can sometimes outperform a solid roller setup. These instances aren’t common, but certain race classes, such as in circle track, sometimes impose limits on valve lift or duration. In such a scenario, solid flat-tappet lifters can be advantageous over solid roller lifters, because they offer quicker initial lobe acceleration very early in the lift curve. Roller lifters can, indeed, handle higher peak ramp acceleration rates, but solid flat tappets have the advantage very early in the lift curve. And, if a racing class limits peak valve lift to, say, .500 inch, then a solid flat-tappet cam might actually perform better than a roller cam.

Solid Rollers for the Street

In many hot rodding circles, solid roller camshafts and street cars don’t mix, and that reputation is well earned. Decades ago, solid roller lifters were plagued with reliability issues, as they were originally designed for high-RPM race use. The only way their needle bearings could be lubricated was from oil thrown up by the crank. Because race cars spend very little time at idle and low RPM, that oiling method worked just fine. However, when people tried to use the lifters on the street, the same lifters that lasted for several seasons in race cars were failing more quickly. Compounding the problem of using solid roller lifters in a street car was that most old roller cams were designed for very high spring loads. This kept everything under control at high RPM with an aggressive cam, but it greatly increased valvetrain load at low speed.

Camshaft manufacturers recognized these problems and pioneered several effective solutions. First, mechanical roller lifters were completely redesigned to include an integrated oil band with a small hole to feed oil down to the needle bearings. Furthermore, the steel used for the axle was greatly increased in strength and redesigned to distribute load more efficiently. Second, new lobe profiles and valve springs were developed specifically for street use. These new profiles perform very well in the 2,000- to 7,000-rpm range while requiring far less spring pressure than older race profiles. Together, these changes make it possible to now run a solid roller cam in a street car without any of the past reliability issues. Although it is recommended to check valve lash every 5,000 to 6,000 miles with solid lifters, the need to do so is greatly reduced with rocker arms that have poly locks.

Retainers prevent the valve springs from shooting off the heads, but because they’re positioned at the very top of the springs, they must accelerate and change directions rapidly while traveling a long distance. In an effort to reduce mass, high-end street and race engines often employ titanium retainers, which can extended an engine’s operating range by 200 to 300 rpm before valve float sets in.

All Gen III/IV small-blocks feature stamped-steel rocker arms with roller trunions. LS7s utilize a 1.8:1 ratio, and all other LS motors have a 1.7:1 ratio. The rocker arms are extremely durable and capable of handling 7,000 rpm.

Aftermarket rocker arms have trunions that are beefier than those on factory units in order to handle higher valve spring loads. Additionally, their bodies are contoured for additional clearance for larger-diameter springs. They’re constructed from both aluminum and steel. Aluminum rockers are lighter, easier to manufacture, and provide a dampening effect on the valvetrain. However, they have a more limited life cycle. (Photo courtesy of Comp Cams)

Valve Springs

The valve springs sit in recesses, or pockets, machined into the cylinder heads and provide tension upon the valves. Valve springs force the valves shut against the seats until they’re compressed by the rocker arms, at which point the valves open. Valve springs attach to the valves with retainers and locks, which center the springs around the valves and keep them in a slightly preloaded state. This load is known as the spring’s seat pressure, and the load the spring provides at maximum valve lift is called open pressure.

The position of the valve springs in relation to the rest of the valvetrain helps put their importance into perspective. During compression, the springs are responsible for keeping the lifters in contact with the cam lobes so they can precisely follow the contour of the ramps. As the springs rebound after the point of maximum valve lift, their job is to close the valves in a controlled fashion while preventing them from bouncing off their seats. Accomplishing both of these functions requires having just enough spring pressure.

Valve spring pressure is determined by several factors, chief among them are spring rate and load. The rate of a valve spring is simply the force required to compress it 1 inch. For instance, a spring that compress 1 inch under 100 pounds of force has a spring rate of 100 pounds per inch. The spring rate, combined with how much the spring is compressed (load), determines valve spring pressure. For illustrative purposes, let’s take a look at a set of Comp Cams springs designed specifically for LS-series small-blocks, part number 26921. They feature a spring rate of 408 pounds per inch, which, at an installed height of 1.770 inches, yields 135 pounds of seat pressure. Compressed to a height of 1.120 inches, the spring load increases to 400 pounds of open pressure.

Compared to aluminum units, steel rocker arms are heavier, but that extra mass provides an increase in durability and life cycle. Conversely, they’re more difficult to manufacture and harder on the rest of the valvetrain. (Photo courtesy of Comp Cams)

In a stud-mount rocker arm arrangement, the stud bears the brunt of the load imparted by the valve spring and pushrod. In a shaft-mount rocker system, the rocker pivots around a central shaft that’s bolted to the cylinder head. With this setup, the shaft absorbs most of the valvetrain load, reducing deflection and increasing durability and high-RPM stability. (Photo courtesy of Comp Cams)

A shaft-mount rocker arm system makes it easy to compensate for pushrod offset, because a rocker can easily be moved onto different parts of the shaft. Additionally, they’re available in just about every ratio imaginable.

Because spring diameter, wire thickness, and the number of active coils affect the spring rate and, therefore, pressure, aftermarket manufacturers are constantly juggling these variables around to establish a wide variety of pressures for a diverse range of applications. Nevertheless, because wire diameter and the number of coils are built into a spring and can’t be changed, the only two factors relevant to engine builders are the diameter of the spring and how much the spring is compressed after installation. That’s because spring compression, or installed height, can be adjusted after the spring has been installed on the cylinder head. Using shims to adjust the installed height serves as a handy fine-tuning tool in achieving target spring pressure. As for spring diameter, the size of the valve spring pocket determines the maximum-diameter spring that can be used. Larger-diameter springs provide more pressure, but they also require the spring pockets to be machined wider, and there’s a physical limit to how much the pockets can be opened up.

With stock Gen III cathedral-port cylinder heads, a 1.250-inch-outsidediameter valve spring is as large as you can go. The valve spring pocket can safely be machined to about 1.450 inches, but anything larger runs the risk of breaking into the ports. Factory rectangle-port castings have more commodious pockets and can be enlarged safely to approximately 1.550 inches. Even so, with the very heavy-duty spring pressures that aggressive camshafts require, sometimes running a larger-diameter valvespring just isn’t enough. Consequently, aftermarket manufacturers offer dual valve springs that feature a small inner spring that fits inside the primary spring to increase pressure.

Ultimately, optimizing valve spring pressure is a delicate balancing act, as too much pressure will place undue stress on the valvetrain, and not enough pressure will compromise valvetrain stability. Load is just one of the many factors that needs to be addressed when selecting valve springs, and more isn’t always better. A lower-mass spring with less load often performs far better than a fat spring with more load. The trick is to use a spring that offers just enough pressure to get the job done, and not a pound more. As with camshaft selection, there is no universal rule of thumb to follow when it comes to choosing the right valve springs. Instead, opting for springs that are proven performers in applications similar to yours will usually suffice. On the other hand, if your combo isn’t exactly mainstream, it’s not a bad idea to seek expert advice to avert potentially catastrophic engine failure. Camshaft manufacturers have thousands of hours invested into developing valve springs, so it makes sense to tap into their expertise.

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Retainers

Even on a head as mild as a stock LS6 cylinder head, the valve springs exert 90 pounds of seat pressure. So even at 0 rpm, the valve spring needs to be cinched tightly in place to prevent it from launching off the cylinder head. That’s the job of the retainer, which sits on top of the valve spring and locks into a notch machined into the valve stem with a set of valve locks. Like most production engines, the LS-series small-block utilizes steel retainers that do a fine job in the 6,000 to 7,000 peak RPM that they’re designed for. Beyond that point, however, reducing retainer mass just a few grams can extend an engine’s peak RPM dramatically.

Mast Motorsports offers drop-in, shaft-mount rocker arms that are direct replacements for the factory units. Instead of having eight rocker arms riding on a central shaft, the Mast design features a shared shaft for each pair of rockers. The design is compact enough to fit under stock valve covers.

Titanium is the most commonly used material for lightweight retainers, and they weigh roughly 40 percent less than standard steel retainers. This decrease in mass alone is enough to add up to 200 rpm before an engine starts floating the valves. Although that can be the difference between winning and losing in a competitive racing class, the increased cost of titanium retainers makes them cost-prohibitive for most street/strip motors. A set of standard steel retainers can be had for $50, while equivalent titanium units ring up a $250 tab.

To bridge that gap, Comp Cams has recently introduced a new line of lightweight tool steel retainers that offer the best of both worlds. These retainers tap into the company’s NASCAR connections, and according to Comp, each one weighs just 2 to 4 grams more than a titanium retainer. Furthermore, extensive testing has revealed that they are just as durable. The best news is the price, as Comp’s tool steel retainers cost $150 for a set of 16.

Rocker Arms

A rocker arm is responsible for converting the upward motion of the pushrod into the downward motion that pushes the valve open. To accomplish this, a rocker pivots like a see-saw on a trunion, which acts as a fulcrum. The stiff valve spring pressure and high RPM associated with aggressive camshaft grinds place tremendous loads on the rocker arms. The upward force of the lifters and pushrods works against the pressure of the valve springs, and steeper cam lobe profiles and stiffer springs only compound the situation.

Fortunately, the factory GM rocker arms are excellent pieces of hardware. All LS-series small-blocks come equipped with 1.7:1 roller rocker arms from the factory. The only exception is the LS7, which uses 1.8:1 roller rockers. Compared to those of the Gen I small-block Chevys, most of which were equipped with stamped 1.5:1 rockers, Gen III/IV rocker arms are more like race hardware than typical factory equipment.

Most aftermarket and factory LS rocker arms incorporate a roller tip that presses down on the valve stem. At high lift, a plain tip has a tendency to place unwanted side loads on the valve stem, and a roller tip helps alleviate this effect.

The pushrods represent one of the biggest areas for potential deflection, because they bear the opposing forces imparted by the valve springs and cam lobes. If a pushrod isn’t stiff enough, reciprocating motion that should be transferred to the rocker arm instead flexes the pushrod, thereby reducing the effective valve lift and duration. Stiffer pushrods feature thicker walls, which increases weight, but the benefits in valvetrain stiffness are worth the tradeoff.

Stock LS rockers perform extremely well at engine speeds up to 7,000 rpm. For many stroker combinations, it’s not even necessary to replace the stock rockers with aftermarket units. They do have their limits, however, and with elevated valve spring pressures and sustained 7,000-plus-rpm operation, aftermarket rocker arms are a wise investment. Another drawback of stock rocker arms is that they are not adjustable. That’s great for reducing manufacturing costs in a high-production, volume environment, but in performance applications where the valvetrain geometry has been altered, the only way to adjust the stock rockers is by changing pushrod length and the rocker stand height.

The three primary advantages that aftermarket rockers offer over their stock counterparts are reduced deflection, lower mass, and a higher multiplication ratio. Deflection can be reduced using several different methods. Aftermarket rocker arms are built using stronger alloys than those in production units, which reduces flex. Also, production and entry-level aftermarket rocker arms are pedestal-mount designs that attach to the cylinder heads using bolts or studs. In this type of arrangement, the stud is often the area of the valvetrain that’s most prone to flex. The aftermarket offers several solutions, with companies, such as Comp Cams and ARP, offering stiffer 3/8-inch rocker studs that replace the factory bolts.

Most pushrods have a standard 180- degree radius on each end. In applications exceeding .750-inch lift, it’s sometimes necessary to upgrade to pushrods with a 270-degree radius to prevent interference between the pushrod end and the pushrod cup in the rocker arm.

A decades-old technique is to bolt a girdle on top of the rocker arm assembly. This dramatically reduces deflection and stabilizes the valvetrain. With quality aftermarket rocker arms, studs, and a girdle, a pedestal-mount rocker system can operate safely at 8,000 rpm. Even so, such an arrangement is fairly uncommon in a typical LS stroker build, because, in recent years, more cost effective shaft-mount rocker arm systems have entered the market.

Shaft-mount rockers pivot on a centrally mounted shaft that’s bolted to the cylinder head. The body of the rockers actually slides around the shaft, decreasing friction and increasing mounting stiffness. Although shaft mount rockers don’t make horsepower in and of themselves, they allow an engine builder to make more power by providing a stable, high-RPM valvetrain platform to work with. An entry-level shaft-mount rocker system costs about $800 to $1,000, which is only marginally more than the combined cost of pedestal-mount rockers, aftermarket studs, and a girdle. Furthermore, stud girdles also require using a valve cover spacer, which is another strike against a pedestal-mount rocker system in a high-RPM application.

Rocker Ratio

Because valve lift is simply lobe lift multiplied by the rocker arm ratio, there are several lobe-and-rocker ratio combinations that can be used to achieve a target valve lift figure. Some engine combinations utilize lots of lobe lift with a relatively conservative rocker arm ratio, and others feature conservative lobe lift and a very aggressive rocker arm ratio. One combo isn’t necessarily better than another, and there is a time and place for each. Adding the acceleration speed with the rocker is easier on harmonics and valvetrain stability in relation to RPM. With a higher ratio, the rocker is responsible for valvetrain acceleration, which allows for a gentler cam lobe ramp design. Generally, high ratios can be used to open the valve off the seat more quickly, and lower ratios can be used to stabilize a valvetrain that is out of control. Using a lower rocker ratio reduces the load on the pushrod and thereby helps increase stiffness. Ultimately, cam manufacturers design their lobe profiles around specific rocker ratios in mind, so for the average hot rodder, the issue isn’t worth splitting hairs over.

Any time the lifter height, block deck height, gasket thickness, cylinder head deck thickness, or valve stem length is altered, the pushrod length must also be changed. Pushrod length determines the rocker-tip-to-valve stem geometry. The ideal pushrod length places the rocker tip at the center of the valve stem tip at mid lift.

Pushrods

In some respects, pushrods represent the inherent inefficiency of mounting a camshaft in the middle of the block and having it transfer the reciprocating motion of the cam lobes and lifters all the way to the top of the cylinder heads. Breakthroughs in valvetrain technology have helped overcome this setback, and improvements to the pushrod itself are part of that equation. As with the rest of the valvetrain, pushrods must be stiff to resist deflection and accurately translate cam motion to the valves, but they also need to be lightweight to reduce inertia.

To accomplish this, pushrods come in a dizzying array of steel alloys, such as 1010, 4130, and 4340. The other two ingredients to pushrod stiffness are diameter and wall thickness. All stock LS-series small-blocks come equipped with 5/16-inch pushrods, except for the LS7, which has 3/8-inch pushrods. Although factory pushrods work fine under normal operating conditions, sustained high RPM operation and a few missed shifts can bend them up rather quicker. Consequently, aftermarket pushrods from companies, such as Comp Cams, Manley, Trick Flow, and Isky, are highly recommended in all stroker motor combinations. In addition to using superior alloys, aftermarket pushrods are offered in both 5/16- and 3/8-inch diameters with wall thicknesses ranging from .080 to .125 inch.

Because the length of the pushrod determines the rocker-arm-to-valve-tip geometry, a pushrod’s length is just as important as its stiffness and mass. Block deck height, cylinder head deck thickness, rocker arm design, camshaft base circle size, lifter design, and valve stem length all affect the correct pushrod length of an engine combo. Optimizing pushrod length allows the rocker arm tip to press on the center of the valve stem, and aftermarket manufacturers sell tools that make it easy to measure for the proper length. Tools to check pushrod length are basically threaded rod assemblies that can be varied in height until the correct pushrod length has been determined. Most of the time, the correct-length pushrods are offered as off-the-shelf items, but when they aren’t, manufacturers offer custom-length units at a very reasonable price.

The single-roller factory LS2 timing chain offers outstanding performance at a dirt-cheap price. It can handle more than 400 pounds of open valve spring pressure and has proven to be reliable beyond 7,000 rpm.

The cam phaser assembly used in VVT Gen IV small-blocks is nothing more than a rotor within a stator. The cam gear rotates via chain as usual, but the rotor and vanes don’t physically come into contact with it. Instead, a cushion of oil on both sides of the rotor determines where the camshaft will be positioned in relation to the crankshaft. (© GM Corp.)

Timing Sets

In order for an engine to operate properly, the camshaft’s rotation must be perfectly synchronized with the crankshaft. It’s up to the timing set to get the job done, but the stiffer valve springs used in high-performance engine combinations increase load and, therefore, the potential for deflection. The good news is that there are dozens of factory and aftermarket timing set options that can easily handle the most demanding of engine combinations.

On the factory front, GM redesigned the timing chain assembly with the introduction of the LS2 in 2005. The single roller LS2 unit features a stronger alloy steel and thicker side plates for significant improvements in strength over the Gen III design. This chain has proven reliable in applications having slightly more than 400 pounds of valve spring open pressure.

The purpose of the valvetrain is to open and close the valves, but the valves themselves must also be lightweight to avert valve float. To accomplish this, valves are built with hollow stems, and in extreme cases, they are whittled into shape out of a slab of titanium. (© GM Corp.)

When setting the installed height of a valve spring, it’s important to check the valve seal- to-retainer clearance. Not doing so can cause the retainer to smack into the seal and destroy it. At maximum valve lift, there should be a minimum of .050 inch of clearance. (© GM Corp.)

At anything beyond 400 pounds, an aftermarket timing set buys cheap insurance. They’re offered from companies, such as Comp Cams, Trick Flow, SLP, Manley, and Katech, in both single- and double-roller applications. Generally, double-roller sets are more durable, but some single-roller timing sets are also extremely durable. Aftermarket timing sets have adjustable cam and/or crank gears, making it possible to advance or retard the cam manually.

In addition to offering them as a traditional timing chain setup, aftermarket companies also offer timing sets as belt drives and gear drives. Belt drives are usually mounted outside of the timing cover, which allows for quick and easy timing changes without removing the timing cover or water pump. Also, this feature greatly simplifies the cam swap process. Additionally, belt drives reduce frictional losses and oil windage while helping to dampen engine harmonics. Their biggest downside is that they cost two to three times as much as a chain drive setup. Gear drive timing sets use a series of gears between the cam and crank sprockets to synchronize them. They offer the ultimate in durability and timing precision, but they also generate lots of noise.

Timing Covers

From the factory, LS-series small-blocks come with four different types of timing covers. The Gen III timing cover, also known as the LS1/LS6 cover, is the most basic and has no provisions for a cam sensor. The standard Gen IV timing cover, which is used on most non-VVT Gen IV engines, is similar to the Gen III unit but with an integrated cam sensor. The LS7’s dry sump oiling system uses a larger pump assembly, and it requires its own timing cover for extra clearance. The VVT-equipped Gen IV timing cover has an electric solenoid assembly built in that actuates the oil control valve in the camshaft.

All factory timing covers are durable cast-aluminum units, and there’s no real performance advantage to upgrading them. However, with some aftermarket double-roller timing sets, it’s necessary to grind down the factory timing cover for additional clearance. Two-piece aftermarket timing covers don’t offer much in the way of performance gains, but they do make swapping out cams much easier.

Although it’s possible to calculate spring pressure by hand using simple arithmetic, it’s far more practical to simply look up a manufacturer’s published valve springs specs or to measure spring pressure in a testing tool.

Written by Stephan Kim and Posted with Permission of CarTechBooks

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By controlling how much, how long, and when the intake and exhaust valves open and close, the camshaft determines how much horsepower an engine makes and the RPM range in which that power is concentrated. The camshaft also profoundly impacts gas mileage and emissions quality, which probably aren’t very high on the priority list for hot rodders, and it also affects idle quality and low-RPM drivability, which are huge concerns for any street driven vehicle.

What makes proper camshaft selection so critical is that going too big or too small can ruin an otherwise perfect engine combination. Too conservative of a cam won’t allow an engine to take full advantage of the cylinder heads’ airflow capabilities. Too aggressive of a cam can compromise low-speed drivability so badly that you wonder why you spent so much time and money maximizing the cubic inch total in the first place. As with cylinder head design, camshaft theory is an extremely complex science that involves dozens of inter-related variables. It’s quite possible that there are even fewer true camshaft experts than there are cylinder head gurus, and that’s really saying something.

All LS-series small-blocks come equipped with a hydraulic roller camshaft from the factory. Thanks to improvements in modern valvetrain technology, this combines the convenience and low maintenance of hydraulic lifters with the high-RPM potential once reserved for mechanical lifters.

The good news is that you don’t need to know how to design the perfect lobe profiles in order to pick the ideal camshaft for your stroker motor project. Just learning the basics of camshaft theory will get you in the ballpark, and consulting with camshaft manufacturers and engine builders will get you the rest of the way there.

Camshaft manufacturers have invested thousands of hours into designing hundreds of off-the-shelf cam grinds that complement the vast majority of engine applications. Oftentimes, an off-the-shelf grind out of a catalog works remarkably well, and at roughly $400, it’s not terribly expensive to experiment with different camshafts, if necessary. If you do need to spec out a custom cam, chances are its design is based upon an off-the-shelf grind that’s been slightly modified to suit the specific demands of your application. Either way, the consumers are the direct beneficiaries of the massive R&D efforts of the major camshaft manufacturers, such as Comp Cams, Lunati, Isky, GMPP, and Edelbrock. Just learn the basics, and you’ll be on your way to selecting the perfect camshaft for your stroker buildup.

The camshaft determines how long the valves stay open and how far they move off their seats, as well as when these events take place. These factors profoundly impact how much power is produced, in addition to where that power is concentrated in the powerband.

Although LS enthusiasts looking for maximum high- RPM power usually upgrade from a hydraulic roller cam to a solid roller cam, a solid flat-tappet cam is also a very capable performer. It’s lighter and offers superior high-RPM valvetrain stability compared to a hydraulic roller cam, which yields greater power potential. Likewise, its flat noses offer quicker initial lift acceleration than a roller design, making it an appealing option for racing classes that place restrictions on maximum duration or lift. (Photo courtesy of Comp Cams)

Cam Effects

Although camshaft dynamics is an extremely complex subject, a few basic universal truths of cam theory can help simplify understanding the role a cam plays in overall engine performance. Replacing a stock camshaft with a larger aftermarket unit having longer duration and higher lift almost always yields an increase in horsepower. Likewise, the larger the camshaft, the higher in the RPM band an engine produces peak horsepower and torque. And because horsepower is simply torque multiplied by RPM, moving the torque peak higher in the RPM range increases horsepower every single time.

The drawbacks of big camshafts are that they increase emissions output and decrease low-RPM torque, throttle response, and intake manifold vacuum at idle. Again, tailpipe emissions and gas mileage probably aren’t big concerns for the typical hot rodder, but compromised low-speed torque requires shorter gearing, and an engine that doesn’t produce adequate idle vacuum won’t be able to actuate a power brake system. Furthermore, extremely aggressive camshaft grinds also accelerative wear and tear on the rest of the valvetrain components. Combating this with a heavier-duty valvetrain drives up cost considerably.

That said, like every other aspect of engine building, choosing the right camshaft is all about balance. Having an irrational phobia for duration and lift is a good way to guarantee that an engine never produces respectable power and torque. For instance, it makes no sense whatsoever to invest thousands of dollars in a set of top-notch cylinder heads and a forged rotating assembly capable of handling 9,000 rpm if the parts combination is going to be hampered by a dinky hydraulic roller cam that isn’t capable of fully exploiting an engine’s airflow and RPM potential. Furthermore, different drivers have different tolerances for low-RPM surge and choppy idle quality, so what’s considered an aggressive cam or a tame cam is purely subjective. Although a high school kid may find a lopey 230-at-.050 cam charming and intoxicating in a 346-ci motor, an older and more mature enthusiast might find the same cam unstreetable in a 396.

Varying tastes and tolerances aside, what can’t be disputed is that larger cubic inch engines reduce the adverse effects of a more aggressive camshaft. For example, a camshaft that struggles to idle and suffers from very poor throttle response in a small-displacement motor idles like stock and yields tire-shredding low-end torque in an engine that’s 100 ci larger.

Camshaft grinding is an extremely precise process in which tolerances are held to thousandths of an inch. In order to endure higher valve spring pressures and steeper ramp acceleration rates, solid roller camshafts are usually ground from a durable billet core.

Ideally, the effects of altering duration and lift would be independent from each other, but this isn’t possible, because both are determined by the shape of the cam lobe. Increasing lift increases the distance the lifter must travel around the cam lobe, thereby increasing duration. A good rule of thumb is to select duration based on an engine’s operating RPM range, and lift should be based on a cylinder head’s airflow at high lift.

Expanding upon that example, let’s say there are two engines, with identical camshafts, cylinder heads, intake manifolds, and compression ratios, but one measures 383 ci and the other measures 427 ci. The peak horsepower output between the two is similar, but the 427 produces far more low- and mid-range torque and manifold vacuum while peaking at a lower RPM. That equates to a far more streetable package that places less stress on the valvetrain components, enhancing durability, and enables running taller gearing for improved gas mileage. Consequently, camshaft selection must be closely matched to an engine’s displacement and intended usage. Simply changing the camshaft can transform driving characteristics from that of a stock-caliber rebuild to a low RPM street cruiser to a dual-purpose street/strip machine, or a high-RPM race engine.

Lobe Profile

At the risk of pointing out the obvious, a camshaft is a shaft fitted with eccentric cam lobes, and lobe lift is the difference between the radius of the cam lobe’s base circle and the height of the eccentric. On an OHV engine like the LS-series small-block, the camshaft is mounted inside the block, and one lobe is designated for each valve, for a total of 16 lobes. The eccentric shape of the cam lobes enables them to convert the rotating motion of the camshaft into reciprocating motion. It’s this reciprocating action that pushes up on the lifters, pushrods, and rocker arms, thereby opening and closing the valves. How far the cam lobes push open the valves is referred to as “lift,” and the length of time the valves stay open is called “duration.”

The shape of the cam lobes establishes lift and duration, and consequently, they are interdependent. For instance, grinding down a cam lobe to reduce lift also reduces duration. Likewise, the maximum amount of lift that can be ground into a cam is ultimately limited by cam duration. That’s because increasing lift without increasing duration creates a steeper lobe profile, and there’s a physical limit to the rate of ramp acceleration that both the camshaft and lifters can handle. Under ideal circumstances, camshaft design would allow for isolating the effects of duration and lift from each other in terms of how they affect power, but that simply isn’t the case. Even so, understanding the relationship between duration and lift is a useful tool in the camshaft selection process.

The distance between the peaks of the intake and exhaust lobes establishes a camshaft’s LSA. Although duration and lift dictate how much power an engine will produce, the LSA is a fine-tuning tool that changes where in the RPM range horsepower and torque is concentrated.

If duration, lift, and LSA were the only factors in camshaft dynamics that influenced performance, then one manufacturer could reverse engineer another company’s R&D efforts by simply duplicating their published camshaft specs. The truth of the matter is that there are far more variables involved in designing a camshaft. Two camshafts with identical duration and lift figures can have vastly different acceleration curves. The challenge for manufacturers is figuring out how hard the lobes can push the valvetrain without creating excessive deflection and introducing valve float. (Illustration courtesy of Comp Cams)

Duration

Of the multitude of variables that go into designing a camshaft, duration has the most profound impact on power production. Because a camshaft rotates at half the speed of the crank, duration is expressed in degrees of crankshaft rotation. This represents how long the valves stay open in relation to crankshaft rotation. For instance, a cam that has 250 degrees of duration at .050-inch lift stays open for about 250 out of the 360 degrees that it takes the crank to make one complete revolution. At low RPM, when there is plenty of time to fill the cylinders with air, short-duration camshafts perform very well. However, as RPM increase, and the amount of time available to fill the cylinders decreases, a short-duration cam literally chokes off an engine’s air supply, and horsepower plummets accordingly.

The concept of time in relation to cylinder filling might seem awkward at first, but it’s actually very easy to conceptualize. Consider that at 2,000 rpm, the intake valves open and close roughly 17 times per second. At 6,000 rpm, however, that figure increases to 50, giving the incoming air charge far less time to fill the cylinders with air each time the intake valves open and close. Because longer-duration camshafts hold the intake valve open longer, thus improving cylinder filling, they improve horsepower and torque output at high RPM. Additionally, they also extend the RPM at which peak power is produced. For example, swapping out a 220-at-.050 cam in a 408 stroker motor with a 240-at-.050 cam increases the horsepower peak from about 5,500 rpm to 6,000 rpm.

On the other hand, the same long duration camshaft that works so well at 5,000-plus rpm sacrifices low- and mid-range torque compared to a shorter duration cam. In order to maximize cylinder filling, it’s common for the intake valve to stay open even after the piston passes BDC on the intake stroke. As a result, the intake valve doesn’t close until after the piston begins traveling back up the bore during the compression stroke. That might sound like a bad idea at first, but at high RPM, the intake air charge packs enough energy and velocity to continue filling the cylinder even after the piston passes BDC.

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Unfortunately, that’s not the case at low RPM, when the incoming air charge simply lacks adequate velocity to do so. Consequently, a portion of the intake air charge is pushed back past the intake valve and into the intake manifold. The subsequent drop in cylinder pressure accounts for the loss in low-RPM torque in a long-duration camshaft. To compensate for this, it’s very common practice to increase the static compression ratio to increase cylinder pressure and minimize low-RPM torque loss. At the end of the day, duration determines both the power potential of an engine and the range of RPM in which it produces that power.

At maximum valve lift, the piston is nowhere near TDC. With very aggressive cam grinds, the piston often travels halfway down the bore at maximum intake valve lift. As a result, piston-to-valve clearance is more a function of duration than it is maximum lift.

Overlap

During the four-stroke cycle, there is a brief period when both the intake and exhaust valves are open at the same time. As the piston travels up the bore during the exhaust stroke, the intake valve opens before it reaches TDC. This gives the intake charge more time to fill the cylinders at high RPM and increases the scavenging effect imparted by the exiting exhaust gases. At high engine speeds, the inertia of the combustion gases escaping into the exhaust port helps pull additional air through the intake port and into the cylinder.

On the other hand, overlap isn’t always a good thing. At low RPM, the scavenging effect of the exhaust is insignificant. Consequently, when the intake valve opens near the end of the exhaust stroke, when residual cylinder pressure is still present in the cylinder, exhaust gas flows past the intake valve and reverts back into the intake manifold. This is what gives performance camshafts a lopey idle.

Overlap is essentially the distance, in cam degrees, between the peaks of the intake lobe and the exhaust lobe. These peaks, known as the intake and exhaust centerlines, are measured in degrees of crankshaft rotation and establish the lobe separation angle (LSA) of the cam. For instance, if a cam has an intake centerline of 108 degrees ATDC and an exhaust centerline of 112 degrees BTDC, averaging the sum of both figures yields an LSA of 110 degrees. In other words, the distance between the peaks of intake and exhaust lobes in such a cam would be 110 degrees of camshaft rotation. Decreasing the lobe separation by moving the intake and exhaust lobe peaks closer together increases overlap. As with duration and lobe lift, the LSA of a camshaft can’t be changed without regrinding the lobes.

Adjustable timing sets allow altering the installed centerline of a camshaft, thereby advancing or retarding the valve events in relation to crankshaft rotation. Comp Cams’ adjustable billet timing set has nine keyway slots in the crank gear for up to 8 degrees of latitude. (Photo courtesy of Comp Cams)

Although camshaft duration determines the operating RPM of an engine, changing the LSA can be used to further fine-tune the operating characteristics of an engine within its powerband. It seems simple enough, but tightening or widening the LSA as a tuning tool can get tricky, because its effects are dependent upon duration.

On a small cam with roughly 210 degrees of intake duration at .050-inch lift, a tighter LSA improves top-end horsepower at the expense of idle quality and low-end torque. That’s because generous overlap improves scavenging at high RPM, but it also increases reversion at low RPM. Conversely, widening the overlap with a short-duration camshaft tends to improve idle quality and low-RPM torque at the expense of top-end power.

However, gauging overlap solely by a cam’s LSA can be a bit deceptive. As duration increases, overlap increases, even if the LSA isn’t changed. For example, a 260-at-.050 cam has much more overlap than a 225-at-.050 cam, even if both are ground on a 112-degree LSA. In other words, the actual overlap of a camshaft—measured in crankshaft degrees—takes precedence over the lobe separation angle.

This fact is important to remember as duration at .050 inch tappet lift increases. The same 112-degree LSA that idles so smoothly in a 210-at-.050-duration cam will yield a very choppy idle in a 260-at-.050-duration cam. The substantial overlap in the 260-at-.050-duration cam will not only sacrifice low-end torque, it can also dilute the intake charge enough to reduce high-RPM horsepower. Such a cam usually still produces excellent peak horsepower numbers, but the power curve drops off very sharply after that point. Consequently, with long-duration camshafts, a wider LSA can result in a broader, more flexible powerband that drops off much more gradually after peak power. And in competitive racing classes, power after peak is almost as important as peak horsepower.

Between 1997 and 2006, all LS-series small-blocks used cam cores that attached to the timing gear with three bolts. However, GM began phasing in single-bolt camshafts in 2006. Since VVT-equipped Gen IV small-blocks use a single cam bolt that doubles as an oil control valve, GM presumably started phasing in single-bolt cam cores throughout the LS lineup to cut down on production costs. All VVT-equipped LS motors have a single-bolt cam, but not all LS motors with single-bolt cams have VVT. Fortunately, swapping out a three-bolt cam for a single-bolt setup, and vice versa, is as easy as pairing the camshaft with a matching single-bolt timing set.

Studying the specs of factory Gen III/IV camshafts thoroughly reinforces this point. The 5.7L LS1 used in 2001–2002 F-bodies came equipped with a 196/207-at-.050-duration camshaft ground on a 116-degree lobe separation angle. On the other hand, the 7.0L LS7 utilizes a 211/230-at-.050 cam with a 120.5-degree lobe separation angle.

Compared to the typical aftermarket cam, factory LS-series camshafts have far wider lobe separation angles to improve idle quality and clean up emissions output. This is particularly important in the LS7, as its duration specs are quite aggressive in the world of factory cams, which was necessary in order for GM engineers to achieve their target horsepower and operating RPM range. To compensate for the inherent increase in overlap that its longer-duration specs yield, the LS7 cam is ground on a substantially wider LSA than the LS1’s. The LS7’s wider LSA also helps mask the detrimental effects on idle quality and emissions output that its longer duration naturally creates. Because idle and emissions quality aren’t major concerns in a hot rod application, aftermarket cams can get away with much more overlap and tighter lobe separation angles.

Measuring Duration

Camshaft manufacturers publish both advertised duration figures and duration at .050-inch tappet-lift specs. The advertised duration figure is always bigger, and although it seems strange, there’s a good reason why two different duration specs are necessary.

Due to the acceleration rate of a cam lobe’s ramp, it’s difficult to determine the precise moment at which the lifter starts climbing the ramp. As a result, camshaft manufacturers start to measure duration at a predetermined amount of lifter, or tappet, rise. For example, Comp Cams begins measuring duration once the lifter rises .006 inch above the base circle. Obviously, the lower this figure is, the longer the duration specs appear to be, even though the duration hasn’t actually changed. This makes the cam look bigger on paper than it is. Because camshaft manufacturers can measure advertised duration at any lift point they choose, it makes it very inaccurate to compare advertised duration specs among different manufacturers. Recognizing this problem, camshaft manufacturers agreed to use duration at .050-inch lifter rise as the industry standard for measuring duration. Doing so allows engine builders and enthusiasts to accurately gauge the duration figures of camshafts among different manufacturers. As a result, it’s widely accepted that advertised duration numbers aren’t nearly as important as duration at .050 figures.

Although that may be the case, it doesn’t mean that the duration figures below .050 inch tappet lift are entirely irrelevant. Low-lift numbers between .001 and .020 inch tell an engine builder a great deal about engine vacuum and throttle response, and high-lift numbers greater than .200 inch are more indicative of power potential. The .050-inch number is relatively easy to measure with a dial indicator and degree wheel, which explains why it’s the universal industry standard. Additionally, it does the best job of predicting the operating range of a given lobe in a specified application.

It’s important to remember, however, that duration at .050-inch tappet lift isn’t the same as the actual amount of time the valve stays open. The actual duration of the intake valve—or how long the valve remains unseated between its opening and closing events—is affected by tappet lift below .050 inch, as well as rocker arm ratio. This is one of the reasons why two camshafts with identical duration, lift, and LSA can perform very differently on the dyno and on the street.

Lift

Compared to duration, cam lift is relatively straightforward. Lobe lift is simply the difference between the radius of the cam’s base circle and the height of the eccentric portion of the cam. For example, a factory 2001 LS6 cam has a base circle radius of .760 inch, and the distance between the base circle centerline and the highest point on the intake cam lobe is 1.068 inches. Subtracting the base circle radius of .760 inch from the cam lobe height of 1.068 inches nets .308 inch of lobe lift. This simple illustration explains why high-lift camshafts typically have smaller base circles.

Reducing the size of the base circle while leaving the cam lobe height unchanged increases the lobe lift. That’s because decreasing the base circle diameter increases the distance between the top of the cam lobe and the base circle radius. Using this approach, let’s imagine that the 2001 LS6 cam’s base circle radius was reduced from .760 inch to .720 inch while its 1.068-inch cam lobe height remained unchanged. This would effectively increase lobe lift from .308 inch to .348 inch. Furthermore, smaller base circles are also necessary to prevent the connecting rods from contacting the camshaft in engines with stroker crankshafts.

Single-bolt camshafts are also used in newer non-VVT LS-series engines. Aftermarket companies offer camshafts and timing sets for both three- and single-bolt configurations. An engine equipped with a single-bolt camshaft can be converted to a three-bolt arrangement, and vice versa, by swapping over to a matching timing set. (Photo courtesy of Comp Cams)

In the aforementioned example, marginally increasing lobe lift might seem rather insignificant. However, valvetrain dynamics suggests otherwise. As the cam lobe pushes up on the lifter and pushrod, the rocker arm acts as a see-saw and converts this upward motion into downward motion. During this process, it also multiplies the lobe lift. Consequently, valve lift is the product of lobe lift multiplied by the rocker arm ratio. With the exception of the LS7, all factory Gen III/IV small-blocks utilize a 1.7:1 rocker arm ratio. Therefore, increasing lobe lift from .308 inch to .348 inch increases valve lift from .524 to .591 inches, which is substantial in anyone’s book.

While duration specs are based on the target operating RPM range of an engine, lift is based upon airflow through the cylinder heads. This makes it very easy to select the proper amount of lift for a camshaft. For instance, if a cylinder head achieves peak airflow at .650-inch lift, it should be matched with a camshaft that has at least .650-inch valve lift. In extreme applications, however, things can get more complicated. Some engine builders contend that it’s sometimes possible to increase horsepower by opening the valves beyond the point where the cylinder heads back up. Such an application might have a .800-inch lift cam even though the cylinder head airflow starts dropping off at .700-inch lift. The justification is that, beyond a certain point, the flow bench can’t accurately replicate the operating conditions inside an engine. In other words, in extreme high-airflow, high-RPM conditions, the piston sucks down on the intake charge much harder than the electric motor in a flow bench can draw air in through the ports.

Because GM’s slick VVT system is hydraulically actuated, it requires channeling oil through the center of the camshaft. To facilitate this, VVT cam cores are drilled hollow.

Nonetheless, one aspect of lift that can’t be disputed is how much of it can now be packed into a relatively short duration camshaft. Compared to cams of just 20 years ago, modern bumpsticks allocate much greater lift over much shorter duration cycles. In the past, relatively long-duration camshafts were necessary in order to hit a target valve lift to reduce stress on the valvetrain. That’s because steeper lobes place greater loads on the lifters, pushrods, rockers, valve springs, and the lobes themselves. Cylinder heads of the day rarely flowed well beyond .500-inch lift, so this wasn’t a big deal. However, as cylinder head technology improved, as evidenced by factory LS-series castings that flow well past .600-inch lift, it became necessary to improve valvetrain durability. Fortunately, camshaft and valvetrain manufacturers met the demand, and now it’s possible to stuff tons of lift over a steep, short-duration lobe. This gives both the drivability that was once missing in large cams and the power that was difficult to achieve with short-lift cams.

Piston-to-Valve Clearance

When planning a new engine build, or upgrading to a larger cam in an existing combo, it’s critical to check for adequate piston-to-valve clearance. As the term suggests, there must be enough clearance between the valves and piston crown near TDC in order to prevent severe damage to the valvetrain, cylinder heads, and short-block.

Generally, piston manufacturers recommend a minimum of .080-inch clearance between the intake valves and piston crown and .100-inch clearance for the exhaust valves when using steel connecting rods. Due to the increased stretch of aluminum rods, they require an additional .030 inch of clearance. Hot rodders instinctively examine the maximum lift figures of a cam to try to determine if there will be adequate piston-to-valve clearance, but the issue at hand revolves more around duration than lift. The reason for this is simply because when the piston is at TDC, the intake valve is nowhere near peak lift. In fact, at TDC, the intake valve is just starting to move off its seat.

Verifying this line of thinking is as easy as looking at the intake centerline angle of a cam’s published specs. To illustrate the point, let’s take a look at one of the most aggressive LS-series camshafts in Comp Cams’ catalog, its XFI286R113 grind. This beastly solid roller cam boasts duration specs of 251/256-at-.050 and .660/.655 inch valve lift. Despite the fact that it packs a massive amount of lift, especially for a small-block, a quick look at the intake centerline angle reveals that the intake valve doesn’t reach peak lift until 110 degrees ATDC. At that point, the piston is nowhere close to TDC and has actually descended about halfway down the bore. That means that even with a stock 3.622-inch LS1 crank, a piston would be nearly 1.811 inches down the bore. Worrying about whether or not the cam’s peak lift—in this case .660 inch—is enough to smack into the piston is awfully silly, considering that the piston would be almost 1.811 inches down the bore. And that’s before you even take the angularity of the intake valve into account. This resoundingly reinforces the point that piston-to-valve clearance has very little to do with peak valve lift.

To feed oil to the VVT system’s phaser assembly, oil is routed from a groove cut into the number-2 cam journal to the oil control valve that bolts inside the cam snout. This simple arrangement allows supplying hydraulic pressure to actuate the system without any modifications to the block, which makes it very easy to retrofit VVT to non-VVT engines.

On the other hand, it’s safe to assume that a cam with 251 degrees of intake duration, regardless of lift, would probably cause piston-to-valve interference issues, unless big valve reliefs were cut into the piston. During the time it takes the crankshaft to make one complete 360-degree revolution, a 251-degree cam leaves the intake valve open for roughly 70 percent of that cycle. That means that the intake valve is closed for just 109 degrees, or 30 percent of time, for each revolution of the crank. Additionally, the actual valve duration is even longer than the duration figure at .050- inch tappet lift.

Unfortunately, although long-duration camshafts increase the potential of piston-to-valve interference, there is no single spec that gives a definitive answer on whether or not it will be an issue. Variations in block deck height, cylinder head casting tolerances, head gasket thickness, combustion chamber depth, and piston shape all affect piston-tovalve clearance. Consequently, the only way to accurately check for it is during the engine assembly process. A builder checks it by placing a piece of clay on top of the pistons, bolting the cylinder heads down, and then rotating the crank over by hand several times. Removing the cylinder heads and inspecting the clay clearly reveals whether or not piston-tovalve interference is present.

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Valve Events

Cam duration and lift determine how long and how much the valves open. Granted they’re the two most important variables in the horsepower equation when it comes to camshafts, but they don’t offer any insight as to when the valves open and close. In a four-stroke internal-combustion engine, there are four valve events: intake valve opening (IO), intake valve closing (IC), exhaust valve opening (EO), and exhaust valve closing (EC). When each of those events takes place, it is collectively known as valve timing, and each plays a role in the shape of the power curve.

Because lift and duration can’t be changed without regrinding a camshaft, the only parameter that can easily be tweaked once the lobe profiles have been finalized is the cam timing. Although when the valve events take place in relation to the position of the crankshaft, and therefore pistons, may seem rather inconsequential, engine builders have proven otherwise over the decades. Of the four valve events, intake valve closing most profoundly impacts horsepower output. In fact, some engine builders say that intake closing is more important than the other three valve events combined.

The phaser assembly in VVT engines replaces the cam gear. It features an internal rotor and stator assembly, which enables the camshaft to be advanced or retarded. An oil control valve that doubles as the cam bolt adjusts hydraulic pressure in the phaser assembly, which advances or retards the camshaft.

The opening and closing of the intake valves determine how much air can be drawn into the cylinders on the intake stroke. With a typical performance camshaft, it’s not uncommon for the intake valve to close up to 60 degrees ABDC. Extremely long-duration cams delay intake closing even farther. This isn’t ideal at low RPM, as the pistons push the air/fuel mixture back past the intake valve and into the intake manifold, which hurts low-end torque and idle quality.

On the other hand, delaying the intake closing point is exactly what an engine needs at high RPM, and simple physics dictates why this is the case. Although the piston tries to push the air/fuel mixture back past the intake valve as it travels up the bore, at high RPM the inertia and velocity of the intake charge exceeds the upward pressure exerted by the piston. So even though the piston is moving up the bore after BDC during the intake stroke, the inertia of the intake charge continues filling the cylinder with air. By nature, long-duration camshafts delay the intake valve closing point, which is largely responsible for the increase in horsepower they yield over shorter-duration cams.

One downside to GM’s VVT system is that excessive valve spring pressure can overwhelm the hydraulic pressure inside the phaser assembly, sending it into full mechanical retard. To prevent this from happening, Comp Cams and Mast Motorsports offer restrictors that can be fitted into the phaser assembly to limit the movement of its internal rotor to 20 to 30 degrees.

A VVT timing cover is easily identified by the big hump in its upper section, which houses an electric solenoid that controls oil flow through the phaser assembly. The solenoid—which interfaces with the camshaft position sensor—operates via pulse width modulation, since it must react instantaneously to inputs received by the PCM.

The oil control valve fits behind an electric solenoid mounted in the timing cover. When the solenoid applies pressure to the valve, oil exits out of the valve’s multiple orifices and into the phaser assembly. By manipulating pressure, the solenoid can put more pressure on one side of the phaser assembly than the other, causing it to rotate.

To put into perspective how late the intake valve closing point can be delayed in a long-duration camshaft, let’s re-examine Comp Cams’ XFI286R113 solid roller grind. This 251/256-at-.050 cam has an intake valve closing point of 74 degrees ABDC. That means the intake valve doesn’t close until the piston is almost half way up the bore during the compression stroke. Expanding upon this example, many race-only cams have more than 280 degrees of duration at .050-inch tappet lift, pushing the intake valve closing point even farther into the compression stroke. To put it succinctly, with the tremendous airflow potential of modern cylinder heads, and the high-RPM potential of today’s short-blocks and valvetrain hardware, never underestimate the effects of inertial charge filling and the dividends in horsepower they offer.

In contrast, the other three valve events also affect power production, but they aren’t nearly as important. The exhaust opening point is most commonly accepted as the second most important valve event, as it determines the LSA of a camshaft. Retarding exhaust opening improves bottom-end torque, and advancing it allows spent fumes to exit the cylinders sooner, which generally improves high-RPM power. Because the intake valve opens before TDC during the exhaust stroke, it affects overlap. An earlier IO increases overlap, thereby sacrificing low-end torque for top-end power. Delaying the IO does the exact opposite. Likewise, the exhaust valve closes after TDC during the intake stroke.Consequently, an early EC decreases overlap and boosts low-end torque, but it doesn’t allow sufficient time for exhaust gases to escape out of the cylinder at high RPM, decreasing top-end power. A late EC has the opposite effect. Although IO, EO, and EC all alter horsepower and torque production in some way, their effects are largely inconsequential compared to IC.

Timing Tricks

As with duration and lift, the four valve events can’t be changed independently of each other without regrinding a camshaft. They can, however, be changed at the same time. During engine assembly or dyno tuning, advancing a cam involves turning it a few degrees clockwise in relation to the crankshaft, thereby advancing when the valve events take place; retarding the cam involves turning it a few degrees counterclockwise to delay the valve events. In other words, advancing or retarding the cam simply changes the installed intake centerline in relation to the crank.

GM’s VVT system uses surprisingly few VVT-specific parts. All you need to retrofit VVT onto any Gen III or IV small-block is a timing cover, a phaser assembly, a 58-tooth reluctor wheel, and an oil control valve from an L92/L99, all of which can be purchased for well under $300. Other requirements include a VVT-specific camshaft and a computer from a factory equipped VVT car that has the necessary programming tables to control the phaser assembly. A VVT retrofit is quite justifiable if you’re building an LS motor from scratch to drop into a muscle car, since most of the retrofit-specific hardware is stuff you’d have to buy anyway.

Programming the VVT system requires obtaining a PCM from a VVT-equipped car and reprogramming it with software from HP Tuners or EFI Live. Another alternative is to use Mast Motorsports’ stand-alone M-90 PCM and wiring harness, which has the necessary tables to control the factory VVT system.

For example, if a cam is ground on a 109-degree intake centerline, but it’s installed at a 112-degree intake centerline, it has been retarded three degrees. To facilitate quick-and-easy timing adjustments, most aftermarket timing sets have multiple keyway slots ground into the crank sprocket. Advancing the cam generally improves low-end torque and throttle response while sacrificing peak horsepower, and retarding the cam decreases low-end torque while increasing peak power. This is because advancing the cam closes the intake valve sooner, and retarding the cam delays the intake closing point.

In theory, changing all four valve events in unison isn’t an ideal situation. Ideally, the intake and exhaust lobes should be phased independently, but that requires either a DOHC valvetrain or a trick cam-in-cam layout like the one in the 2008-and-up Dodge Viper V-10. Fortunately, the other three valve events are so inconsequential compared to intake closing that it’s nothing to split hairs over.

For instance, exhaust opening is commonly accepted as the second most important valve event, as it determines the LSA of a camshaft. Retarding the exhaust opening improves bottom-end torque, and advancing it allows spent fumes to exit the cylinders sooner, which generally improves high-RPM power with long-duration camshafts. This is exactly the opposite of what happens when advancing or retarding intake closing, which means that optimizing intake closing actually compromises the exhaust opening point. Nonetheless, the effects of intake closing are so much more profound that it really doesn’t matter much at all.

Variable Valve Timing

The obvious limitation of valve timing adjustments is that after an engine is built and installed into a car, the only way to make addition adjustments is to tear into the motor again. Furthermore, aftermarket timing sets typically limit the latitude of adjustment to roughly six degrees. New car manufacturers recognized this problem long ago, and as a result, variable valve timing (VVT) systems have been used in production cars for more than 20 years. GM got in on the action, too, with the Gen IV L92 smallblock in 2007. The L92 was the first production GM small-block to utilize variable valve timing, and the system has since been installed on the L99 and LY6.

The beauty of the system is its simplicity. GM’s VVT setup features a hydraulically actuated phaser assembly that’s integrated into the cam gear. Essentially a rotor that rotates inside of a stator, the phaser assembly uses oil pressure to move the camshaft in relation to the timing chain and crankshaft. Using instructions from the engine management computer and cam position sensor, an electric solenoid mounted inside the timing cover presses upon a hydraulic valve bolted into the nose of the cam. This manipulates oil flow into the phaser assembly to advance or retard the cam. GM’s VVT system is very flexible, and it allows advancing the cam 7 degrees and retarding it up to 45 degrees, for a total of 52 degrees of latitude.

Although GM uses VVT technology as a means of boosting fuel economy and cleaning up emissions, in performance applications, its primary purpose is to optimize the intake closing point. This allows advancing the cam at low RPM to boost torque and retarding it at high RPM to increase top-end horsepower for tremendous flexibility in broadening up the power and torque curves.

Perhaps the key to the system’s seamless performance is advances in modern electronics that make its technology possible. Because the phaser assembly is essentially a rotor that moves inside of a stator, hydraulic pressure is the only thing preventing the cam from twitching around erratically. Even when the cam phasing is locked in one position, the engine management software is constantly adjusting oil pressure into the phaser to keep the cam in a fixed position. Otherwise, the pressure exerted on the phaser from the valve springs could force it into full mechanical retard, as the system relies on hydraulic pressure to overcome the force exerted by the valve springs. In fact, once valve spring pressure exceeds 380 pounds of open pressure, the VVT systems starts losing control of the cam phasing beyond 5,000 rpm. Consequently, there is a practical limit to how aggressive cam duration and valve spring pressure can be when using GM’s VVT system in a performance engine build. Nonetheless, hot rodders have already used the factory VVT system in stroker builds, producing well in excess of 600 hp with ultra-broad powerbands that non-VVT motors can only dream of.

When installing a camshaft, it’s a good idea to cover it with a liberal coating of assembly lube. Although roller cams do not require a break-in period, the lube helps keep friction in check until oil starts to flow through the engine during the initial start-up period.

Single- vs. Dual-Pattern

With production cylinder heads, the exhaust ports always flow less than the intake ports. Unlike the intake ports, which rely on the pressure differential created by the piston to draw in air during the intake stroke, the exhaust ports benefit from the pistons physically pushing exhaust gases out of the cylinders during the exhaust stroke. Consequently, it simply isn’t necessary for the exhaust ports to flow as well as the intake ports. Nonetheless, to compensate for this disparity in airflow, engine builders often use camshafts with more exhaust duration than intake duration. These are referred to as dual-pattern cams, and bumpsticks that have the same duration and lift specs on both the intake and exhaust lobes are known as single pattern cams.

As no surprise, GM uses dual-pattern cams on all LS-series small-blocks. However, there is a big difference between the amount of duration split used on GM’s cathedral-port and rectangle-port heads. For example, a stock LS2 features a 204/211-at-.050 cam, and a stock LS7 cam measures 211/230-at-.050. The reason why the LS7 needs 19 degrees of intake/exhaust split compared to the LS2’s meager 7 degrees of split is because the LS7’s exhaust is relatively weak. A stock LS2/LS6 cylinder head flows roughly 183 cfm on the exhaust side and 260 cfm on the intake side for an exhaust/intake ratio of 70 percent. In comparison, a stock LS7 head flows about 220 cfm through the exhaust ports and 370 cfm through the intake ports for an exhaust/intake ratio of 60 percent. As the two heads illustrate, the amount of duration split in a dual-pattern must always be matched to the flow rate of both the intake and exhaust ports.

Written by Stephan Kim and Posted with Permission of CarTechBooks

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Without question, a durable, big-inch short-block is the foundation of every stroker motor combination. Even so, stout blocks, forged rotating assemblies, and premium machine work don’t mean squat without a set of high-flow cylinder heads capable of feeding those hungry bores with a constant supply of air. Sure, it’s tempting to splurge on high-dollar aftermarket blocks and rotating assemblies forged from exotic alloys, but matching a marginal short-block with a killer set of heads yields a combo that handily stomps a mega-buck short-block paired with a mediocre set of heads.

Because internal-combustion engines are nothing more than glorified air pumps, the cylinder heads that flow the most air while maintaining excellent air velocity make the most power. Consensus among engine builders is often elusive, but the importance of cylinder heads in the overall horsepower equation is one universal truth that everyone agrees upon.

The problem is that cylinder head theory is a very complex subject, and it’s impossible to make it simple. The top cylinder head designers in the world have decades of experience under their belts, because, quite simply, it takes decades of practice to firmly grasp the complex science of cylinder head theory and design. Reading one chapter in a book can’t substitute for years upon years of massaging ports and reshaping combustion chambers, but it will most definitely assist in the selection of the best cylinder heads for any given LS stroker combination. Unlike professional engine builders, who must be at the forefront of cylinder head technology to stay in business, typical hot rodders only have to select the proper heads for their engine combination instead of actually designing them, and that’s a great luxury to have.

Simple physics dictate that as bore diameter and stroke length increase, an engine’s airflow requirements also increase. With the recent influx of aftermarket blocks that are continually pushing the envelope of displacement, cylinder head manufacturers have been forced to keep pace. With the latest LS race heads flowing in excess of 450 cfm, even the biggest of engines rarely run out of breath.

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Outstanding cylinder heads are what give the Gen III/IV small-blocks such a huge power advantage over the competition from the factory, and GM is constantly improving upon the design as the LS platform continues to evolve. In fact, GM’s current crop of rectangle-port L92 and LS7 castings put its original cathedral-port LS1 and LS6 castings to shame, even though less than a decade separates when each respective design was first introduced to the market. Factor in the dozens of aftermarket cylinder head offerings that are now available, and the power potential for stroker motors of all applications and sizes—from 383 to 500-plus ci and everything in between— is truly staggering.

Appetite for Air

Four-stroke internal-combustion engines utilize pressure differential to fill their cylinders with air and fuel. As a piston moves down the bore during the intake stroke, it creates an environment where air pressure inside the cylinder is less than the air pressure outside the motor. It’s this difference between ambient pressure and cylinder pressure that pushes air past the throttle body, through the intake manifold and heads, and then into the cylinder. Expanding upon this fundamental concept offers two important revelations. First, as displacement and the volume inside each cylinder increases, so does an engine’s appetite for air. Second, as an engine’s operating RPM range increases, its airflow requirements also increase. Consequently, because stroker motors throw both additional cubic inches and higher RPM into the mix, the airflow demands necessary to properly feed them are staggering.

Cylinder head airflow is measured in cubic feet per minute, or cfm, and the natural inclination for many enthusiasts is to bolt the heads with the highest flow figures onto their short-block. Unfortunately, that can often lead to disastrous results in a street motor. Cylinder heads that post the highest advertised flow numbers can have oversized intake ports that sacrifice low- and mid-lift airflow for greater peak cfm at higher valve lift. That’s fine for all-out race motors that spend most of their time at 6,000-plus rpm, but it results in a mismatched combination with poor throttle response and compromised low- and mid-range torque in a typical street/strip application.

Airflow is measured on the flow bench by mocking a set of valves in the heads and then measuring flow at various lift points. Street heads tend to back up or go turbulent above .600-inch lift, so it’s imperative to optimize airflow and low- and mid-lift. With race heads, on the other hand, all that matters is high-lift airflow.

Raising the intake port provides a straight path from the intake port entrance to the intake valve, thereby improving airflow. This effect is enhanced with flat valve angles. The challenge for head designers is raising the intake port as much as possible without making the valve spring pockets too thin.

The theory that polishing a port wall improves airflow is nothing more than a myth. In fact, some roughness is desirable to help create a boundary layer of stagnant air that sticks along the port wall. This reduces the coefficient of friction for air passing over the boundary layer and improves flow. Likewise, the boundary layer reduces the effective cross-section of the port, improving velocity. Having ridges perpendicular to the port can also creates turbulence to help keep fuel in suspension without hurting airflow, which is a big perk in carbureted motors. Hand-finishing the port walls with 80-grit cartridge rolls nets a surface finish that’s close to ideal.

The obvious question, then, is how to precisely determine the airflow requirements for any given engine combination. Again, there is no simple answer. Taking a scientific approach requires determining an engine’s intended use (drag, endurance, etc.), volumetric efficiency, RPM at peak horsepower, the valve area needed to achieve target airflow, optimum air speeds throughout the induction system, volume of the overall induction system, and an engine’s resonant tuning characteristics. Once those variables have been established, they can be plugged into a series of complex mathematical equations to precisely determine the airflow needs of a particular engine combo.

Quite honestly, that’s more information than the typical hot rodder cares to process, but it’s valuable information to have. Through thousands of hours of flow bench and dyno testing, cylinder head manufacturers have done most of the homework for you. Most offer heads in a plethora of port volumes and chamber sizes compatible with virtually every engine combination conceivable.

The problem is that with the infinite number of ways in which an engine can be configured, there are no simple rules of thumb to follow. For instance, it’s easy, yet often inaccurate, to make generalizations regarding optimum port volume in relation to displacement. Although a 205-cc cylinder head may work fine for the majority of 346- to 396-ci small-blocks, a 7,500-plus-rpm drag race motor with a solid roller cam can certainly benefit from larger 215-cc ports. Conversely, although a massive 454-ci stroker might have the sheer size necessary to warrant a set of monstrous 245-cc heads, if it’s destined to power a heavy muscle car with tall gears and a modest 6,000-rpm peak power target, it will perform much better with smaller 230-cc ports. For the average enthusiast, a very unscientific, yet extremely effective, method of selecting the ideal cylinder heads for an engine is to consult cylinder head manufacturers, experienced engine builders, and fellow hot rodders who have built and tested similar combinations to the one you’re putting together. That, plus the information outlined in this chapter, will help point you in the right direction.

Port Volume

The volume of the intake ports is one of the most common points of reference used to determine the airflow and horsepower potential of a cylinder head. Because an intake port features a series of complex contours and curves, the best method of measuring its volume is to turn a head on its side, with the intake valve in place and the intake port facing upward, and then fill it up with water using a graduated cylinder. Although comparing port volumes can be a useful tool when trying to determine the ideally sized cylinder head for an engine combination, it does have its limitations. Relocating the intake port entrance has a tendency to lengthen or shorten the port, which can dramatically increase or decrease port volume without impacting a cylinder head’s airflow potential. That’s because port volume actually has a negligible effect on how well an intake port can move air.

In practice, the most accurate gauge of an intake port’s performance potential is the size of its cross-sectional area. Enlarging a port’s cross-section can dramatically increase airflow while just marginally increasing port volume. For instance, porting a set of stock LS1 head castings can often increase peak airflow figures from 240 to 320 cfm, but the process might only increase port volume from 200 to 225 cc. Conversely, lengthening a port while maintaining the same cross-sectional area is unlikely to impact airflow much at all. Larger-displacement motors, or smaller motors operating at high RPM, generally benefit from larger ports with larger cross-sections, and small-displacement motors that turn modest RPM are better off with smaller cross-section ports.

In essence, comparing the intake port volumes of two-cylinder heads is only valid if they share similar port architecture. For example, an 18-degree Gen I small-block Chevy head has a significantly larger port volume for any given cross-sectional area than a 23-degree head. Therefore, making a direct comparison between the two is futile, and the same applies to different Gen III/IV castings.

The short-turn radius of the intake port is the area where the port floor drops off right before the valve. Raising the intake port takes a load off the short-turn radius, allowing head designers to create a more gradual transition for improved airflow.

The upper ridge that gives cathedral-port heads their unique shape was actually a design compromise. The location of the pushrods limited how wide GM engineers could make the ports, so they had to create a ridge at the top to achieve their desired port volume. This tall-and-skinny port shape resulted in very low port entrance height, which severely compromised flow above .650-inch lift.

Taking cues from the C5R race heads, GM was able to increase the port volume and cross-sectional area of the LS7 heads by moving the intake pushrod over to the side. That allowed for a more conventionally shaped rectangular intake port.

Compared to their cathedral-port forebears, the rectangle-port L92 and LS7 castings used on the latest Gen IV small-blocks feature vastly different overall port architecture. The raised intake port locations on the L92 and LS7 heads effectively lengthen the ports, resulting in a dramatic increase in port volume. Whereas the intake ports on factory cathedral-port LS1 castings measure 200 cc, the L92’s rectangle-port heads feature a 260-cc intake port volume. A difference of 60 cc suggests that the L92 castings were designed to feed a motor nearly 100 ci larger than the 346-ci LS1. However, the truth of the matter is that GM’s Gen IV rectangle-port heads are bolted to motors (L92, LS3, and L99) that are only 30 ci larger

Likewise, GM also uses these 260-cc heads on the LY6, which is just 18 ci larger than an LS1. This merely reinforces the point that port volume figures can only be used as a point of reference when comparing heads with similar port architecture. In the wake of Gen III/IV small-blocks, that means port volume should only be used to compare cathedral-port heads to cathedral-port heads and rectangle-port heads to rectangle-port heads.

Flow vs. Velocity

In a high-winding race motor, the only thing that matters is maximizing peak cfm at high valve lifts. On the other hand, designing a cylinder head for a street/strip engine is a much trickier proposition. In these dual-role applications, high-RPM horsepower is still important for the occasional jaunt down the dragstrip, yet low- and mid-RPM performance is arguably the most important design consideration, because that’s where street/strip motors spend most of their time. Furthermore, because race engines don’t need to last tens of thousands of miles, they employ extremely aggressive camshaft profiles to optimize airflow above .600-inch valve lift. That approach significantly reduces valvetrain durability, so street engines must make do with camshafts featuring much more conservative peak valve lift figures.

Consequently, building a street/strip motor that offers a flexible powerband requires designing a cylinder head with respectable peak cfm, in addition to outstanding port velocity and airflow below .600-inch valve lift. Due to the fact that impressive peak airflow figures often come at the expense of port velocity, and vice versa, balancing these two opposing forces is the biggest challenge facing any cylinder head designer.

Make no mistake that peak airflow through an intake port is extremely important. In a street motor, however, it’s just not as important as air velocity. Some head designers contend that air speed is 10 times more important than raw flow numbers. Reducing air velocity by 10 percent can sacrifice 40 percent of the wave and ram energy that dynamically fills the cylinders. Additionally, blind fixation on peak-cfm figures can actually lead to situations in which an engine has cylinder heads that flow more air than it can actually use.

Compared to the LS-series small-block’s original cathedral ports, the rectangle ports used on LS7- and L92-style heads are much shorter and wider. As a result, GM was able to raise the intake port entrance for dramatic improvements in high-lift airflow.

Moving the intake pushrods over to the side to make room for the rectangular port design required GM to design offset intake rocker arms to maintain proper valvetrain geometry. The C5R heads address this problem with shaft-mount rocker arms, but for high-volume production engines, that simply isn’t an option. Instead, LS3/L92 engines incorporate offset intake rockers.

In performance applications, a 45-degree valve seat is the most common angle, as it offers a nice balance of airflow and durability. In a typical three-angle valve job, a 45-degree primary cut is matched with a 30-degree top cut and a 60-degree bottom cut. Serdi-style tools allow machining all three angles at the same time.

For instance, if a set of 375-cfm cylinder heads is bolted on a short-block that can only use 325 cfm, the engine will not only fail to achieve the power potential of that 375 cfm, but it will also fail to reach the power potential of the 325 cfm that it really needs. That’s because the port design necessary to achieve 375 cfm of airflow sacrificed critical air speed in the induction system. The end result is a low air-speed induction system that can’t properly fill the cylinder by means of dynamic inertia, and, as a result, the engine will never reach its full power potential.

Furthermore, oversized ports needlessly make an engine combination lazy and soft at low RPM while providing no appreciable benefit at higher RPM. Having extra airflow isn’t always bad, but it can’t come at the expense of air speed, and the ports must be sized properly. Matching the airflow needs of an engine with properly sized cylinder heads is the key to taking full advantage of every last CFM of air.

In a street engine, a good rule of thumb is to choose the smallest head that flows enough air to meet its horsepower target and properly feed the cylinder at the desired RPM range. In other words, the goal of a head is to move as much air as possible through as small a port as possible. A simple way to look at it is if cross-sectional area of a port is increased and flow increases, then velocity hasn’t been compromised. On the other hand, if a port is opened up and flow doesn’t increase, then velocity has been compromised. It’s a delicate balancing act, and air velocity is not uniform throughout a port. There are average velocities and localized velocities, and air moves faster toward the center of the port where friction from port walls doesn’t affect it as much. The trick is minimizing the differences between localized velocities.

Many of the secrets to finding good low- and mid-lift flow are in the combustion chamber design, valve job profile, and the actual shape of the valve itself. Back-cut valves are a must, and time must be invested in trying different angles, as well as different widths of those angles. Additionally, the actual width of a 45-degree seat needs to be considered, and the short-turn radius height and shape also play a smaller role. It’s a give and take; really strong peak numbers can sacrifice a lot of low- and mid-lift flow, and really strong low- and mid-lift numbers may knock more off the peak than you may be willing to accept.

Many of the secrets to finding good low- and mid-lift flow are in the combustion chamber design, valve job profile, and the actual shape of the valve itself. Back-cut valves are a must, and time must be invested in trying different angles, as well as different widths of those angles. Additionally, the actual width of a 45-degree seat needs to be considered, and the short-turn radius height and shape also play a smaller role. It’s a give and take; really strong peak numbers can sacrifice a lot of low- and mid-lift flow, and really strong low- and mid-lift numbers may knock more off the peak than you may be willing to accept.

A properly designed combustion chamber is essentially an extension of the valve job. Using this approach, combustion chambers in modern cylinder heads tend to resemble a figure-8. The more efficient a chamber, the less timing advance an engine needs, which reduces pump losses.

Valve Angle

The valve angle of a cylinder head is often the topic of discussion in bench racing circles, so it makes sense to explain what it is and how it affects overall airflow dynamics. If the valve stems were placed perpendicular to the deck surface of the block, they would be positioned at a 0-degree angle. Due to underhood installation constraints, this is very difficult to achieve. Consequently, in an OHV engine with an inline valvetrain, like the LS-series small-block, the valves are angled toward the outside of the cylinder bore. In other words, as the valves open, they move closer to the exhaust manifold side of the block.

Many aftermarket manufacturers now offer heads with CNC-machined combustion chambers. In the not-so-distant past, engine builders had to hand blend the chambers, which dramatically increased labor and costs.

The inherent drawback of this layout is that air entering the cylinder from the valve seat area of the head tends to shroud up against the bore, which hinders airflow. To combat this, cylinder head designers are always trying to flatten out the valve angle as close to vertical, in relation to the deck surface, as possible. This allows the incoming air charge to move freely down the bore instead of crashing into the cylinder wall. Furthermore, lower valve angles free up additional space, which allows for the fitment of larger-diameter valves. Other benefits include smaller, more efficient combustion chambers; decreased chamber burn time; reduced pumping losses; and a lower propensity for detonation or pre-ignition.

It should come as no surprise, then, that GM engineers opted for a very flat 15-degree valve angle when designing the Gen III cylinder heads. To put that figure into perspective, keep in mind that the Gen I small-block utilized a 23-degree valve angle, and when it comes to Mouse motors, cylinder heads with a 15-degree valve angle are considered full-race hardware.

Naturally, many hot rodders equate a flatter valve angle to a superior port design, but that’s not always the case. Valve angle is merely one of dozens of variables that distinguish an excellent cylinder head from a mediocre cylinder head, and the reason why a flatter valve angle isn’t always better is actually very simple. Any time the valve angle is reduced, it must coincide with a raised intake port entrance. Flattening the valve angle without raising the intake port entrance increases the angle that the incoming intake air charge must negotiate at the short-turn radius.

For example, the Achilles’ heel of stock cathedral-port LS castings is their relatively low ports. That, combined with their flat 15-degree valve angle, means that airflow drops off dramatically after .600- inch valve lift. GM wisely addressed this shortcoming with its rectangle-port L92 cylinder heads. Raising the ports allowed engineers to take full advantage of the heads’ low valve angle and dramatically improve airflow. Although the L92 heads share the same 15-degree valve angle as their cathedral-port forebears, their raised intake ports flow 320 cfm, compared to the LS1 heads’ 240 cfm. The moral of the story is that although valve angle is an important design element to any cylinder head, it’s foolish to judge the merit of a head based on valve angle alone.

Angle of Attack

The relationship between the incoming air and the back of the intake valve is sometimes referred to as the angle of attack. Due to its added height, a raised-runner head simply has a better angle of attack, or vantage point, for a straighter shot to the back of the valve. The added height also reduces the angle that the incoming charge must negotiate at the short-turn radius. As a result, the geometry of a raised-runner port is superior to that of a similar non-raised runner design, because it allows additional airflow and a higher terminal velocity before it stalls or backs up. The lower the port gets, the more the air speed increases at the short-turn radius, and the more critical its shape becomes.

Lower ports force head designers to lower the air speed in order to get the air to negotiate the short turn, which wastes energy. Ideally, the short-turn radius needs to be shaped to have the highest air speed throughout the RPM range without disrupting the boundary layer, which is a layer of stagnant air surrounding the port that reduces the coefficient of friction of the air passing over it.

The extremely flat 12-degree valve angle of the LS7 heads yields very shallow combustion chambers for quick and efficient burning of the air/fuel mixture. Shallow chambers help maintain a more homogenized air/fuel mixture. (© GM Corp.)

Although they’re considered yesterday’s news by some, factory GM cathedral-port heads still perform exceptionally well. These stock LS6 castings were ported by SAM to flow more than 350 cfm, and they helped power a 1999 Camaro shop car to mid-9-second ETs.

An inherently weak spot on factory GM cathedral-port castings is the valve spring pocket. Removing too much material from the port ceiling can break through the casting and into the spring pocket. Aftermarket heads have extra metal in the spring pocket area to avoid this problem.

In essence, the short-turn radius controls the shape of the power curve. Standing it up to increase air velocity increases low-end torque and sacrifices top-end power. Laying it back to reduce air speed compromises low-end torque and increase top-end power. Not only do raised intake ports offer more latitude in shaping the short-turn radius, they also yield smaller, fast-burn combustion chambers and offer a better overall induction system path and design. Additionally, raised runners keep the air/fuel mixture in suspension far better, because there’s less frictional loss and fuel fallout. An air/fuel mixture entering the combustion chamber from a straight, high-port induction path is more homogeneous and burns faster, producing more power with better brake-specific fuel consumption numbers.

Many of the benefits of any raised-runner design are often overlooked. Due to the higher port inlet locations of a raised-runner head, the intake manifold runner length is naturally increased, as the space between the left and right port banks also increases and allows the manifold designer more room for a smoother turn radius from the plenum to the intake manifold runners in a single-plane-style intake. Also, in addition to having runner shape advantages, a raised-runner intake manifold has a much better approach angle from the manifold exit to the runner entrance of the cylinder head. This is somewhat irrelevant for fuel-injected motors, but a definite advantage for the growing number of enthusiasts building carbureted Gen III/IV small-blocks.

Valve Seat Angle

After air has traveled through the intake manifold, down the intake port, and around the short-turn radius, its final stop before entering the cylinder is the intake valve. The valve opens and closes against a seat machined into the head, and the angle of the valve seat plays an important role in overall airflow dynamics. Higher seat angles give up some flow at low lift, but once lift increases and the valve curtain area opens up, a greater angle is better for performance. With cams that have less than .400-inch lift, a lower angle might be better, but with any moderate amount of cam lift at all, a higher angle always enhances airflow.

A 45-degree valve seat angle is very common, and any angle greater than that makes more power. However, there are some tradeoffs. As the seat angle increases, durability decreases. That’s why lower angles are common in many production motors where durability is more of a concern, and just about all diesel engines have 30-degree seats. Typically, 50- to 55-degree seats sacrifice 10 to 15 percent of flow from .200- to .400- inch lift. However, in race applications it’s foolish to sacrifice high-lift flow for low- and mid-lift flow, because that’s not where power is produced. Some of the top engine builders in the country, such as in NHRA Pro Stock and NASCAR Sprint Cup, don’t even turn the flow bench on until .300- to .400-inch lift.

Also, the improved high-lift flow of bigger angles allows opening up the venturi, because at that point the venturi becomes the restriction. A ton of energy is lost when air exits from the port into the cylinder, so a bigger venturi helps maintain that energy. Designing a port is all about area relationships, and you always want to maintain the valve area as the restriction, not the port. In other words, you don’t want a weak port with 50- to 55-degree seats. A weak port with a valve seat area that flows well creates lots of turbulence, which hurts flow. The more skilled the head designer, the less that is lost by going with a higher angle seat.

Combustion Chambers

With all the hoopla over port architecture and valve angles, an area of cylinder head design that gets overlooked quite frequently is the combustion chambers. Doing so leaves a lot of horsepower on the table, as the shape of the combustion chambers profoundly impacts airflow. In fact, many head porters agree that combustion chamber design is more important than port design itself, and any time a valve job is performed, the chambers must be reshaped to take full advantage of the increase in airflow. The reason for this is because as air transitions from the tight confines of the intake port into a comparatively large cylinder, it experiences a tremendous loss in energy and velocity.

The purpose of a well-designed combustion chamber is to keep air velocity even around the entire circumference of the valve and decelerate the intake air charge at a controlled rate to minimize this inevitable loss in velocity and pressure. Head designers refer to this dynamic as pressure recovery. Although different heads require different types of chambers, and there is no single shape that’s best for all heads, the goal is for the combustion chamber to be an extension of the valve seat area all the way into the cylinder. Following this principle, with wedge heads, a well-designed chamber has a tendency to be shaped like a heart or a figure-8.

A properly shaped combustion chamber is a balance of pressure recovery, wet flow, and flame travel, as these three factors can separate a good cylinder head from a great cylinder head. A chamber that is laid back too far causes a total loss of pressure recovery, flow control, and poor fuel dispersion inside the cylinder. Additionally, the combustion chambers can be highly sensitive to minor changes in shape. Something as simple as milling the heads to increase compression can cause a complete loss of pressure recovery and substantially reduce airflow. Additionally, peak volumetric efficiency is reduced when the pressure recovery is undermined by a chamber that has been laid back too far.

Fortunately, managing the three key factors in combustion chamber design is far easier with flatter valve angles, which explains why Gen III/IV castings have such outstanding chambers from the factory. With very low valve angles, the chamber can come right off the valve seat like a venturi. In contrast, higher valve angles require a deep concave chamber to assist with pressure recovery. Deep concave chambers often have poor wet flow characteristics and reduced pressure recovery, due to valve shrouding.

Just how good are the factory L92/LS3 heads? By adding nothing more than a 225-at-.050 cam, Gen IV enthusiasts are picking up an additional 65 hp. Some hot rodders feel that the 260-cc intake runners on these heads are far too large to use on small-displacement stroker engines, but that simply isn’t the case. It’s worth noting that GM uses the L92 castings on the LY6 small-block, which measures just 364 ci. Furthermore, the LY6 produces 382 ft-lbs of torque and is installed on 3/4-ton trucks, illustrating that the cross-sectional area of the L92 castings is well suited for smaller engines, despite the large port volume of the heads. (© GM Corp.)

More than any other factor, the shape of the chamber determines how efficiently a motor burns the air/fuel mixture. Efficient chambers disperse the air/fuel mixture very evenly throughout the cylinder, resulting in even combustion and brisk flame front propagation. As a result, cylinder heads with efficient combustion chambers require less ignition advance, which reduces pumping losses and increases horsepower output. A typical Gen III/IV small-block with ported factory heads or aftermarket castings needs just 26 to 28 degrees of total timing. On the other hand, an iron-headed big-block with massive 118-cc combustion chambers might require as much as 50 degrees of advance.

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Factory Cathedral-Port Heads

When the LS1 was introduced in 1997, perhaps the most shocking departure from the Gen I small-block Chevy was its unique cathedral-shaped intake ports. Not only were they much taller and narrower than the rectangular ports Chevy enthusiasts had grown accustomed to, they also featured a triangular ridge at the very top of the port. Many people assumed some sort of voodoo science-inspired the intriguing port shape, but in reality, the design was a product of GM engineers tying to hit their target port volume within a limited amount of space. By nature, the passages inside an OHV cylinder head are crowded close together, as the ports, pushrod holes, and coolant passages must snake their way through the head without intersecting each other. The section of the head where the pushrod tubes run adjacent to the intake ports is referred to as the pushrod pinch area. When designing the LS1 heads, GM engineers realized that the pushrod pinch area constricted the cross-sectional area of the intake ports too much, and the only way they could hit their target port volume was by creating a unique cathedral-shaped design.

History lessons aside, stock GM cathedral-port heads offer exceptional airflow, combustion efficiency, and horsepower potential. In stock trim, 200-cc LS1 castings flow 240 cfm. That’s plenty of airflow to support more than 450 hp, and it’s right on par with a set of aftermarket 23-degree Gen I heads. Despite the recent influx of aftermarket castings that have hit the LS scene, porting factory cylinder heads is still an excellent choice for hot rodders on a tight budget. With the potential to easily exceed 300 cfm in the hands of a skilled porter, factory cathedral-port cylinder heads move plenty of air to feed a stout 400-plus-ci stroker combination.

LS1 Heads

Although factory cathedral-port LS1 heads have been superseded by GM’s newer rectangle-port heads, they’re still extremely popular for stroker engine builds for several reasons. Far more Gen III/IV small-blocks have left the factory with cathedral-port heads than rectangle-port heads, making them plentiful and inexpensive. Additionally, the large 2.165/1.590-inch valves fitted to GM’s rectangle-port L92 heads require a minimum bore size of 4.000 inches. So, if you’re building a small-bore LS engine combo, the L92 castings simply aren’t an option. Fortunately, that’s not a big deal, because stock LS1 heads can move some serious air. With quality porting, a nice valve job, and larger 2.055/1.570-inch valves, it’s not uncommon for these heads to flow more than 320 cfm. Several companies, such as Total Engine Airflow and Patriot Performance, offer porting services that boost stock LS1 castings past the 300-cfm mark for $1,000 to $1,500 in fully assembled trim.

Many aftermarket blocks are offered with a six-bolts-per-cylinder bolt pattern, but they are still compatible with standard four bolt heads, as long as the upper and lower bolts aren’t used. An option is to weld tabs onto the heads, so that they can be bolted to the extra holes on the block.

Throughout the LS1’s eight-year production run, its cylinder head castings saw several minor revisions. Heads manufactured in 1997 and 1998 had provisions for perimeter-bolt valve covers, and heads produced from 1999 to 2004 were cast for center-bolt valve covers. Some LS1 heads were sand cast, and others were die cast, but there is little to no difference in performance between the various casting numbers. All LS1 heads feature 200-cc intake ports, 70-cc exhaust ports, 67-cc combustion chambers, 2.000-inch intake valves, and 1.550-inch exhaust valves.

LS6 Heads

Manufactured from 2001 to 2004, the LS6 heads—identifiable by their “243” casting number—are essentially an improved variant of the LS1 cylinder heads. The biggest improvement of the LS6 castings is that they feature a raised port floor, which yields a more gradual approach angle at the short-turn radius. Furthermore, the LS6’s intake ports have an enlarged midsection and raised roof to even out localized air velocity fluctuations. On the exhaust side, the port is .125 inch higher than the port on the LS1 heads, and it is D-shaped instead of oval-shaped. These tweaks help boost intake airflow to 260 cfm. Additionally, LS6 heads have slightly larger 210-cc intake ports, 75-cc exhaust ports, and 64-cc combustion chambers. Compared to the LS1 cylinder heads, where the LS6 castings really shine is at high valve lift. In ported trim, the LS6 heads can flow in excess of 350 cfm, making them highly desirable for stroker motor buildups.

GM also equipped the LS2 and LS4 with the 243 casting. Other than having valves that are slightly heavier than the ones used on the LS6, the 243 castings off the LS2 and LS4 are virtually identical to the LS6 heads. A slight variation of the 243 casting is the “799” casting that came equipped on the high-output L33 and LH6 5.3L Vortec truck motors. These heads are essentially a carbon copy of the 243 castings, and they are highly coveted by hot rodders.

AFR cylinder heads are famous for their high-velocity ports that keep on chugging up top. The heads yield extremely broad and street-friendly powerbands that can more than hold their own at the track. The company’s new V2 series heads are revised versions of its original LS castings; the new heads boast improved low and mid-lift flow, as well as an extra 7 to 10 cfm of peak airflow.

Although the growing popularity of five-axis CNC machines has made CNC-ported cylinder heads much more affordable, head casting technology has improved dramatically to enable the production of competitive heads. Trick Flow offers 225-cc heads that flow 305 cfm, and they are much cheaper than the typical CNC head.

LQ4/LQ9 Heads

As the story goes, once Cadillac engineers caught wind of the LS6 engine development program, they knew they had to integrate some of the same technology into Cadillac’s flagship SUV, the Escalade. The results were the LQ4 and LQ9 small-blocks, which are basically 6.0L iron LS motors topped with LS6 heads. The only difference is that the LQ4/LQ9 heads have larger 71-cc combustion chambers that yield a lower compression ratio. Consequently, the LQ4/LQ9 cylinder heads flow every bit as well as the vaunted LS6 design.

Thanks to their large combustion chambers, which allow for lower static compression ratios, the aluminum LQ4/LQ9 castings have carved out a niche in forced-induction circles, where it’s common practice to install a set of LQ4/LQ9 castings on an LS1 short-block to reduce the compression ratio to about 9.5:1. That makes them perfect for hot rodders looking to add a turbocharger or a supercharger to a factory short-block. The black sheep of the 6.0L lot are the early iron “873” LQ4 castings manufactured from 1999 to 2000. Although they flow just as well as their aluminum counterparts, their iron construction makes them 40 to 50 pounds heavier and much more difficult to port.

4.8L/5.3L Truck Heads

To create the 4.8L and 5.3L Vortec truck engines while retaining the basic Gen III architecture, GM engineers reduced the LS1’s 3.900-inch bore to 3.780 inches. This effectively reduced displacement to create the 5.3L; by decreasing the LS1’s 3.622-inch stroke to 3.267 inches, the engineers created the smallest of the Gen IIIs, the 4.8L.

Because reducing an engine’s bore and stroke also decreases the compression ratio, GM fitted the small-displacement Vortec motors with smaller 61-cc combustion chambers. Along with the reduced bore size, these motors were fitted with smaller 1.890/1.550-inch valves. Other than these tweaks, the 4.8L/5.3L cylinder heads are virtually identical to the 5.7L LS1 castings.

The smaller valves do, in fact, reduce airflow to roughly 220 cfm, but with massaged ports and larger seats and valves, the Vortec heads flow just as well as the LS1 heads. Hitting 320 cfm with these castings is no problem, and thanks to their smaller combustion chambers, they’re very popular in naturally aspirated engine buildups, due to the increase in compression ratio that they offer. Using a set of 5.3L heads in lieu of LS1 heads typically boosts static compression by .75 point. That means hot rodders can hit their target compression ratio without having to remove as much material from the deck surface. The 4.8L/5.3L were produced in several casting variations, but there’s no real performance difference among them, and the most common casting numbers are “862” and “706.” So, despite the fact that they originally served duty in a lowly truck application, these castings are excellent performers.

Factory Rectangle-Port Heads

As great as they may be straight from the factory, the production cathedral-port LS cylinder heads still left plenty of room for improvement. Their biggest design flaw was a relatively low intake port entrance in relation to their flat 15-degree valve angle. This created a very sharp corner at the short-turn radius, causing the intake port flow to go turbulent at roughly .600-inch valve lift. GM addressed this issue to a certain degree with the LS6 castings by reshaping the port floor for a more gradual transition at the short-run radius, but high-lift flow still left something to be desired. Enter the factory rectangle-port cylinder heads used on the LS7, LS3, L92, and L99, which represent a profound departure from their cathedral-port counterparts.

Right out of the box, the factory LS7 heads flow 360 cfm, making them an exceptional value for stroker small-block buildups. With some quality hand porting, these heads are capable of delivering up to 400 cfm of airflow. That’s better than many big-block heads on the market. (© GM Corp.)

The progenitor to the production rectangle-port castings was actually conceived to power the factory-backed C5R Corvette road racing program in the American Le Mans Series. The team won multiple championships, largely attributable to the potent powerplants under their hoods. When creating the C5R cylinder heads, engineers completely revised the intake port architecture by raising the floors and roofs and implementing an even flatter 11-degree valve angle. This gave the incoming air charge a much straighter path to the back of the intake valve for a substantial improvement in high-lift airflow.

The cathedral port’s tall and narrow shape offers a limited cross-sectional area. This bottleneck ultimately limits how much air can flow through the port. In contrast, the rectangle ports used on the C5R heads offer far greater cross-sectional area, port volume, and flow potential. Although the standard cathedral-port design is still used in the majority of production Gen III/IV small-blocks, GM is phasing in its latest rectangle-port castings in a growing number of applications.

LS7 Heads

Regardless of engine make, GM’s LS7 cylinder heads are the greatest factory small-block castings ever built. Conceived to catapult the Corvette Z06 to the top of the supercar stage in 2006, the 427-ci LS7 small-block produced 505 hp and spun effortlessly to 7,000 rpm. Feeding that much displacement and RPM forced engineers to design a set of cylinder heads that borrowed heavily from GM’s C5R racing program.

On the original cathedral-port LS1 castings, maximum port volume and cross-sectional area was limited by the pushrod pinch area. Packing a symmetrical port layout between each pair of valves only compounded matters. To work around this issue, GM simply moved the pushrod passages over to the side to create more space for the intake ports, which allowed engineers to create more traditionally shaped rectangle ports. This also enabled raising the intake port entrance to make the most of the LS7’s 12-degree valve angle.

The stock LS9 cylinder heads share the same port and chamber design as the L92/LS3 units, but they are cast from a more durable A356-T6 material to handle the rigors of forced induction. One of the most noticeable differences is the LS9’s reinforced rocker stud bosses. (© GM Corp.)

Edelbrock’s Victor LSR cylinder heads aren’t for the faint of heart or budget. These raw castings feature unfinished ports and combustion chambers, so designing them is up to the end user. The reward for all that hard work is immensely powerful ports capable of flowing more than 450 cfm. (Photo courtesy of Edelbrock)

Cutting down on production costs is always a design consideration at the OE level, so in order to retain the same valve spacing as in other Gen III/IV small-blocks, GM implemented offset intake rocker arms in the LS7. Consequently, the final spec sheet for the LS7 heads resembles an all-out race head: CNCmachined 260-cc intake ports, 86-cc exhaust ports, 70-cc combustion chambers, 2.20/1.61-inch valves, and 360 cfm of airflow. Further porting can push flow figures to the hallowed 400-cfm mark. That’s pretty darn close to big-block territory, and at a hair under $3,000 for a set of fully assembled LS7 heads from your friendly GMPP distributor, they’re an exceptional value.

L92 Heads

At first glance, the factory L92 cylinder heads look like a slightly detuned variant of the vaunted LS7 castings. Like the LS7 units, the L92 castings feature raised rectangular port entrances measuring 260 cc and 70-cc combustion chambers. Differences include the standard Gen III/IV 15-degree valve angle, smaller 2.165/1.590-inch valves, and as-cast ports, as opposed to the CNC-machined ports on the LS7 heads. Because of this, the L92 heads don’t flow quite as well as the LS7s, but at 330 cfm, they’re not far behind.

The most shocking figure of all is their price. Because the L92 heads were designed to be produced in much greater volume than the LS7 castings, they sell for just $1,000 fully assembled, a price that includes the mandatory offset rocker arms. That makes them the best value by far out of all the factory and aftermarket LS-series cylinder heads on the market.

The design of the L92 heads actually predates the conception of the LS7 heads. When the LS7 engine program began, engineers had a horsepower target of 500, but they weren’t sure how many cubic inches the motor would eventually displace. Consequently, the first sets of prototype heads were designed for use on cylinder bores smaller than the massive 4.125-inch bores engineers ultimately settled upon. Once GM decided to move forward down the 4.125-inch-bore path, the small-bore prototype heads were deemed inadequate for delivering their airflow objectives. As a result, they developed a new head with bigger valves and CNC-machined ports.

Fortunately, the small-bore heads didn’t go to waste. GM was working on a replacement for the LQ4 and LQ9 in its heavy-duty truck applications, and engineers realized that the small-bore prototype heads from the LS7 engine program would work perfectly in meeting their 400-hp target. So, in an interesting turn of events, cylinder heads that eventually made their way into heavy-duty truck motors are direct descendants of the factory C5R racing program.

Once engineers knew that the L92 heads were destined for large-volume production, they shifted their focus to reducing manufacturing costs. This is why they retained the standard 15- degree valve angle, which helps maintain the same pushrod length as in other Gen III/IV engines. Moreover, the biggest factor in keeping costs down is that, unlike the LS7 heads that are CNC-ported, the L92s are delivered as cast. As no surprise, porting a set of L92s can yield airflow figures of 370 cfm, right on par with the LS7 heads.

With the success of the L92 heads, GM began installing them on engines across the car and truck lines, most notably in the C6 Corvette and fifth-gen Camaro. Engines equipped with these heads include the LS3, L92, L99, and LY6. Additionally, the rectangle-port castings used on the supercharged LS9 and LSA are essentially L92 heads built from a more rugged alloy for forced-induction duty. Perhaps the most appealing aspect of the L92 castings is that they work on bore diameters as small as 4.000 inches, whereas the LS7 heads require a minimum bore size of 4.100 inches.

C5R Heads

The early days of hopping up the LS-series small-block were a rat race between cubic inches and cylinder head development. At first, ported factory LS1 and LS6 castings flowed more than the typical street/strip engine could reasonably use. However, as machinists mastered the art of sleeving factory aluminum blocks, and displacement figures surpassed the 400-ci mark, the stock cylinder heads began limiting ultimate horsepower in all-out race applications.

In addition to greater airflow potential, aftermarket cylinder heads boast an all-around superior casting design. Common features include thicker deck surfaces, raised valve cover rails, and beefier port walls.

It wasn’t until AFR released the first aftermarket LS-series cylinder heads in 2004 that enthusiasts had a reasonably priced alternative to the factory castings. Before that long-awaited day came to pass, the only other option for big-inch, high-RPM race engines was the GM C5R cylinder heads. Like the LS7 heads that borrowed heavily from the design of the C5R heads, port architecture was dramatically improved. The intake ports were raised as high as possible without interfering with the valvetrain, and they were widened out substantially to create a rectangular shape. Likewise, the heads were cast from a more durable 355-T7 aluminum.

From GM, the C5R castings are delivered unfinished with no valve seats, valve guides, or valvetrain components. Because professional head designers are often forced to add epoxy to certain areas of the ports to create the ideal shape, the C5R heads feature unfinished ports with tons of material that can be ground away to create the perfectly shaped port. The intake ports measure just 210 cc out of the box, but they can be enlarged by 50 to 60 cc. The same goes for the tiny 30-cc combustion chambers, which are also unfinished.

Some aftermarket heads slightly alter the factory valve location. This requires a custom stand bolted between the rocker arm bosses and the rocker arms, which are supplied with the cylinder heads, to maintain proper rocker-tip-to-valvestem geometry. (© GM Corp.)

Most factory and aftermarket rectangle-port heads require a minimum bore diameter of 4.000 inches. Mast Motorsports doesn’t think that the big-bore guys should have all the fun, so it also offers its LS3 heads for 3.900-inch-bore engines. These impressive castings flow 353 cfm, despite being limited on valve size. (© GM Corp.)

Although all this extra material gives porters lots of flexibility in designing ports and chambers that are ideally matched to a given race application, the C5R heads require a substantial amount of labor just to make them useable. In essence, with full-race heads like the C5Rs, a head porter is forced to design ports and chambers from scratch, and only race teams with very deep pockets can even afford to use them. Likewise, a set of bare C5R castings costs $3,800, and they also require a custom intake manifold. Even though they’re capable of flowing in excess of 400 cfm, the price and labor requirements of the C5R heads puts them out of reach for most enthusiasts, especially since GM and the aftermarket are now offering heads that deliver similar performance at a much lower price. If you stumble upon a set of C5R heads that have already been prepped, then they might be worth the investment. Otherwise, there are much better performance values on the market.

Aftermarket Heads

Although it took nearly seven years for the first aftermarket Gen III cylinder heads to hit the scene, parts catalogs are now packed full of them. They range from mild 205-cc castings intended for mild, stock-displacement, hydraulic roller cam applications all the way up to full race heads capable of supporting more than 1,000 naturally aspirated horsepower. Obviously, for anyone looking to crack quadruple digits in power, aftermarket cylinder heads are a must. However, because ported factory GM heads can flow just as well as entry-level aftermarket castings, why even bother with aftermarket hardware?

There is a time and a place for both, but aftermarket heads generally offer several advantages. Compared to factory heads, most aftermarket units are cast from a more rugged alloy for enhanced durability. Additionally, aftermarket castings typically utilize thicker deck surfaces for better gasket seal, particularly in power adder applications, along with raised valve cover rails for increase valvetrain clearance. Other common enhancements include reinforced rocker stud bosses and thicker port walls.

GMPP offers several versions of its LSX cylinder heads. Common features among them include .625-inch-thick decks, thicker port walls, and a six-bolts-per-cylinder bolt pattern. Port and chamber designs are based on the L92 castings.

Unlike the production LS7 heads that come CNC-ported, GMPP’s LSX-LS7 units are delivered as cast. With the benefits of a 12-degree valve angle and raised intake ports, these heads give professional porters lots of flexibility in designing powerful ports.

All this adds up to a cylinder head that’s superior to a factory unit in almost every regard. Furthermore, the cost of porting stock heads and fitting them with quality valvetrain components can run up a tab that’s just as expensive as an aftermarket casting.

Perhaps the most important factor to consider is that putting a ported stock cylinder head up against an out-of-the-box aftermarket casting is an apples-to-oranges comparison. Although it’s possible for a ported stock casting to match the performance of an aftermarket head, a ported aftermarket head puts a stocker to shame. For instance, although a ported factory LS6 head typically tops out at around 320 cfm, a ported aftermarket cathedral-port casting can easily exceed 350 cfm. Much of this is attributable to the fact that aftermarket heads have thicker port walls, especially in the critical areas around the pushrod tubes and the valve spring pockets. This enables a skilled porter to achieve the cross-sectional area and port volume necessary to push an aftermarket head well beyond the capabilities of a stock casting.

As with factory cylinder heads, aftermarket castings are available in both cathedral- and rectangle-port designs. For the ultimate in performance, rectangle port heads still have the edge, but it’s not nearly as big of a gap in the realm of aftermarket heads.

In the past few years, there has been an influx of cathedral-port heads with massive intake runners designed for big-inch, high-RPM engine combinations capable of supporting 750-plus hp. By taking full advantage of their meaty port walls, these monster cathedral-port heads—from companies such as AFR, Dart, and Trick Flow—move upwards of 370 cfm of air. So, although a rectangle-port head may sometimes offer a slight edge in performance, that’s not always the case. Interesting, too, are the exotic canted-valve heads that have recently entered the marketplace; they offer monstrous ports approaching 300 cc and more than 420 cfm of airflow.

Needless to say, regardless of brand, it’s hard to go wrong with any aftermarket cylinder head these days. Unlike 20 years ago, when enthusiasts had to pick between good heads and junk heads, now the challenge is trying to pick the best heads out of an assortment of heads that offer outstanding performance. And that’s a very good problem to have.

AFR

The first company to release all-new aftermarket Gen III cylinder heads in 2004, Air Flow Research offers some of the best street/strip castings available today. Powerful, high-velocity ports that combine respectable peak CFM with outstanding low- and mid-lift performance distinguish AFR heads. On the street, where a broad powerband takes precedence over high-RPM power, that’s exactly what you want out of a cylinder head.

It all started with AFR’s original 205-cc Mongoose LS1 castings, designed for 3.900-inch-bore motors. Despite having ports that are only 5 cc larger than those of a stock LS6 casting, the 205-cc AFRs flow an additional 70 cfm for an advertised total of 298. Common sense says that such a dramatic increase in airflow through a stock-sized intake runner translates to high-velocity ports that promote low-RPM cylinder filling. Flow figures aside, however, AFR’s claim to fame is producing cylinder heads that seem to consistently outperform their advertised cfm numbers. With a mild hydraulic roller camshaft with 220 to 230 duration at .050-inch lift, the 205-cc AFRs routinely produce 550 to 600 hp.

AFR recommends its 205-cc cylinder heads for applications ranging from 346 to 396 ci. Although AFR’s 205-cc heads are the most competition-proven Gen III/IV heads on the market, the company has recently improved upon them with is new 210-cc V2 castings. These heads offer further-improved low- and mid-lift flow and velocity in addition to an extra 8 to 10 cfm of peak flow.

For larger-displacement, higher-RPM engine combinations, the AFR lineup also includes 215-, 230-, and 245-cc cathedral-port castings. Like the 210-cc V2 cylinder heads, the 230-cc units are an updated version of AFR’s original 225-cc castings that utilize a raised intake port entrance for improved efficiency. For the ultimate in cathedral-port performance, AFR’s 245- cc heads flow an impressive 360 cfm. These heads have undergone tons of R&D, more so than most competing designs, and these blur the lines between cathedral- and rectangle-port heads, as they flow just as much as a set of factory LS7 castings. As many Gen III/IV engine builders will attest, it’s hard to go wrong with a set of AFR cylinder heads.

Bore diameter, valve size, and valve angle determine whether or not valve-to-cylinder-wall interference will be an issue. To maintain proper clearances, it’s very important to pay attention to a cylinder head manufacturer’s recommended minimum bore diameter.

All Pro

As aftermarket blocks give hot rodders the flexibility to build bigger displacement motors, it forces cylinder head development to keep pace. All Pro’s 12-degree, rectangle-port LS7 cylinder heads prove the point. Featuring 285-cc intake ports, 2.200/1.600-inch valves, a thick .750-inch deck surface, and CNC-machined ports and combustion chambers, the All-Pro heads flow a very respectable 410 cfm. With a flow figure like that, they’re ideally suited for big-inch stroker combos in excess of 430 ci. Other highlights include reinforced rocker stud bosses, large 1.625-inch-diameter valve spring pockets, and compatibility with six-bolt aftermarket blocks.

Dart

Few aftermarket companies can boast as rich of a racing heritage as Dart. Company founder Richard Maskin cut his teeth building championship-winning NHRA Pro Stock engines, and the lessons learned on the track have inevitably trickled down into Dart’s aftermarket product line.

A CNC machine is only as good as the person who programmed it, and the first step in the CNC porting process is to create a prototype port by hand. Next, a space-age coordinate measuring machine, called the FARO arm, creates a virtual computer model of the cylinder head. By simply running a probe across the surface of the ports and chamber, the FARO arm exports data points into your computer.

As the probe is moved throughout the ports, a 3-D model is created on-screen, which enables the operator to gauge how much progress is being made. Since the probe inevitably passes over the same spot multiple times, redundant data points can be filtered out after digitization is complete. A huge perk of digitizing the ports is that dimensions that are difficult to calculate manually, such as the average cross-sectional area, can be precisely measured.

Dart’s Pro 1 Gen III/IV lineup features cathedral-port cylinder heads that retain the factory 15-degree valve angle, and they are offered in 205-, 225-, and 250-cc configurations. The 205-cc castings come equipped with 2.020/1.600- inch intake and exhaust valves and 62-cc combustion chambers, as well as an advertised peak flow number of 290 cfm. Dart’s 225-cc castings have a slightly larger 2.050-inch intake valve, and they flow a hair over 300 cfm. To keep costs down, both the 205- and 225-cc cylinder heads have as-cast ports. The heads can be purchased for $1,800 fully assembled, making them two of the most affordable sets of aftermarket heads available. At the top of the Dart LS1 pyramid are the company’s 250-cc CNC-ported cylinder heads. Highlights include 2.080/1.600-inch valves, beefier valve spring pockets, and CNC-machined ports, combustion chambers, and bowls. At $2,100 fully assembled, they’re also an excellent value.

Edelbrock

With one of the most impressive foundry facilities in the country, Edelbrock uses its unparalleled resources to cast quality cylinder heads for virtually every domestic engine platform in existence. Naturally, the company offers a number of different cylinder head options for the LS-series small-block.

Edelbrock’s 203-cc Performer RPM castings were designed in conjunction with Lingenfelter Performance Engineering, and they feature reinforced spring pockets, CNC-ported runners and bowls, 2.020/1.570-inch valves, and 65-cc combustion chambers. Advertised flow ratings spec in at 320 cfm, and they cost $2,600 for a fully assembled set.

Although Edelbrock’s Performer RPM heads flow plenty for the vast majority of enthusiasts, the company also offers one of the most serious castings in the entire Gen III/IV market. Designed strictly for competition engines, Edelbrock’s Victor LSR cylinder heads offer what is arguably the most flow potential of any LS casting on the market. The heads are cast utilizing a hot isostatic pressure process, in which the heads are placed in a vacuum chamber to remove porosity and contaminants. After that, they are pressurized at 30,000 psi with nitrogen during heat-treating. The end result is an extremely durable head that can handle the most abusive of environments.

However, the keyword to remember with these heads is “potential,” as the LSRs are pure race castings that arrive unfinished. That means it’s up to end users to design their own custom ports and combustion chambers from scratch. Because the heads have extra thick port walls and space for massive 2.250/1.600-inch valves, some of the first race shops to get their hands on these heads have been able to coax 450- plus cfm out of the LSRs. That’s not just big-block Chevy territory; that’s Big Chief Rat motor territory. Nonetheless, that kind of potential comes at $2,400 for a pair of raw castings, so the entry price point is reasonable. However, factor in the labor involved with designing and machining custom ports and chambers, and the total development costs for a finished set of LSR heads can easily cost 10 times as much.

The path the CNC tool travels to create the desired port shape is called the tool path. After creating a 3-D model of the ports and chamber, the projected tool path is put into motion on-screen to check for potential hang-ups. The tool path is then run through a post-processor to write the actual program, which is called the G-code. Different CNC machines have different axes and ranges of travel, so post-processing software is specific to each machine.

So, although it’s all in good fun to admire the airflow potential of the Victor LSR heads and appreciate just how far the LS engine platform has advanced, unless you own a five-axis CNC machine and have your own in-house R&D department, forget about bolting these beasts up to anything but a mega-dollar race engine. That said, as more race shops get their hands on the LSR castings and put forth the initial investment in R&D to develop port and chamber designs for them, it is quite possible that they may someday be within reach for the deep-pocketed sportsman racer.

GM Performance Parts

During the 50 years that separate the genesis of the Gen I small-block Chevy and the Gen III small-block platform, engineers and racers learned a thing or two about building performance engines. That partially explains why Gen III/IV engine development— both at the OE level and in the aftermarket—has advanced at such a blistering pace. What can’t be forgotten, however, is the influence of GM’s internal Performance Parts division, which simply didn’t exist in 1948. By leveraging the stacks of R&D data compiled while designing production parts with the vast resources of GM, the company’s Performance Parts division has been at the forefront of Gen III/IV aftermarket parts development since day one. Just as GMPP’s LSX blocks upped the durability and displacement ante, its new lineup of cylinder heads improves upon production castings that are already excellent performers.

The digitizing process is repeated for the combustion chamber. Because the chambers have very complex contours with tight radiuses, it’s often necessary to attach a tiny 3-mm probe onto the FARO arm. The digitized chamber data can also be sent to the piston manufacturer to help create a dome that fits perfectly inside the chamber.

With symmetrical heads, such as GM Gen III/IV castings, the prototype ports and combustion chamber are cloned for the remaining three cylinders. This eliminates the need to design each and every port one at a time, and it ensures that every single port and chamber in the heads are identical, a feat that’s nearly impossible when porting by hand. The port on the left is a hand-ported port, and the others have been cloned using computer software based on bore spacing of the block. The software is so precise that it allows modifying a hand-ported surface even further.

Currently, GMPP offers five different LSX rectangle-port cylinder heads, for everything from stock-displacement engines to drag race and circle track applications. All LSX heads are cast from a durable 356-T6 aluminum alloy and feature 5/8-inch-thick decks and beefier port walls. GMPP’s entry-level aftermarket head, the 250-cc LSX-L92, is essentially an improved version of the stock L92 castings designed for enthusiasts with small 3.900-inch-bore engines. To maintain small-bore compatibility, they’re fitted with 2.000/1.550-inch valves.

In comparison, the LSX-LS3 heads retain the stock L92’s 260-cc ports and 2.165/1.590-inch valves. The LSX-LS9 heads are the same as the LSX-LS3’s, but with the addition of titanium intake valves and sodium-filled exhaust valves. An improved version of the stock LS7 cylinder heads, the LSX-LS7 heads feature a 12-degree valve angle, giant 2.200/1.610-inch valves, 270-cc as-cast ports, and six-bolt-per-cylinder bolt pattern for use on aftermarket blocks.

The upper echelon of GMPP’s aftermarket cylinder heads is an evolution of the C5R castings. These monsters come in two configurations—for circle track and drag racing applications—and are appropriately named LSX-CT and LSX-DR. Common features include an 11-degree valve angle, 1.625-inch valvespring pockets, raised valve cover rails, a spread-port exhaust configuration, CNC-machined ports and combustion chambers, and intake ports that have been raised an impressive 10 mm.

In addition to mapping out the ports and chamber, the FARO arm creates virtual planes based on the angle and position of the deck, intake port, exhaust port, and valve cover rail surfaces. The dowel locations and front and rear surfaces of the head are also mapped. This enables the FARO arm to orient the location of the ports and chambers in relation to the rest of the head.

The digitized data from the Verisurf software is run through a post-processor to create lines of G-code compatible with the Haas CNC machine. That data is then loaded, via a USB drive, into the CNC machine’s control station, which interprets the code, using Mastercam software, to determine the tool path. On a five-axis mill, the place where the A- and B-axes meet is called the point of origin. The final step before machining begins is to make sure that the computer program and the CNC machine have the same point of origin.

The LSX-CT heads utilize epic 302-cc intake ports, 45-cc combustion chambers, and 2.200/1.610-inch valves. They flow more than 420 cfm, and they can support 850 hp. The LSX-DR heads are even meaner, with 313-cc intake ports, 50-cc combustion chambers, 2.280/ 1.620-inch valves, and 450 cfm of airflow capable of producing 900 hp.

As their cavernous ports and tiny combustion chambers suggest, both the LSX-CT and LSX-DR heads are intended strictly for race motors running race gas. Aside from stunning airflow, what makes these heads even more appealing is that GMPP offers matching intake manifolds for them, which eliminates the need to custom fabricate an expensive sheet-metal unit. The bottom line is that the LSX-CT and LSX-DR castings are extraordinary heads for extraordinary applications.

Mast Motorsports

Founded in 2007, Mast Motorsports is a newcomer to the performance aftermarket, but the company has already established a reputation for quickly and efficiently developing cutting-edge hardware. Mast developed the industry’s first aftermarket rectangle-port Gen III/IV cylinder heads, and it has since expanded its lineup to envelop everything from stock 3.900-inch-bore engines to all-out race motors.

Offered in three basic configurations, for 3.900-, 4.000-, and 4.125-inch bore engines, Mast’s rectangle-port LS3 heads flow 353, 370, and 390 cfm, respectively. These 12-degree heads are fully CNC-ported, and they are compatible with the factory LS3 intake manifold and rocker arms. Additionally, the Mast catalog includes an LS7 casting that flows an impressive 395 cfm through a 274-cc intake port that’s only a hair larger than stock. For serious race motors, Mast also has a canted-valve LS7 head that flows more than 420 cfm.

To further expand its product line, Mast acquired Performance Induction Specialties. The consolidated resources of both highly respected companies have created a veritable powerhouse in the Gen III/IV cylinder head market. The Mast cylinder head catalog includes a comprehensive lineup of 11-degree cathedral-port castings ranging from 215 to 265 cc. Much like GM’s rectangle-port heads, these unique castings incorporate offset intake rocker arms for improved port architecture and a reduced pushrod pinch area, with peak flow numbers ranging between 320 to 370 cfm.

RHS

A circle track and drag racing powerhouse in the 1970s and 1980s, Racing Head Service went on hiatus for several years before reemerging early this millennium. Apparently, RHS hasn’t missed a beat, and its cylinder heads are better than ever. For Gen III/IV small-blocks, RHS offers Pro Elite cathedral-port cylinders in 210- and 225-cc configurations. Highlights include an 11-degree valve angle, thick .800-inch deck surfaces, and a taller valve cover rail. The meaty deck surface can be milled to reduce combustion chamber size down to 36 cc, and the extra material between the top of the intake ports and the valve cover rail frees up space for a raised roof port design. Despite these improvements, the Pro Elite heads are fully compatible with standard Gen III valvetrain components and intake manifolds.

After all programming has been completed, the CNC machine can go to work. The maximum amount of material removed in one pass should be limited to .25 inch. The step-over distance of the tool path determines the size of the ridges that are usually visible with CNC-ported heads. A .100- to .200-inch step-over is typically used when roughing in a port, and a .010- to .020-inch step-over is common on the finishing pass. SAM cut its Edelbrock LSR heads with an ultra-fine .005-inch step-over, which creates a smooth finish that is almost indistinguishable from that on a hand-ported head. Each port flowed within 1 cfm of each other, ranging between 460 to 461 cfm.

Forced induction is a subject that can be an entire book in and of itself, but suffice it to say that turbocharging an LS-series small-block yields stunning performance. Thanks to their high-flow heads, LS-series small-blocks produce outstanding horsepower figures at low boost levels on pump gas. Fastlane’s 72-mm turbo system for LS3 Camaros lays down 585 rear-wheel hp at just 7 psi of boost.

RHS’s 210-cc heads are designed for engines with a minimum bore of 3.900 inches and feature 2.040/1.570-inch valves. According to RHS, they’re good for 320 cfm and can support more than 600 hp. The 225-cc castings are compatible with bore sizes of 4.000 inches and up, and they are fitted with larger 2.080/1.600-inch valves. With an advertised flow rating of 328 cfm, RHS says they’ll support in excess of 650 hp. At $1,500 for a set of fully assembled 210-cc castings, RHS’s Pro Elite heads are a great value and undercut the price of many ported stock heads.

Trick Flow

Although the company started by manufacturing small-block Ford components, Trick Flow now offers a range of cathedral-port GM Gen III/IV cylinder heads that’s one of the most diverse in the industry. All Trick Flow heads have a flatter-than-stock 13.5-degree valve angle, and the company offers a top-notch head for every conceivable application.

Although the smaller-displacement 4.8L and 5.3L LS small-blocks have been widely ignored by most aftermarket manufacturers, Trick Flow recognized they’re growing in popularity and created a cylinder head specifically for these applications. The company’s GenX Street 205 head is a small-runner casting that works incredibly well on 3.7800-inch bore engines. The heads feature 58-cc chambers and 2.00-inch intake valves that complement their small 205-cc runner size very nicely and clear the factory bore. All of Trick Flow’s 205-cc heads come with fully CNC-machined ports and combustion chambers, and they can also be used on 3.900-inch stock-bore LS1s. According to Trick Flow, a set of its 205-cc castings will out-flow stock LS6 castings, and in-house testing of an otherwise stock 5.3L with a 216/220-at-.050 netted 456 hp and 425 ft-lbs of torque.

Moving up the ladder, Trick Flow offers 215-cc heads with 2.040/1.575- inch valves for 3.900-inch-bore engines, and 225-cc heads with 2.055/1.575-inch valves for 4.000-inch-bore applications. Designed for 400-plus, big-bore motors, Trick Flow’s 235-cc heads are fitted with 2.080/1.600-inch valves and flow 340 cfm out of the box. For motors displacing upwards of 440 ci, the 245-cc castings utilize 2.100/1.600-inch valves and include provisions for six-bolt blocks. For the truly exotic engine combos that require even more airflow potential, Trick Flow has recently released an unfinished LSX-R casting that can support up to a 265-cc intake runner that pushes preconceived notions of cathedral-port heads to the envelope.

In addition to CNC-ported heads, Trick Flow also offers 220-cc units that incorporate the company’s “Fast as Cast” intake and exhaust runners. To create these heads, Trick Flow started with one of its CNC-ported cylinder heads, and then it made intake and exhaust port tooling based off the port shapes. Thanks to modern casting technology that allows locating the intake and exhaust cores to tight tolerances, the port shapes are much more precise than was possible just 10 years ago. With as-cast ports and CNC-machined chambers and bowls, the 220-cc heads flow almost as well as CNC-ported heads for 30 percent less money. Offering 305 cfm of flow for $1,700 in fully assembled trim, the as-cast 220s boast excellent performance for the dollar with plenty of room to grow.

Written by Stephan Kim and Posted with Permission of CarTechBooks

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Unlike the Gen I small-block, the oil pump on Gen III/IV engines mounts to the front of the block and is driven directly off the crankshaft. The pump housing and drive gear slide over the crankshaft snout, and the pump gear is rotated by the crank keyway. By eliminating the camshaft-driven oil pump driveshaft used in the Gen I small-block, the Gen III/IV arrangement reduces drag placed on the valvetrain, as well as deflection and pumping losses.

Oil lubricates virtually every moving part in an engine. A thin film of oil is the only thing preventing those components from seizing up, so it’s critical to maintain proper flow, volume, and pressure. The crankshaft main and rod journals are two of the highest friction areas in an engine. The surface area of the journals, the loads placed upon them during the combustion process, and RPM all dramatically increase friction.

The pump draws oil from the pan through a pickup tube, and after it’s pressurized, oil is sent down the main gallery on the driver side of the block en route to the oil filter. The oil then travels up the back of the block to the main feed gallery, which runs through the lifter bores. From there, oil trickles down to the main bearings. Just like the Gen I small-block, LS-series engines direct oil to the cylinder heads through holes drilled into the lifters and pushrods. This lubricates and cools the valve springs and rocker arms, and the oil then drains back into the pan through passages in the cylinder heads and block.

Lubrication Basics

Oil is the only thing preventing the moving parts in an engine from seizing up. Before delving deeper into the specifics of the LS-series oiling system, it’s important to investigate the very basic principles of lubrication, which can be divided into three different states. Hydrodynamic lubrication describes the ideal situation where a continuous film of fluid separates two sliding surfaces. During hydrodynamic lubrication, the viscosity of the oil supports the entire load between moving parts and prevents them from touching.

Oil lubricates virtually every moving part in an engine. A thin film of oil is the only thing preventing those components from seizing up, so it’s critical to maintain proper flow, volume, and pressure. The crankshaft main and rod journals are two of the highest friction areas in an engine. The surface area of the journals, the loads placed upon them during the combustion process, and RPM all dramatically increase friction.

Since the Gen III/IV oil pump is mounted on the front of the motor, it uses a long pickup tube, supported by the center main cap, that runs rearward to draw oil from the pan sump. Many performance Gen I small-block builds do not incorporate a windage tray. In contrast, Gen III and IV small-blocks boast factory installed windage trays.

LS-series engines use the oil pan as a structural component of the block to reduce noise and vibrations. Hence, the pans are very rugged in design and built from cast aluminum. The F-body oil pan, originally installed on 1998 to 2002 Camaros and Firebirds, is the most popular with engine swappers, due to its rear sump location and generous ground clearance.

The extreme opposite end of the spectrum is boundary lubrication, which is the last line of defense before metal-to-metal contact occurs. When oil is squeezed out from between moving parts in high-load areas, such as the main journals and bearings, all that’s left to prevent excessive wear are the anti-wear additives in the oil.

Mixed film lubrication is a little bit of both, where some oil has been squeezed out, but a thin coat of oil is still present.

Each state of oil is present somewhere in the engine, which makes formulating oil very complicated. Ideally, hydrodynamic lubrication would be achieved under all conditions, but because this isn’t possible, it makes the oil much more important.

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Pressure vs. Volume

Oil control is a balance between pressure and volume, and although cars come equipped with pressure gauges, there’s no way for the average hot rodder to measure the volume of oil that’s flowing through an engine. In essence, oil pressure is used as an indirect method of measuring oil volume. As oil is pressurized in the pump, the resistance against the pump outlet creates oil pressure. Elementary physics dictates that pressure must be present in order to move oil through an engine, so in this regard, pressure is a good thing. However, excessive pressure merely increases an engine’s pumping losses and adversely affects power output. Because pressure demands increase with RPM, it’s generally recommended that an oil pump should be able to supply 10 psi of pressure for every 1,000 rpm. In other words, an engine turning 6,500 rpm needs roughly 65 psi of pressure for proper lubrication.

Even so, as long as sufficient volume is present to fill the clearances between moving components and remove heat from the bearings and journals, oil pressure is somewhat irrelevant. This is important to remember in performance engine builds that typically employ looser bearing clearances than stock engines. As clearances increase, a greater volume of oil is needed to maintain a target oil pressure. High-volume oil pumps help increase pressure to the desired level, but they aren’t always necessary. It’s quite possible that a stock pump can supply plenty of volume and pressure in a typical stroker buildup. Furthermore, oil pressure is also affected by oil viscosity and temperature. Thicker oils, and colder oil temperatures, increase oil pressure, but not volume. This merely reinforces the point that although having sufficient oil pressure is important, having proper oil volume is an even greater priority.

Factory GM oil pans are just as impressive on the inside as they are on the outside. They utilize internal strengthening ribs, which double as oil control baffles to prevent oil from sloshing around under cornering loads. (© GM Corp.)

Companies, such as Moroso, Canton, and Milodon, offer aftermarket oil pans built from sheet metal that fit a variety of vehicle chassis for both street/strip and road racing applications. Since road race machines sit low to the ground, oil pans designed for these cars have T-shaped sumps, which provide the necessary oil capacity without compromising ground clearance.

All-inclusive kit to facilitate the installation of any wet sump LS engine into early model GM vehicles such as the early 70s muscle cars and more. X09SP_PA101 (United States)

Oil Pumps

Sticking with the philosophy of not fixing things that aren’t broken, there are only three basic types of oil pumps used by GM throughout the entire LS engine lineup. All Gen III/IV small-blocks use identical oil pumps, except for displacement-on-demand engines, which use higher-volume pumps. Consequently, there is no advantage to taking an oil pump off an LS6 and installing it onto an LS1, or using an LS3 pump on a 6.0L truck motor. The one major deviation within the LS family is the LS7 pump, which is completely different, because it’s designed to feed a dry sump system. The LS7 unit is actually two pumps in one, as it moves pressurized oil like a conventional pump, and it also scavenges air and oil out of a dry sump pan. As a result, the LS7 pump isn’t compatible with any other LS-series small-block, unless a motor has been converted to the factory dry sump system.

One critical measurement that should never be overlooked is the pump pickup-to- pan clearance. If the oil pump pickup sits too close to the bottom of the pan, it will starve the pump of oil and result in inadequate flow and pressure. Engine builders recommend having .375 to .5 inch of clearance.

Every chassis presents its own installation challenges, and in some instances, there might not be enough space for the oil filter. In these situations, a remote filter installation kit can be used to mount the filter virtually anywhere in the engine compartment.

Due to the extreme cornering loads the Corvette Z06 and ZR-1 are capable of producing, the LS7 and LS9 both use the most exotic oiling system ever installed on a GM production car. Their dry sump configuration means that the oil is stored in a separate tank, and the pan merely acts as a drip pan. Oil is pumped out of the storage tank and into the block, where it drips down onto the pan after circulating through the engine. The pump then scavenges the oil and sends it back to the tank, and the process repeats itself. (© GM Corp.)

Aftermarket oil pumps are available from Melling and GM Performance Parts. These units are offered in both standardvolume and high-volume configurations. For engines with loose bearing clearances, these aftermarket pumps can increase oil volume by 10 to 33 percent. That said, a standard-volume pump typically supplies enough oil for the average stroker small-block. Companies, such as Lingenfelter Performance Engineering and SLP, also offer pumps, which are slightly modified versions of the aftermarket or stock replacement oil pumps. Many engine builders opt to port the inlet and outlet passages of the oil pump. This process involves radiusing the inlet and outlet passages of the pump to create a smooth transition point between the block and oil pump pickup. In theory, this is said to improve oil flow volume, but in practice, it has a negligible impact on pump performance.

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Stock Pans

The Gen III/IV oil pan is a unique design in that it serves as a structural member of the engine. This enhances block stiffness and helps reduce vibration. Throughout the LS-series production run, GM has used several different types of oil pans, primarily to account for the installation needs of various types of vehicles. Early LS-series small-blocks featured three basic oil pan designs. They included the F-body pan used on fourth-gen Camaros and Firebirds, the “batwing” oil pan used on C5 Corvettes, and the deep sump oil pan used on trucks and SUVs. As with intake manifolds, each of these pans was designed more for fitment within the chassis than achievement of certain performance benchmarks. Still, each of them offers excellent oil control, and as a testament to the seriousness with which GM approaches oil control, each style pan incorporates a windage tray.

Because its sump location allows the best fitment into a wide variety of chassis, the F-body oil pan is the most popular choice for engine swappers. In contrast, the truck pan’s extremely deep sump doesn’t provide adequate ground clearance in most street machines, so it isn’t a very practical option for anything other than a truck. Due to the Corvette’s very low ride height, engineers had to get creative by kicking the oil pan sump out to the sides to ensure adequate oil capacity and ground clearance. This also means that the Corvette pan provides excellent oil control under high cornering loads, but its unique shape makes it difficult to install into most chassis.

The LS7’s dry sump system sends a frothy mix of oil and air to the storage tank, so baffles and air/oil separators are integrated into the tanks. Without them, the oil would foam up and lose all of its lubrication properties. (© GM Corp.)

Aftermarket companies, such as ARE Dry Sump Systems, manufacture dry sump oiling systems for road racing applications. Aftermarket systems employ an external oil pump that is driven off the accessory drive belt.

Dry sumps aren’t just for road-race vehicles. High-RPM drag racing motors also benefit from them, because they virtually eliminate windage. Katech’s four-stage billet pan has four separate oil pickup points.

As GM installed the LS-series small-block into more cars, it produced several variations of its three early oil pan designs. The LS2 Corvette oil pan did away with the C5 pan’s batwing design, and it features a long 13.5-inch sump section that makes it difficult to fit into most cars. Likewise, the oil pan used on the Cadillac CTS-V features a sump design deeper than the F-body pan but shallower than the truck pan. Even so, it offers inadequate ground clearance for many engine swap applications. The same applies to the LH8 oil pan used on 5.3L-powered Hummer H3s. It has a long sump section that clears most crossmembers, but its deep 7.5-inch sump makes it difficult to install.

Aftermarket Pans

Because extensive research and development (R&D) efforts went into designing factory GM oil pans to make sure they performed well under both longitudinal and lateral loads, and had rugged cast-aluminum construction, the most compelling reason to use an aftermarket oil pan is for improved chassis fitment. Aftermarket pans are typically constructed from aluminum sheet metal, which gives engineers more flexibility in designing a pan because they don’t have to invest in the tooling required to build a cast pan. The number of aftermarket oil pans for the LS platform is staggering, and each is designed to fit in a specific chassis. Although it isn’t feasible to list all of the available aftermarket oil pans here, most pans generally offer a similar list of benefits.

For street cars that see time at the autocross or on the road course, an accumulator system is a nice alternative to stepping up to a costly dry sump setup. Accumulators store several quarts of oil in an air-charged canister. When oil pressure drops below a preset level, the oil is discharged out of the system and into the block. The oil is then replenished after oil pressure recovers. Accumulators are available from several companies, such as Moroso and Canton.

Although oil is primarily thought of as a lubricant, it also helps cool the rotating assembly, valvetrain, and bearings. As a result, it’s very important to keep an eye on oil temperature. Ideally, the oil temperature should stay within 10 to 15 degrees of the coolant temperature.

Well aware of the importance of managing oil temperature, GM installs factory oil coolers on Gen IV small-blocks, such as the LS3, L99, and LS9. The cooler is essentially a small heat sink integrated into the driver side of the oil pan. (© GM Corp.)

Many aftermarket pans are built with the average street cruiser in mind, but companies also offer specialized designs for drag racing and road racing applications. Because drag racing oil pans only need to perform optimally during straight-line acceleration and braking, they tend to have much deeper sump designs. In contrast, road race pans have kicked-out sumps for improved ground clearance. Furthermore, drag racing pans usually have a single trap-door baffle that prevents oil slosh and helps keep the pump pickup submerged during acceleration and braking. Trap doors function by trapping a pocket of oil under normal driving, and then swinging open under high g loads to release a pocket of oil to keep the pickup submerged in oil. Road racing pans take this concept one step further by employing several trap-door baffles for superior oil control under cornering loads.

Dry Sump Systems

Once considered the exclusive territory of full-blown race cars, enthusiasts were in disbelief when GM revealed that the LS7 would feature a dry sump oiling system. Even though dry sump systems have found their way into high-end street cars, they’re still rather exotic by nature. As their name implies, dry sump oiling systems don’t use a conventional oil pan. Instead, oil is stored in an auxiliary tank mounted elsewhere in the chassis, and the oil pan merely functions as a collection bin that catches oil dripping off the crank, rods, and block. A crankdriven pump scavenges this oil, and then circulates it into the storage tank before making its way back into the block.

Although complicated, this arrangement offers several benefits. For one, having no oil in the pan eliminates windage and frees up horsepower. This reduction in windage is why many drag race engines that turn 8,000-plus rpm utilize dry sump oiling systems. Second, storing the oil in an auxiliary tank takes oil slosh completely out of the equation, ensuring an uninterrupted supply of oil to an engine’s critical parts under even the highest of cornering loads. Additionally, the low-profile design of a dry sump pan allows mounting an engine extremely low in the chassis for improved handling. That said, only a very small fraction of LS builds require a dry sump oiling system. If deemed necessary in an autocross or road racing application, the stock LS7 system can be retrofitted into just about any chassis very easily. Furthermore, several aftermarket companies offer dry sump conversion systems.

Oil Coolers

An oil’s ability to properly lubricate moving parts is dependent upon its operating temperature. For every 18- degree increase in oil temp, the oxidation rate doubles. That means that although the difference between 200- and 220-degree oil temps might not seem like a big deal, every last degree of temperature increase has a dramatic effect on oil performance. Consequently, it’s not surprising that GM installed a factory oil cooler on just about every Gen III/IV small-block ever built. Latemodel engines typically operate at higher coolant temperatures than older engines in an effort to reduce emissions output, which explains why factory oil coolers are so common. Although they’re not mandatory on stroker smallblocks, which are typically set to operate at lower coolant temperatures, they’re cheap insurance and help extend oil longevity and performance. Stock oil coolers are usually integrated into a car’s cooling system, so in engine swap applications, it’s easiest to install an oil cooler offered by several aftermarket manufacturers, such as TCI and B&M.

Installing an oil temperature gauge is an easy way to keep an eye on how much heat is in the oil. Under steady state cruising, where a constant supply of air is moving through the radiator and oil cooler, oil temps generally mirror coolant temps. In stop-and-go traffic and heavy acceleration, however, oil temperature usually gets much higher than coolant temperature. Without a gauge, trying to figure out oil temp is just a guessing game.

Written by Stephan Kim and Posted with Permission of CarTechBooks

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As with crankshafts, the past 15 years have produced an influx of affordable aftermarket connecting rods. Consequently, enthusiasts have more choices than ever, and it’s no longer costprohibitive to step to some aftermarket forgings. Quality aftermarket rods are now so affordable, in fact, that tried-andtrue practices, such as shot-peening and reconditioning stock connecting rods, are a thing of the past. Rugged 5140 forged steel aftermarket rods can be had for under $300, and $600 buys a set of H-beams that will handle up to 1,400 hp. Connecting rods come in a variety of shapes and sizes, and they are made from several different materials, so it pays to study before laying down cash on a set of rods for your stroker project.

Except for the LS7 and LS9, all stock LS motors rolled out of the factory with powdered metal rods. Although they’re capable of standing up to 500 hp, the stock rod bolts become marginal at 6,500 rpm. The cost of new rod bolts, resizing the big end, and press-fitting the rod back onto the piston is just as much as an entry-level forged I-beam rod, so stock connecting rods are destined for the scrap heap most of the time.

Stock Rods

Factory Gen III/IV connecting rods incorporate a very durable design that has proven to be reliable up to 500 hp. They’re built from powdered metal, a process that involves packing powdered steel into a mold, heating it, and then forging it into the shape of a rod. A parting line is then machined onto the big end of the rod before cracking off the cap. The result is a cap that fits perfectly into the grooves of the rod when bolted down. Stock LS rods are actually stronger than the coveted “pink” rods used in select Gen I small-blocks.

GM didn’t cut any corners when developing the LS7, and it fitted the engine with titanium connecting rods. At just 434 grams each, their low mass is essential for reducing rotating weight at the LS7’s 7,000-rpm peak engine speed. (© GM Corp.)

GM manufactured LS connecting rods in three different lengths. Rod length is measured from the center of the big-end bore to the center of the smallend bore. The vast majority of Gen III/IV small-blocks—including 5.3L, 5.7L, 6.0L, and 6.2L car and truck motors—utilize 6.098-inch rods. Of these, most feature press-fit piston wrist pins. The exceptions are the rods used in 6.0L Vortec truck motors, which have floating pins and a slightly thicker beam. With the launch of the Gen IV small-block in 2005, GM began phasing in rods with bushed small ends in order to support full-floating wrist pins. Otherwise, all 6.098-inch factory LS rods are very similar.

To minimize production costs, GM also manufactures a longer 6.275-inch rod for 4.8L Vortec truck motors. Because these smaller engines are equipped with a shorter 3.267-inch stroke, a longer rod allows GM to use the same piston casting for the 4.8L as for the 5.3L. One of the latest iterations of factory GM rods is also the most extreme. To keep reciprocating mass to a minimum, GM developed a brand-new 6.067-inch titanium connecting rod for the LS7. These ultra-lightweight rods tip the scale at just 464 grams, about 30 percent lighter than stock powdered-metal rods, which is part of the reason why the LS7 revs freely to 7,000 rpm. At more than $400 for each rod, however, the LS7 forgings are extremely expensive for anyone considering bolting them to a stroker motor.

Due to the increase in power and cylinder pressure that results from strapping a supercharger onto the LS9, GM had to raise the rod bar once again. Compared to the LS7 rods, the LS9 rods are also built from titanium, but their beams are larger near the big end. Unlike earlier LS rods, the LS9 units attach to the pistons with floating wrist pins that are secured by lock rings. Dowels help locate the rod caps, which is a feature usually exclusive to aftermarket rods. An interesting side note is that the LS9 rods are built by Austria-based Pankl, a leading supplier of Formula One engine components. (© GM Corp.)

For most hot rodders, aftermarket forged steel rods offer the best balance of strength and cost. As with crankshafts, 4340 steel has become the standard for aftermarket rod forgings. Due to the minor difference in price between 4340 steel and weaker alloys, such as 5140 and 4130, these lesser materials are rarely used anymore. These 4340 H-beam rods from Scat feature a lightening hole right above the big end housing to reduce mass.

The only benefit aluminum rods have over steel rods is a weight reduction of 25 percent. Although that’s rather significant, it comes at the price of substantially reduced tensile strength and fatigue life. Not surprisingly, they’re rarely used in street motors and are best suited for drag race engines.

Because stock connecting rods are adequate to the 500-hp mark, they are a viable option for a budget-oriented stroker build. However, the factory rod bolts become marginal once engine speeds approach 6,500 rpm. This is because every time the piston is pulled back down the bore after it reaches TDC, tremendous loads are placed upon the rod bolts. In fact, rod bolts are subjected to the greatest amount of stress in the entire engine. Consequently, in any performance application where the stock rods are re-used, the bolts must be replaced with quality replacements from a company, such as ARP. Doing so requires machining the inside housing diameter of the big end of the rods, because new fasteners may change their shape.

Unlike stock-style rods that are held together with bolts and separate nuts, most aftermarket rods employ cap screws that thread directly into the rods themselves. This means that no part of the rod protrudes into the shoulder, which improves rod bolt clearance around the camshaft. Aftermarket cap screws and bolts are available in tensile strength ratings ranging from 190,000 to 280,000 psi.

The biggest drawback to re-using stock connecting rods is labor costs for resizing; they can come close to the price of new aftermarket forgings. Re-sizing a set of eight connecting rods costs about $80. Most stock LS rods must be press-fit onto the pistons, which racks up another $80 in labor. Throw in the $100 that quality rod bolts will set you back, and the total cost to recondition a stock rod is almost enough to buy a set of forged I-beam aftermarket rods.

Forging Materials

Thanks to the efficiency and low cost with which aftermarket connecting rods can be manufactured in today’s market, cast rods are virtually nonexistent. As with crankshafts, forged steel alloys are most common, and extremely exotic and expensive materials, such as titanium and aluminum, are also available. There are tradeoffs in strength, weight, and cost with each, so it’s important to know the difference between each material in order to select a rod that’s both durable and affordable.

The three most popular grades of steel used to manufacture aftermarket connecting rods—in ascending order of strength—are 5140, 4130, and 4340 alloys. Just as with crankshafts, higher grade alloys offer advantages in tensile strength and ductility. This is due to their higher concentration of carbon, nickel, and chrome content. Typically, entry level aftermarket rods are forged from 5140 or 4130 steel, and pricier high-end rods are built from a premium 4340 alloy. After the forging process, aftermarket rods are heat-treated, shot-peened, and stress-relieved to further enhance durability. They’re also fitted with rod bolts or cap screws that offer up to 280,000 psi of tensile strength.

The majority of aftermarket rods are H-beams (bottom), which have led many people to assume that they are stronger than comparable I-beams (top). However, this isn’t always the case, as companies, such as Oliver, offer some of the most durable rods around and use an I-beam design exclusively. Lunati offers these premium 6.125-inch I-beams, which are just as strong as its H-beams and 60 grams lighter.

Blurring the line between H- and I-beam rods, Compstar now offers hybrid H/I-beam rods that incorporate elements of both designs. They feature triangulated big and small ends that increase beam thickness by 25 percent with only a 6-percent weight penalty. Compstar recommends using its H/I-beam rods in applications exceeding 1,000 hp.

For mild street applications up to 600 hp, 5140 and 4130 steel rods are sufficient, but 4340 rods are advisable at anything beyond that point. With the increasing affordability of 4340 steel, however, 5140 and 4130 alloys are becoming less prevalent, even in entry-level rods. Interestingly, the number of entry level connecting rods in the $300 range on the LS market is extremely limited compared to their higher-end alternatives, with Scat and Eagle being the primary players this arena. This is probably due to the fact that the factory Gen III/IV rods are very strong, and most performance LS stroker builds can easily approach or exceed the 600-hp mark where premium rods are a necessity. Not surprisingly, Scat and Eagle sell more of their premium grade rods than their entry-level rods.

Two of the more exotic materials used to manufacture connecting rods are aluminum and titanium. Available from aftermarket companies, such as GRP and Howards Cams & Racing Components, and popular in racing applications where low mass takes precedence over ultimate strength and long-term durability, aluminum rods can be forged or cut from blocks of billet. Because they weigh 25 percent less than a steel forging, aluminum rods reduce reciprocating mass, and, therefore, improve power output. However, they only offer half the tensile strength of steel, and they have a much shorter fatigue life. That means that as aluminum rods accumulate mileage and are subjected to repeated heat cycles, they tend to stretch, harden, and weaken over time. Additionally, aluminum rods must be made bulkier than their steel counterparts to compensate for their lower tensile strength. Consequently, they are unsuitable for street motors, and they must frequently be checked for stretching in race motors. According to some rod manufacturers, the fatigue life of an aluminum rod is 90 percent lower than that of a steel rod. For these reasons, aluminum rods are most frequently used in high-dollar drag race engines that are rebuilt multiple times per season.

Striking a balance between steel and aluminum is titanium, which offers the highest strength-to-weight ratio of all materials used to build connecting rods. The most commonly used material for automotive applications is 6AL-4V titanium, an alloy that contains 6 percent aluminum and 4 percent vanadium for improved machinability. Although titanium’s tensile strength is 15 percent lower than that of steel, it’s 30 percent lighter. As a result, titanium rods are often used in applications that operate at extremely high RPM, such as 9,500-rpm NASCAR Sprint Cup motors, 18,000-rpm Formula One engines, and even 7,000- rpm LS7s. Offered by aftermarket companies, such as Crower and Cunningham, the biggest drawback of titanium rods is price. A set of eight titanium rods usually costs twice as much as a set of comparable steel rods. Furthermore, despite their extra weight, steel rods are more than up to the task of enduring sustained engine speeds above 8,000 rpm.

Although the term “billet” doesn’t refer to any specific material and can more accurately be described as a manufacturing process, billet is worth discussing to determine where it falls in the hierarchy of connecting rods. A billet rod starts out as a single ingot of forged steel, aluminum, or titanium and is machined into the final shape of the rod. The primary advantage of this manufacturing technique is customizability. Because they don’t rely on very expensive forging dies and presses to pound them into shape, billet rods can be manufactured in custom lengths much more easily. With a forging, building a custom-length rod requires building a new set of dies, which is cost-prohibitive. Billet steel rods are machined from a purer, more highly refined alloy than most forgings. Additionally, they feature a longitudinal grain flow with excellent molecular bonding properties for enhanced strength. Although billet steel rods aren’t as strong around the big end as forged steel rods, due to the absence of a circular grain flow, they’re much more resistant to the formation of surface cracks. Overall, billet steel rods are stronger than forgings, but also cost twice as much.

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Carrillo is known for making some of the most durable H-beam rods on the market, but the company also offers A-beam rods. They look similar to I-beam rods, but with more beam material around the big end. The company recommends using them in moderate-load applications.

Many production engines, including early LS small-blocks, use wrist pins that are press-fit onto the small end of the rods. The problem with this arrangement is that if the pin ever seizes up in the piston, it will impede the rod’s ability to pivot and snap it in half. Most aftermarket rods have a bushed small-end bore that can accommodate a full-floating pin. The bushing acts as a bearing, allowing the rod to float around the wrist pin on a thin film of oil. Floating pins must be secured inside the pistons with lock rings.

Like all fasteners, rod bolts distort as they are tightened. Unlike most fasteners, if a single rod bolt fails, it can destroy an entire engine. Consequently, simply torquing down a rod bolt to spec won’t suffice, because the friction of the bolt rubbing against the rod cap will make torque measurements inaccurate. A far more precise method of making sure the rod bolt is properly torqued is to measure bolt stretch. Bolt manufacturers, such as ARP, publish target stretch figures with all of their rod bolts. Attaching a stretch gauge to the bolt and slowly tightening the bolt until the target stretch is achieved ensures optimal bolt performance.

Rod Shape

The shape of a connecting rod is just as important as the material it’s made from. Like most factory rods, Gen III/IV small-block units utilize an I-beam design; aftermarket rods are shaped into both I-beams and H-beams. Making a blanket statement as to which design is stronger wouldn’t be accurate, as each offers distinct advantages. Most rod manufacturers agree that I-beams are lighter and stronger in compression, because they distribute stress more evenly throughout the rod. Conversely, H-beam rods are heavier and can withstand greater tensile loads, due to their bulkier design. In theory, these attributes would indicate that I-beams are better suited for forced-induction and nitrous applications, and that H-beams are more durable in high-RPM race motors. In practice, however, this isn’t the case. Most companies that sell connecting rods manufactured overseas market their I-beams as entry-level offerings and their H-beams as premium offerings rated at higher horsepower limits. As a result, many hot rodders have concluded that H-beam rods are a stronger overall design. However, this isn’t always the case, as manufacturers, such as Manley and Lunati, have recently introduced heavy-duty I-beam rods that are rated at a higher horsepower level than their own H-beam rods.

For instance, Manley rates its standard weight 4340 steel H-beam rods at 800 hp and 8,000 rpm and its standard-weight Pro Series I-beam rods at 850 hp and 8,500 rpm. Furthermore, companies, such as Oliver, have been manufacturing some of the highest-quality and most durable connecting rods for decades. They’re used in the most demanding racing applications, including everything from circle track cars to monster trucks to sprint cars. Interestingly, Oliver utilizes an I-beam design exclusively in all of its rods. Consequently, both I-beam and H-beam rods can be designed to handle serious abuse, and generalizing as to which is stronger is futile.

When measuring rod bearing clearance, the caps must first be torqued down in a vise. Next, the dial bore indicator must be positioned on the vertical surfaces of the big end housing. The reason for this is because the bearings have a slight horizontal taper.

Although maximizing rod length is a topic of much debate, many of the top engine builders in the country assert that it isn’t something to lose sleep over. Extensive dyno testing by NHRA Pro Stock teams has proven that a higher rod-to-stroke ratio makes no difference in power output. Any ratio of 1.55:1 and 1.85:1 works well in most street engines.

Although “I-beam” and “H-beam” refer to the general design of a rod, that doesn’t mean that all H-beams and I-beams are shaped the same. Some offer more clearance than others. Due to the tight clearance between the rods and camshaft in stroker motors, most aftermarket connecting rods have profiled shoulders to buy some extra space. Likewise, the rod bolts can also compromise clearance. As a piston approaches TDC, the top of the rod bolt nears the cam, and as a piston approaches BDC, the bottom of the rod bolt nears the crankcase and oil pan. To combat this problem, aftermarket rods are often fitted with low-profile bolts. Some companies take it one step further by using cap screws in lieu of bolts, which thread directly into the big end of the rod instead of relying on a separate nut.

Regardless of the particular style of fastener that is used, aftermarket rods come equipped with heavy-duty bolts or cap screws. ARP offers 8740 chromoly fasteners rated at 220,000 psi of tensile strength, and some rod manufacturers offer their own proprietary fasteners rated at up to 280,000 psi. Considering that they’re the most highly stressed fastener in an entire engine, quality rod bolts are cheap insurance.

Rod Length

In stroker builds utilizing a standard deck block, 6.100-, 6.125-, 6.200-, and 6.250-inch rods are most common. Longer 6.460- and 6.560-inch rods are used more frequently in tall-deck motors with longer strokes. Like those in the Gen I small-block, LS crankshafts incorporate 2.100-inch rod journals. This means that Gen I and Gen III/IV connecting rods are interchangeable, because they share the same big-end housing diameter. One important difference is that Gen I rods have a smaller .927-inch small-end bore than the .945- inch bore on rods used in stock LS motors. Consequently, using Gen I rods in a Gen III/IV motor requires matching them with pistons that have a smaller .927-inch small-end diameter. Although most aftermarket LS rods are built with the smaller Gen I small-end, some are offered in a .945-inch housing bore to make them compatible with pistons that use stock-size LS wrist pins.

After going through the effort of setting the rod bearing clearance, it’s easy to overlook checking the rod side clearance. The distance between the sides of the rods should be checked with a feeler gauge and measure .010 to .012 inch. To a certain degree, this controls how much oil drains back into the crankcase.

One of the most controversial topics in engine building is the supposed benefits of maximizing connecting rod length. Most people tend to overgeneralize this issue, and legions of hot rodders firmly believe that using a longer connecting rod yields dividends in horsepower over a shorter rod. An entire chapter can be written on this topic alone, but suffice it to say that the benefits of using a longer connecting rod over a shorter rod are two-fold.

Proponents contend that a longer connecting rod increases high-RPM horsepower, because it forces the piston to move more slowly away from TDC during the power stroke. In theory, this allows for a greater buildup of cylinder pressure and increases power. Longer rods also reduce the angle of the connecting rods in relation to the pistons. This relieves side loading and friction from the piston skirts, which is said to improve both durability and horsepower. Unfortunately, very few engine builders and hot rodders have gone through the painstaking efforts necessary to test this theory. Interestingly, those who have—including GM engineers and the Pro Stock engine builders at Reher-Morrison—aren’t convinced that longer connecting rods provide any performance benefits at all.

A more accurate way to compare connecting rod length between different engine combinations is by looking at their rod-to-stroke ratios. For example, most LS small-blocks that have a 3.622- inch stroke use a 6.098-inch connecting rod, and this equates to a rod-to-stroke ratio of 1.68:1. From a mathematical standpoint, a couple thousandths of an inch of rod length doesn’t impact the ratio much at all. In an exhaustive series of dyno tests that Reher-Morrison performed on NASCAR engines for GM, the shop varied the rod-to-stroke ratio from 1.48 to 1.85:1. In the test, mean piston speeds were in the 4,500- to 4,800-fps range, and painstaking measures were taken to minimize variables. The result was zero difference in average power and no difference in the shape of the horsepower curves. According to Reher-Morrison, one could have laid the curves over each other without being able to distinguish the difference between the different rod-to-stroke ratios on paper.

Although the effects of varying rod-to-stroke ratios might not always be evident on the dyno, there are some pitfalls of going above and below a certain point. At anything below a 1.55:1 ratio, rod angularity is extreme enough that it increases the side loading of the piston by a noticeable margin, increases piston rock, and increases skirt load. Although this doesn’t necessarily change the actual power an engine makes, it does accelerate wear. Conversely, above a 1.80:1- or 1.85:1 ratio, there’s so little piston movement at TDC that it hurts the ability of the pistons to draw in air on the intake stroke. Compensating for this requires advancing the cam, or decreasing the cross-sectional area of the cylinder head ports or intake manifold to increase air velocity. When it comes to rod length, the biggest mistake a hot rodder can make is compromising an entire engine combination by trying to achieve a target rod-to-stroke ratio. Consequently, according to Reher-Morrison, from a performance standpoint, connecting rods are nothing more than pieces of metal that connect the pistons to the crankshaft. It’s as simple as that.

Manufacturer Choices

Trying to distinguish one manufacturer’s connecting rods from another based on looks alone can be difficult. To help you sift through the dozens of aftermarket rods available, here are some of the most well-known LS rods on the market for both street and strip applications. With such a broad selection of quality aftermarket rods now available, reconditioning stock rods is a thing of the past.

Callies

As with its premium crankshafts, Callies’ Ultra connecting rods are 100 percent American made. Forged from a proprietary 4340 Timken steel alloy, the units are premium rods with a premium price of roughly $1,300 per set. That investment buys hardware replete with innovative design features capable of withstanding more than 1,000 hp. The Ultra rods have gussets around the cap screws to help fortify the big end of the rod. Profiled shoulders improve clearance in long-stroke engines, and the cap screws are rated at 260,000 psi. Additionally, the Ultra rods’ I-beam design reduces mass over a comparable H-beam. Covering a wide range of stroke lengths and block deck heights, they’re available in 6.100- to 6.560-inch lengths.

Compstar

Although Compstar was conceived as Callies’ Sportsman line in 2004, the company puts tremendous effort into ensuring top-notch quality in its entry-level products. Forged overseas to keep costs down, Compstar rods start at $400, making them an excellent value. All Compstar rods are forged from 4340 steel, and they are offered in I-beam, H-beam, and H/I-beam configurations. Common features throughout the Compstar line include premium ARP 2000 rod bolts, chamfered and honed pin bushings, and stress-relieved surfaces. According to Compstar, its rods clear 4.000-inch-stroke cranks and cams with up to .660-inch lift without any modifications.

Compstar’s most affordable rods are its I-beams, which can support 650 hp and cost less than $400 per set. They feature a fortified shoulder, a large parting line footprint, added material around the pin collar, a twin-rib cap, and ARP L19 cap screws. For engines that require a stronger rod, Compstar’s H-beam rods come in lengths ranging from 6.100 to 6.560 inches. Officially, they have no published horsepower rating, but engine builders routinely push them past 850 hp without a problem. For forced-induction and nitrous motors producing more than 1,000 hp, Compstar offers a unique H/I-beam hybrid rod design in a 6.125-inch configuration. The added strength comes from a 25-percent-thicker beam and triangulated big-end and pin bores.

Eagle

As with its crankshafts, Eagle offers a diverse lineup of connecting rods to suit virtually all power levels and engine combinations. The entry-level SIR I-beam rods are forged from 5140 steel, and they can handle up to 700 hp. They come in 6.125-, 6.200-, and 6.250-inch sizes. At $300 for a set, they pack some serious value. For not much more money, Eagle’s H-beam rods can handle much more horsepower. Equipped with standard rod bolts, Eagle H-beams are rated at 750 hp. Upgrading to ARP 2000 bolts increases that figure to 1,100, and stepping up to a set of ARP L19 bolts increases the horsepower rating to 1,400. Eagle’s premium H-beams cost just $550, which explains why overseas rods have become so popular in recent years. As far as strength per dollar is concerned, Eagle rods are hard to beat.

Lunati

Lunati’s connecting rods for LS-series small-blocks come in three different trim levels. Its entry-level H-beams are forged overseas from 4340 steel, and they are rated at 700 to 800 hp. The rods are finished in-house, and then they’re heattreated, shot-peened, and stress-relieved. They’re offered in just one size, 6.125 inches, and sell for $630. For just $30 more, Lunati’s Superlight H-beam rods have many of the same features as its standard-weight H-beams, but they are 75 grams lighter (680 vs. 605). The reduced reciprocating weight they offer makes them well suited for circle track and land-speed engines in which prolonged high-RPM operation is the norm. At the top of the Lunati totem pole are its Pro Series connecting rods. These Ibeams are made in America from aerospace-grade 4340 steel. At $1,300, they’re a bit on the pricey side, but the benefit is that they can handle well in excess of 1,000 hp. In fact, Lunati says that they’ll handle just about anything you can throw at them. Available in 6.125- and 6.300-inch lengths, the Pro Series rods are an excellent choice for extreme power adder applications.

Manley

Throwing a monkey wrench into the I-beam-versus-H-beam debate, Manley positions its I-beams at the very bottom and top of its connecting rod lineup. Manley’s Sportsmaster I-beam rods are forged overseas from 4340 steel and measure 6.100 inches in length. The rods are shot-peened, stress-relieved, and heat treated. Additionally, the Sportsmaster rods are profiled around the main cap area to remove stress risers and reduce mass to just under 600 grams. They’re rated at 550 hp and cost $650 per set.

Manley’s mid-level 4340 H-beam rods are also forged overseas. They measure 6.125 inches and are available with ARP 8740 or ARP 2000 bolts, for power ratings of 725 and 775, respectively. Interestingly, they’re priced cheaper than the Sportsmasters at $600.

At the top of the heap are Manley’s Pro Series 4340 I-beam rods, which are manufactured in the United States. Priced at $1,500 a set, these 6.125-inch rods can handle more than 1,000 hp. In addition to brute strength, the Pro Series rods are very light at just 609 grams.

Scat

Like many aftermarket companies, Scat forges its rods overseas, and then it performs final machine work in the United States. To ensure quality control and exacting tolerances, Scat rods are machined with modern diamond tooling, and they are heat-treated using temperature-controlled cooling.

Scat’s entry-level Pro Comp I-beam rods feature 4340 steel construction and are offered in 6.100- and 6.125-inch lengths. They’re lightweight at roughly 600 grams, and they are available with ARP rod bolts or cap screws. Rated at 550 to 650 hp, they’re best suited for mild naturally aspirated engine combinations, and they are budget priced at $325 per set.

Scat’s premium Gen III/IV rod offering is its Pro Sport 4340 H-beams. Available in 6.100- and 6.125-inch lengths, Scat’s H-beam rods are rated at 800-plus hp, making them ideal for forced-induction and nitrous motors. ARP 8740 cap screws come standard, and ARP 2000 fasteners are optional. With prices starting at $450, the Scat H-beams offer excellent durability for the money.

Written by Stephan Kim and Posted with Permission of CarTech Books

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