How to Choose Performance Pistons for Gen IV LS Engines

The OEM pistons for LS  engines are all cast hypereutectic construction except for the LS9 with its factory-forged pistons. Don’t be alarmed about the “cast” term. Unlike earlier cast pistons, hypereutectic pistons feature a dense casting process with high silicon content. They’re stronger than old-school cast pistons, and they’re more stable in terms of thermal expansion and contraction. The dimensional stability allows the factory to run tighter bore clearance. Hyper pistons are fine for up to about 500 to 550 hp generally. Beyond that, or if combustion pressures will rise due to the use of forced induction (supercharging or turbocharging) and/or nitrous oxide injection, you’ll need to upgrade to forged pistons.

 

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.

 

 

 

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.

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.

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.

 

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.

 

 

Static Compression	Boost	Effective Compression
8.0:1	8 psi	12.4:1
9.0:1	4 psi	12.1:1
9.5:1	4 psi	12.1:1
10.0:1	2 psi	11.9:1
11.0:1	2 psi	12.5:1

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.

 

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.

 

 

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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This Tech Tip is From the Full Book, LS GEN IV ENGINES 2005 – PRESENT: HOW TO BUILD MAX PERFORMANCE. For a comprehensive guide on this entire subject you can visit this link:

LEARN MORE ABOUT THIS BOOK HERE

SHARE THIS ARTICLE: Please feel free to share this post on Facebook Groups or Forums/Blogs you read. You can use the social sharing buttons to the left, or copy and paste the website link: https://www.lsenginediy.com/how-to-choose-performance-pistons-for-gen-iv-ls-engines

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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 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.

 

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.

 

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.

 

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 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.

 

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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