When it comes to building a powerful engine, there is a hierarchy to the parts choice from most to least critical. The right set of well-designed cylinder heads with high airflow and good chamber design are far and away the parts that set the limit of the engine’s power potential, but no matter how good the cylinder heads could be, there is one part that determines how well they work on any given engine combination: the camshaft.
That’s because not a single thing in the engine matters at all if the cam isn’t correct for the parts choice, or even more importantly, what the engine is intended to do. Cam selection will make or break an engine more than any other single part because it’s the conductor of the orchestra that is the valvetrain. Awesome cylinder head airflow numbers mean nothing if the valves aren’t opening at the right time, high enough, long enough, or in the correct relation to one another.
While it may take years of schooling and an engineering degree to fully comprehend all the intricacies, it’s fully possible to have a solid grasp of camshaft fundamentals that will put you in the top 10 percent of hot rodders and make your next cam selection more than just a guesswork finger scroll through part numbers. It all begins with fully understanding the basics, and most importantly, how those basics apply to you and your car on a practical level.
"…not a single thing in the engine matters at all if the cam isn’t correct for the parts choice…"
Before You Start
There’s a significant difference between building a race engine and building an engine tha
The very first and most critical thing you have to consider when building any engine or selecting a cam is the thing most often glossed over: What exactly are you going to do with this engine? That sounds like a very basic question that you’d assume most guys have established before contacting a builder or ordering parts, but in his experience, Mike Consolo of QMP Racing Engines says the vast majority haven’t been really honest with themselves. The result is almost always a customer who’s not happy with the result.
Being honest with yourself begins with forgetting about the specific horsepower numbers you want, and focusing only on what exactly you’ll be doing with the car during the majority of the time. “The very worst thing you can do is over-cam an engine,” Consolo says. “Lift, duration, and lobe separation angle need to be matched to the true end purpose; getting any one of those wrong can over-cam the engine, which is worse than under-camming.” That’s from both a power and enjoyment perspective. Lots of guys say they want 600-plus horsepower and 10-second timeslips, but unless you truly plan to race a lot, this isn’t the right way to approach the engine build. Don’t build an engine oriented toward high rpm if you aren’t going to be spinning it hard regularly. If you’ll actually be spending the majority of your time behind the wheel street driving and cruising at low rpm and with lots of idling time, but only occasionally autocross or run-what-ya-brung drag racing a handful of times a year, keep that solidly in your mind. Starting from any of those perspectives doesn’t mean you can’t build a powerful engine, but it does dictate the type of cam you should choose. As Crane Cams’ Allen Bechtloff once told us, “The right horsepower and torque at the right place is more important than the peak.” The “right place” correlates with the engine’s intended use, and the vehicle the engine will go in.
Although a cam can always be installed straight-up per the manufacturer’s specs, sometimes
The other half of the honesty equation is realistically addressing the car you’re going to drop the engine into. Just like an engine is a system that needs to work in harmony, so is the car. You’d be surprised how sluggish a high-powered but peaky or high-revving engine can feel in a car with a poorly matched trans, converter, and rearend ratio. Conversely, you’d be surprised how much quicker any power level feels when properly paired. For example, there’s no point in having a large, lopey cam if you want to run an automatic with a stock stall converter. Similarly, sticking with a puny freeway-friendly rear gear ratio that won’t allow a large cam to ramp quickly into its intended rpm range will only result in a soggy feeling off idle. Other important points to establish include whether you need enough vacuum for power brakes or other vacuum-operated accessories, what type of exhaust will you be running, how much the car weighs, what altitude you live at, whether the engine is EFI or carb, does it have air conditioning, and what kind of driveability and idle quality you want.
So the very first thing to do before talking to an engine builder is to create a list answering all of those questions. Then at the top write your realistic expectations of what you’ll be doing with the car 90 percent of the time, and how you would like for it to behave. Underneath that create a full list of the drivetrain, suspension, chassis, wheels, and tires you plan to run. This will help paint an accurate picture for the engine builder.
The Cam Comes Last
To get the most out of a cam, choose it last. A common mistake is to assume that the camshaft choice should come first, and then the engine should be built around that. It’s actually the opposite, Consolo says. “I prefer to build a cam around an engine, not the other way around.” That’s because an internal combustion engine is just an explosion-driven air pump and to determine how and when the valves need to open is determined by the engine’s airflow potential.
Overall cylinder head flow is important, but there are two other points to consider: the flow in the rpm range that you’ll be using your engine in, and the intake-to-exhaust flow percentage. The first one easily explained; it doesn’t matter what the heads flow at .800-inch lift if you’re building a street engine that will stay in the sub .600-inch lift range. On the other side, if cylinder head flow stalls at .600-inch lift, it makes no sense to run a cam that makes .800-inch lift. But it does matter what the numbers look like at the max intended lift and below. The intake-to-exhaust flow percentage is the amount of air the exhaust port can flow versus the intake port. Typically, a head with a high percentage can use a cam with more closely matched intake and exhaust lobe figures. Conversely, a head with a poor ratio between the intake and exhaust flow will benefit from more exhaust duration to evacuate the burned gases.
"Being honest with yourself begins with forgetting about specific horsepower numbers you want, and focusing only on what exactly you’ll be doing with the car..."