Over the years, the SB Chevy V8 evolved and grew in displacement from 265 to 283 to 302 to 327 to 350 to finally 400 cubic inches. Aftermarket blocks and stroker cranks have allowed even more displacement, with some “small blocks” now having as many cubic inches as a big block Chevy (427 and 454 cubic inches). As for Ford, the 289, 302 and 351 Windsor V8s have also enjoyed a long production run, with later renditions of these engines being relabeled the 5.0L and 5.8L from 1982 to 1995.
As these engines evolved, so did their cylinder heads. As displacements and power outputs grew, so did the size of the intake runners and valves. The best production performance head for the early 265/283 cid Chevy V8s was the Power Pack head with 1.72? intake valves and 1.50? exhaust valves (which are very puny by today’s standards).
A far better cylinder head was the “double-hump” production head for mid-1960s 327 engines that came with 2.02? intake valves, 1.60? exhaust valves, 160 cc intake ports and 64 cc combustion chambers. The best casting numbers for the double-hump head were 461 and 462, which had better flowing ports than the later 441 and 882 versions of this head.
Then Chevy came out with the famous Bow Tie performance heads for the small block, which were even better than the original double-hump heads. These were the hot set up until the mid 1980s, when aftermarket cylinder heads began to appear that out performed the original Chevy castings.
In 1986, Chevy upped the ante with their first aluminum cylinder heads on the L-98 Corvette engine. The aluminum heads were 40 lbs. lighter than cast iron heads, and offered a modest improvement in power. Better yet, the aluminum heads could be bolted onto older engine blocks. Even so, the new factory aluminum heads could not achieve the same performance level as many aftermarket heads that were available at the time.
The next major development was the introduction of the L31 Vortec cylinder heads in 1996. Though originally designed for truck engines, Vortec heads had 170 cc intake runners, and somewhat smaller 1.94? intake valves and 1.50? exhaust valves. But the flow characteristics of this cast iron head proved to be even better than anything before it thanks to the high velocity ports. Thus, Vortec heads became a good upgrade for 350 small block V8s making up to 400 horsepower. The Vortec heads differed from earlier designs by using center-bolt valve covers, and a different intake manifold configuration (8-bolt manifold instead of the previous 12-bolt manifold).
Later LT1 and LT4 engines (GEN II) featured a reverse-flow cooling system, so the cylinder heads on these engines are not interchangeable with those on the earlier GEN I blocks. And the LS1 and later heads that came out in 1997 are an entirely different design, though one aftermarket supplier of engine blocks (World Products) now offers a slightly modified SB Chevy engine block that allows late model LS Chevy heads to be bolted onto the block. The combination is said to be good for 30 horsepower.
As time goes on, it’s getting harder and harder to find stock performance heads for small block Chevy and Ford engines that are still in good usable condition. Many of these heads have been rebuilt more than once, and many have succumbed to cracks or wear that cannot be easily repaired.
That’s one of the reasons why aftermarket performance heads have prospered. They are brand new castings that are not in short supply. You can buy them anywhere and often at a cost that is less than what it would cost to completely rebuild an old cylinder head.
The other reason why aftermarket cylinder heads have proliferated so in recent years is that most of these heads offer much better out-of-the-box performance than even the best factory castings. Aftermarket heads are purpose-built heads designed for specific kinds of performance applications. That means port volumes, port locations, valve sizes, valve angles and combustion chambers can all be optimized for maximum performance.
Computer Numeric Controlled (CNC) machining is also readily available as an option with many aftermarket performance heads today. CNC heads are often touted as being better than as-cast heads. But that’s not always true. CNC machining is just another way to finish a casting. If the profile of an as-cast head has already been optimized to deliver the best flow possible, CNC machining it won’t make it any better. Likewise, if the CNC mapped profile that is machined into a head does not flow as many cubic feet per minute (CFM) of air as a well designed cast port, there’s nothing to be gained.
Where CNC machining does provide an advantage is when a stock casting needs to be opened up or reshaped to improve its flow characteristics. CNC machining is much faster, easier and less expensive than hand porting a cylinder head, and can easily replicate with a high degree of accuracy almost any port configuration that has been developed, tested and refined on a flow bench. But like any machining operation, the tooling tolerances must be tightly controlled to achieve consistent results.
Choosing The Right Cylinder Head
With so many different aftermarket performance cylinder heads available, how do you choose the best head for an engine you are building? Do you go for the head that delivers the highest flow numbers? That’s what some people do, and they often end up being disappointed because bigger is not necessarily better depending on the application.
Do you have to use heads with the same valve angle as the original, or heads that will bolt up to a stock intake manifold or a popular aftermarket intake manifold? That may limit your selection to heads with a 23 degree valve angle for a SB Chevy and heads without raised intake or exhaust ports (which improve flow but may also require special manifolds).
Is cost an issue? If you’re building a budget performance motor for a customer, $1,000 or less may be all he can afford for a pair of aftermarket heads. On the other hand, if your customer has deep pockets and can afford the best, it opens up more possibilities such as CNC-ported heads or even custom heads.
There are a LOT of variables to consider when choosing a cylinder head for a particular engine application. The “best” head will be the one that delivers the most torque, horsepower and throttle response within the target rpm range of the engine.
Street engines spend most of their time at low rpm, so a good street performance engine should be built to deliver maximum torque from 1,500 to 5,000 rpm. The engine should have good throttle response and high intake vacuum for everyday driveability.
A circle track engine, by comparison, also needs good throttle response but a higher torque curve. It depends on the weight of the race car, the gearing, tire size and length of the track.
With drag racing, it’s all about maximum power and acceleration. Bigger is usually better here, with large displacement engines sucking huge volumes of air through high flowing heads.
Aftermarket cylinder heads are available with different intake runner volumes, different port configurations and port heights, and different valve angles. The companies that make such heads typically label their heads according to their intake port volumes or their flow characteristics.
Comparing Intake Runners
Theoretically, a larger intake runner should flow more CFM of air than a smaller volume runner, allowing the head to make more horsepower at higher rpms. But this isn’t always true because the angle and curvature of the port as well as its profile affect the velocity of the air flowing through it. Thus, a well-designed port with a smaller volume may actually flow more CFM of air than a physically larger port.
Airflow is measured at various valve openings on a flow bench. These numbers are often used in advertising to show how much air a particular head is capable of flowing (the implication being that the bigger the CFM number, the better the head and the more power it will make).
There are a couple things to keep in mind here with respect to airflow numbers. The airflow numbers change as the valve opens wider and wider. Peak airflow is typically achieved when the valve is opened as far as it will go. On a racing engine with a high lift camshaft or rockers, the valve opening might be 0.600? to 0.800? or more. But on a street engine, maximum lift might only be 0.500? or so. Consequently, you have to look at the airflow numbers that match the maximum lift of the camshaft and rocker arms you will be using.
The other factor to consider with respect to airflow numbers is how well the head performs at all valve openings. If the flow numbers drop off drastically at less than peak valve lift, the head may not produce as much power as one that delivers good flow numbers at lower valve openings. In other words, the total amount of power produced is not just peak airflow at maximum valve lift, but how much air the head flows at every point as the valve opens and closes.
Let’s say we have two cylinder heads. Head A flows 290 CFM of air at peak valve lift while Head B only flows 275 CFM. But at partial valve lift, Head B has better flow numbers. Which is the better head? The answer may be Head B because the total overall flow is higher even though the peak flow number may not be as good.
Port velocity plays a huge role in determining how much air actually flows into the combustion chamber, which determines how much power the engine makes. There is a relationship between overall port volume, the cross-sectional area of the port, the physical shape of the port and the valve angle that affects the velocity of the air moving through the port.
When the cylinder moves down on the intake stroke and the intake valve opens, atmospheric pressure shoves the air column in the intake manifold and port forward to fill the void in the cylinder. As the air flows into the head and through the port, it will actually speed up as the port constricts and curves toward the valve opening. A well-designed port will take maximum advantage of this venturi effect and allow more air to flow into the cylinder than a poorly designed port that has too much turbulence, or changes shape too abruptly or curves too sharply.
The shape of the intake port just above the valve bowl area makes a big difference in air flow, velocity and volumetric efficiency. At higher velocities, air has a harder time following the short side radius of the intake port. If air pulls away from the surface of the port as it follows the short radius curve into the bowl area, it breaks up the smooth laminar flow and creates turbulence that disrupts airflow. Consequently, less air enters the combustion chamber, volumetric efficiency decreases and the engine doesn’t make a much power as it could.
A well designed intake port will usually have a raised area in the floor of the port just ahead of the short side radius to make it easier for the air to turn the corner. Raising the height of the intake port in the head has the same effect by creating a slightly straighter path to the valve.
If an intake runner is too large, the air column will actually lag a bit when the valve opens, reducing the velocity of the air as it flows through the port. Thus, the larger port may not flow well unless the engine is running at much higher rpms.
The best performing cylinder heads are usually the ones that deliver the most CFM with the smallest intake port displacements. So if you are comparing two different brands of aftermarket heads with identical CFM numbers (assuming the numbers have not be exaggerated for marketing purposes!), but one head has slightly smaller intake runners than the other, go with the smaller head.
Matching port size to engine displacement and rpm, therefore, is absolutely essential for good throttle response and making the most power. For a street performance engine, you want a cylinder head with smaller ports that flow well in the low to mid-rpm range. For a typical 350 small block or a 383 stroker street performance engine, heads with 170 cc to 190 cc runners are probably the best choice. For a larger stroker motor (400 to 450 cubic inches), heads with 200 to 220 cc intake runners should work well.
On the other hand, if you’re building a 400 or larger cubic inch SB Chevy drag motor with a big carburetor or nitrous oxide that will be turning lots of rpms and breathing a lot of air, bigger is better. You’ll probably want heads with big runners (up to 230 cc or larger) and big flow numbers (over 300 CFM).
Something else to consider is the ratio between the size of the intake and exhaust ports. In SB Chevy heads, the exhaust port is often the bottleneck that causes the most restriction. The cross-sectional area of the exhaust port as well as its shape must also be properly sized and optimized to maintain peak exhaust flow. An exhaust port that is too small will cause an obvious restriction, but an exhaust port that is too large can also reduce breathing efficiency by reducing the scavenging effect of the exhaust as it exits
Exhaust flow can be improved by raising the height of the exhaust port (reduces curvature in the port) and by using a D-shaped or flat-floor design.
The angle of the valve with respect to the combustion chamber and deck surface is yet another variable that affects breathing performance and power. The valve angle on stock SB Chevy heads is 23 degrees. But on the newer LS heads and many racing heads, a much shallower valve angle is used along with smaller, more open combustion chambers.
The shallower 15 degree valve angle on LS1 heads allows a taller port while straightening out the entry angle the air must follow as it flows past the valve. The result is more airflow and more power at higher rpms. On the LS2 heads, the valve angles has been decreased to 12 degrees. Many aftermarket heads for SB Chevy applications are available with 18 to 15 degree valve angles, and one (Dart) is now being offered with a 9 degree valve angle.
The volume of the combustion chamber is another important variable because it affects not only the compression ratio of the engine but also breathing. Combustion chambers of 64 cc are fairly common, but available sizes can range anywhere from 50 cc up to 74 cc.
A shallow, more open combustion chamber will have less shrouding around the valves to restrict airflow. So a smaller, shallowed combustion chamber will usually perform better.
A smaller combustion chamber will also increase the compression ratio without having to used domed pistons, which can disrupt the flame front in the combustion chamber. The limiting factor here is the maximum compression ratio a street engine can safely handle on today’s pump gas. For most applications, that would be about 10.5 to 1. For a racing engine burning high octane gasoline or running on alcohol, you can run much higher compression ratios to improve thermal efficiency and power.
In addition to all of the variables already covered, another variable to consider when choosing a set of cylinder heads is their cost. For some people, that’s the most important variable – either because their budget limits how much they can spend on a set of heads, or because they think the more they spend, the more they are going to get.
A good set of aftermarket performance cylinder heads that are properly matched to the engine displacement, camshaft, valvetrain components, intake manifold and carburetion can make a huge difference in how well an engine performs and how much power it produces. Choose the right heads and you’ll end up with a winner. Choose the wrong heads, or mismatch the heads to the engine, cam and carburetion, and you’ll end up with a dog.
Generally speaking, more expensive heads are usually better heads because they typically include CNC porting, higher grade valves and springs, and more development time on a flow bench. You can spend up to a couple thousand dollars for a pair of off-the-shelf performance heads, or tens of thousands of dollars on custom ported heads. The sky is the limit.
Cast iron aftermarket performance heads are generally less expensive than aluminum heads, and are a good choice for many street performance engines as well as lower classes of circle track and drag racing applications. For serious racing, though, aluminum heads are a must because of the weight savings and the repairability of the heads themselves. Damaged or cracked aluminum heads can often be TIG welded and remachined back to like-new condition, saving the cost of having to replace the head.