School of Automotive Machinists founder, Judson Massingill, lives by a simple adage: Get a head, flow a head, stay ahead. That’s because no single component on an engine affects power output more than the cylinder heads, so it pays big time to get educated.
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 flowbench 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 lowand 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 valvespring 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 flowbench 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 valvespring 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 flowbench 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 fourbolt 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 CNCported 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 bigblock 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 valvespring 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 Trickflow—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 Mongose 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-tocylinder- 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 biginch stroker combos in excess of 430 ci. Other highlights include reinforced rocker stud bosses, large 1.625-inch-diameter valvespring 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 valvespring 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 CNCported 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 Barry Kluczyk and Posted with Permission of CarTechBooks
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