Horsepower is one of the most addictive substances on earth — everyone who’s into high-performance vehicles can’t get enough of it. And squeezing the maximum amount of power out of an engine requires careful planning to ensure all of the parts work in unison. Cylinder heads are where you make the power, but selecting the right set can be tricky. Today, we’re going to layout 10 tips to help you get the right heads for your engine with Air Flow Research (AFR).
Consumers can get into a lot of trouble when shopping for cylinder heads if they don’t know what they’re looking for or understand the needs of the engine they’re building. The idea of “bigger is better” really doesn’t jive with buying cylinder heads; in fact, going too big can impact performance in a negative manner. Hopefully, these tips from AFR will help horsepower junkies everywhere make the right choice when it comes time to purchase cylinder heads.
#1: Know Your Application
This tip might seem like a no-brainer, but you can easily lose sight of what you’re building an engine for when you start looking at cylinder head options. Every build is going to be different and has unique requirements for the engine, so you want to tailor your cylinder heads to match the use of the vehicle. Cylinder heads have a variety of chamber sizes, runner sizes, valve sizes, and other variables you need to make sure will be optimized for the application.
Tim Torrecarion from AFR provides some insight into purchasing heads for your application.
“If this is for a street car, the majority of the RPM will be 1,500-5,000. In this scenario, we are going to focus on smaller runners and heads with great low-lift flow numbers. Smaller runners equal higher velocity, so you’ll see better power production earlier in the powerband, perfect for a street car. For racing applications, you need to look at your launch RPM and the RPM you’re at when you go through the lights.”
#2: Have A Budget And Understand What It Gets You
A set budget is actually not a bad thing to have when you’re shopping for cylinder heads. You can easily get lost in the weeds looking at all of the different cylinder heads and options they offer for your engine, so knowing how much you can spend will help narrow that down.
“You really need to think about how much you want to spend before you begin shopping. If your application is not going to take full advantage of a set of CNC-ported heads, you could save a bit of cash and spend that money elsewhere in your build. For example, we offer an Enforcer As-Cast head in addition to our Eliminator line of 100-percent fully CNC-ported heads. For the gearhead looking to increase the power of their current engine over the factory, the Enforcer line is an excellent choice for their weekend cruiser. You’ll still make plenty of power and it’ll save you some money; improving your horsepower per dollar spent,” Torrecarion says.
#3: Know Your Cubic Inches
If you’re debating on how big your engine will be you’ll want to have a final number before you buy cylinder heads. A set of heads can only move so much air; the engine requires air to make power, and if you don’t select heads that match your engine size there will be problems. If you get heads that are entirely too big, the velocity of the air charge is going to be slow, resulting in an engine that feels lazy and unresponsive.
It comes down to math — you need a certain amount of air to make a certain amount of horsepower.- Tim Torrecarion, AFR
You can also select cylinder heads that are too small for your engine — Torrecarion explains why that’s an issue.
“In order to perform at an optimal level, an engine requires a certain amount of air to achieve a given power level. Imagine you are exercising or running a mile, but while doing this you’re wearing a face mask or worse yet, a swimming snorkel. This restriction limits the amount of air that your body can take in, reducing your ability to perform because you are constantly out of breath. The same is true for your engine: down low the motor will feel great and responsive, but as the RPM climbs, the power starts nosing off and it will feel like the engine is hitting a wall the higher the RPM. If the runners are excessively small, the engine may not even rev past a certain level because it has run out of airflow.”
#4: Have An Idea Of How Much RPM You Plan To Turn
The RPM level you spin the engine at has a direct effect on how much power it makes; this also needs to be considered when looking at cylinder heads. There’s a certain sweet spot each set of heads will have for RPM, based on how you’re going to use them.
“If you’re spinning an engine to a higher RPM level you can typically go with a larger runner. With a race application, you’ve probably got a high-lift, high-RPM camshaft. In a street application, you’ll mostly be between 1,500 to 5,000 rpm, with a lift around .500 inches, so there’s no point in buying a head that flows a lot of air at .700- or even .800-inch lift because the engine will never be used under these conditions,” Torrecarion says.
#5: Be Realistic About Horsepower Numbers
The big eye-catching numbers you’ll see when trying to select cylinder heads will be how much CFM they can flow and how much horsepower they can potentially make. While these numbers may seem impressive, they can be a bit deceiving since they’re based on general factors. The heads you’re looking at may not make the power advertised because your engine is different.
“It comes down to math — you need a certain amount of air to make a certain amount of horsepower. A general guideline is CFM times two; if the head is flowing 320 cfm it should support 640 horsepower to the crank. Now, there are a lot of other factors that contribute to this number, such as compression, camshaft, your tune, and so on,” Torrecarion states.
#6: Know What Your Compression Ratio Needs To Be
Your target compression ratio, plus piston volume, will help determine what size combustion chamber is required. For a pump gas engine, you’ll want to be below 11:1. If you plan on using a power adder, it will be even lower…probably nothing higher than 9.5:1. If you’re running E85 or alcohol, these ratios will be different.
“Compression ratio is what’s used to determine the chamber volume, along with piston volume. You’ll need to decide if you’re going to use pump gas, race gas, E85, or other fuel to know what compression ratio you’ll be targeting. You’ll need to decide if you’re going to use a power adder also, so you know how low to run the compression ratio,” says.
#7: Have Piston Information For Your Build
The type and design of piston your engine uses need to be taken into consideration when looking at cylinder heads. You don’t want to have a situation where valves are smashing into the tops of pistons. Torrecarion speaks on how you can avoid piston and cylinder head compatibility issues.
“If you’re doing any type of dome or flat top piston there are reliefs in the piston tops; these reliefs will dictate what size valves they can accommodate. For example, imagine you’re working with an existing short block, with two reliefs in the pistons, and those reliefs can only handle a 1.580-inch exhaust and 2.00-inch intake. In this case, there’s no point in considering a head that has a 2.020 intake or larger and a 1.600 exhaust valve. It won’t work unless you change or modify the pistons you’re using. We run into this situation a lot with customers when they’re looking to upgrade the factory engine in their car…the pistons just weren’t designed for aftermarket heads with larger valves.”
#8: Keep Your Camshaft Specs In Mind
Cylinder heads are the lungs of an engine, but are useless without a brain to control them. The camshaft plays the role of your engine’s brain. The camshaft is in charge of moving the air through the cylinder heads and you need to make sure both work together to maximize horsepower.
“The camshaft grind is going to tell us the RPM range we’re going to be working in. If you look at the cam card the manufacturer provides it will let you know what its operating range is. The lift is another important number to look at. Let’s say your max lift is .625 inches…that’s the lift range you want to focus on when selecting your heads. If you’re building an engine with a .625-inch lift cam and the heads have a sweet spot of .724 inches, it’s a waste because the parts are mismatched. Now, let’s consider a set of heads with smaller runners where the low lift numbers are better with a max lift of .625 inches — even though this set of heads has lower peak flow at .724 inches, it will actually produce a better power curve because it is better suited to the engine you’re putting together,” Torrecarion says.
You really need to think about how much you want to spend before you begin shopping. – Tim Torrecarion, AFR
#9: If You’re Going To Use A Power Adder, Plan For It
Power adders are fun and will really add horsepower to an engine, however, if you’re going to use one it will need to be addressed when you’re looking at cylinder heads. Depending on the type and size of the power adder you choose, the airflow demands of the engine will change. You also will need to look at the materials that you use for valve seats and other items.
“If you’re going to use a power adder, you’ll want a set of heads with a thick deck, a strong casting, made out of A356 aluminum or better. Depending on the application it will also influence your choice of valve springs for the power adder. You’ll also want to pay attention to the material of the valves…something that can handle the increased stress and heat, like Inconel. Be sure to pay attention to the seats, as well. Copper typically is the way to go for more extreme race builds. You need to get the parts that work the best for your specific application,” Torrecarion explains.
#10: Make Sure The Hardware Matches Your Application And Goals
Cylinder heads can be purchased as bare castings where you add all of your own hardware, or you can purchase them ready to bolt on to your engine. If you decide to go with the latter of the two options it becomes important to match the hardware to your application. The power adder tip illustrates how important matching parts to the application is when you’re looking at cylinder heads.
“Another area that needs addressing and is just as important, is your choice of valvetrain parts, i.e. springs, pushrods, rockers, and rocker studs. In a street application, you can get away with lighter springs since the cam won’t be as radical, but with a bigger cam, you’ll need to move to stronger springs. When you increase spring rates this puts more load on the other components. You’ll want to move to a 7/16-inch stud, thicker wall pushrods (.120-inches or better), and possibly even larger in OD like 5/8-inch. Also, consider your rockers — can they handle the spring pressure or even the size of the spring? All of these pieces need to be able to handle the forces of that bigger cam, as well. This is where the saying, ‘you get what you pay for,’ really means a lot. If you’re really going to beat on the engine, you want the best parts possible to reduce the possibility of failure,” Torrecarion says.
As you can see purchasing the right set of cylinder heads takes careful thought and planning and we have only just scratched the surface here. Things can get even more granular when you start looking at an all-out race build. This is when topics like runner length, minimum cross-sectional area, and intake-to-exhaust ratios come into play.
Hopefully, these ten tips will make it easier for you to pick out your next set of cylinder heads when you’re ready to make more horsepower. You can always reach out to the experts at AFR to make sure you’re on the right track before you make a purchase.