The most prevalent reason for engines that do not run as they should be an improper camshaft for the engine and vehicle combination. And that is understandable, because picking the right camshaft can be one of the hardest, most confusing parts of building an engine. If the plan is to restore your engine back to its original factory specifications your job is easy as those specifications can be found in repair manuals for your specific vintage car. The counter guy at the local speed shop/quick lube/food mart is not the best source of information in picking the right camshaft, so knowing the approximate cam specs for your combination is vital to an enjoyable machine, be it full race or full cruise.
The problem is that there are many things to consider when picking the right camshaft, and it involves much more than just the other engine parts. The entire vehicle and the sum of its parts are just as important. Below is an outline of what all those specs and numbers mean in the camshaft catalog and a general explanation of how they affect performance, followed by a rundown of the areas that must be addressed at camshaft selection time. This information is guaranteed to make your next cam choice almost painless, if not completely enjoyable.
Before You Start
The very first and most critical thing you must 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 an extremely basic question that you would assume most guys have established before contacting a builder or ordering parts.
Being honest with yourself begins with forgetting about the specific horsepower numbers you want and focusing only on what exactly you will be doing with the car during the majority of the time. The very worst thing you can do is over-cam an engine. That is 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 is not the right way to approach the engine build. Do not build an engine oriented toward high rpm if you are not going to be spinning it hard regularly. If you will 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-you-brung drag racing a handful of times a year, keep that solidly in your mind. Starting from any of those perspectives does not mean you cannot build a powerful engine, but it does dictate the type of cam you should choose. The right horsepower and torque at the right place are more important than the peak as this correlates with the engine’s intended use, and the vehicle the engine will go in.
The other half of the honesty equation is realistically addressing the car you are going to drop the engine into. Just like an engine is a system that needs to work in harmony, so is the car. You would 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 rear end ratio. Conversely, you would be surprised how much quicker any power level feels when properly paired. For example, there is 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 will not 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 drivability 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 those questions.
What Do You Want the Car to Do?
These are by far the most important considerations not just for cam choice but for building the entire car, and they should be considered before bolting any part onto the car or engine, be it a camshaft or tires. If the engine or car combination is not matched, no camshaft in the world will totally fix the problem. Once you have decided how you want the car to behave, you must build the entire engine and drivetrain to fit.
Everything about the car combination and intended function must be decided upon before choosing the cam. Once this is determined, the following key details must be considered to get exactly the right camshaft: the engine’s compression ratio, the basic power range of the heads/intake manifold/carb/headers combination, the car’s weight, the transmission type (and/or torque converter stall speed), the rear gear ratio and the rear tire size. If you know the actual flow numbers for the cylinder heads, you will be ahead of the game.
Because there is an almost infinite number of combinations of all these parameters, it would take forever to explain the intricacies of how these parts all work together with the camshaft, so we have produced six sample vehicles and called five of the major cam companies for recommendations on what cam they would choose for each vehicle. Compare the cam specs with how radical each car is, and you will get a promising idea of what is needed from the cam. The combinations presented are very typical of many street cars. If yours is close to one of them, choosing one of the cams recommended will guarantee happiness.
How to Pick the Camshaft
There are many variables that will affect how a camshaft will perform in your engine. Before you even start comparing numbers, it is important to understand camshaft design at the fundamental level. Once you have a better understanding of how each number affects the performance of your engine, selecting the right camshaft becomes less of a guessing game and more of a knowledge-based decision.
Picking The Right Camshaft
There is no doubt whether it is daily driven or street and strip, most hot-rodders want their street machines to sound the part. Nothing beats the syncopated music of a choppy idle when you are pulling up to a traffic light. It is more of a side-effect that race engines sound like this because their needs rely on high rpm operation. To run at higher rpm their camshafts need to be ground with a higher lift and longer duration. More valve lift and duration will obviously increase rpm, but on a streetcar one of the most common mistakes is choosing a camshaft that is poorly matched for the engine. The art of matching a camshaft may come easier with experience, but it all comes down to finding the right balance between performance and drivability.
Understanding Camshaft Specifications
There are many numbers and terms used when describing a camshaft’s design that must be understood when choosing a cam. It is good to know exactly how each of these specs affects the engine’s performance, but one of the most important is duration, so pay special attention to that one. Also, look at the recommendations given by the cam companies for our six sample vehicles at the end of this article. Compare how the specs change from one example to the other, then refer to the paragraphs just below to see why one has more duration or lift than the other.
Lift
The cam’s basic function is to open the valves. Lift refers to how far the valve is opened (or lifted) off its seat. A street performance cam will usually have between .450- and .550-inch lift. More lift can increase power, and increased lift without changing duration increases power without affecting the point of peak power on the rpm band. The rocker arms have a direct effect on lift because they do not have a 1:1 lever ratio. A cam that has .318 inch of lobe lift (that’s how far it lifts the lifter) will open the valve .477 inch with 1.5:1 rocker arm (.318 x 1.5 = .477) and .508 inch with 1.6:1 rockers.
A stock engine will tolerate .500-inch lift before the valves hit the pistons or the valve-springs hit coil bind, but any time lift is increased, these clearances should be checked.
Camshaft Duration
Duration is how long the cam holds the valves open. It is expressed in degrees of crankshaft rotation (remember, the cam rotates at half the speed of the crank). A 280-degree-duration cam holds the valves open longer than a 260-degree-duration cam. Holding the valves open longer allows more air and fuel into the engine and allows more to get out through the exhaust. Longer duration (higher number) improves top-end power but almost always sacrifices low-end torque. Lower duration improves low-end torque and makes the car idle better, but it limits top-end power, and you can get only so much valve lift with a short duration cam due to the rate-of-lift limitations of the lifter. Roller cams, which we will discuss below, have the advantage of allowing high rates of lift with relatively short duration.
The confusing thing about duration is the difference between “advertised” and “at .050-lift” duration. At .050-lift duration is measured from the point where the cam moves the lifter up .050 inch until .050 inch before the lifter is all the way back down. Most cam manufacturers differ in where they start and finish measuring for advertised duration. Some start at .004-inch lift, some at .008-inch and some measure it somewhere in between. That is why the .050-lift numbers are the best to go. A 280 cam (advertised duration) from one manufacturer could actually have less at-.050 duration than a 278 cam from another, due to the different points at which the companies measure advertised duration.
Lobe Separation Angle
This is the relationship between the centerlines of the intake and exhaust lobes. A 110-degree lobe separation angle means that the peak opening points of the intake and exhaust lobes are 110 degrees apart. This is ground into the cam and cannot be changed without changing cams. Lobe separation angle is another way of expressing overlap, which is the term formerly used by cam manufacturers. Overlap is the amount of time that both valves are open in the same cylinder. When both valves are open at the same time, cylinder pressure drops. A cam with 106 degrees of lobe separation angle will have more overlap and a rougher idle than one with 112 degrees, but it will usually make more midrange power.
Dual-Pattern Cams
A dual-pattern cam is one that has different duration and/or lift specs for the intake and exhaust. Usually, the exhaust lobes have more duration and lift than the intakes. Depending on the engine, this can be beneficial for engines with poor exhaust-port flow or otherwise-restricted exhaust systems.
Type Of Lifter
A hydraulic-lifter cam is the best choice if the car is to be a daily driver, because it does not need periodic lash adjustments. A solid-lifter cam is beneficial in high-rpm applications (6500 rpm and higher), but it requires a lash adjustment every few months. Hydraulic and solid, non-roller cams are also called “flat-tappet” cams. A roller cam is normally associated with hard-core race engines, but several companies make street rollers that are docile enough for a daily driver, yet still make serious power. Most roller lifters are not of the hydraulic type, so they require adjustment just like a solid lifter. Hydraulic rollers are becoming much more common but have not found much of a following in high-performance circles yet, except in late-model computer-controlled cars. Either type of roller cam and lifter is considerably more expensive than a flat tappet. Also, you cannot mix and match cams and lifters. Because they are ground differently, a solid-lifter cam must use solid lifters and a hydraulic cam must use hydraulic lifters.
Other Considerations When Picking the Right Camshaft
Most street performance cams will range between .450 and .550 inches of lift. But as you climb up to more radical profiles, say .460 inches and up, you should also consider what your budget needs are for the project. To prevent coil bind, changes in clearance, or part failure, high lift camshafts will require an upgrade to the original valve springs, rocker arms and pushrods. If you have power brakes that require vacuum at idle, or a stock transmission and converter, a high lift cam probably won’t perform the best for you. On a near to stock engine a camshaft around .450-inches of lift, 110 degrees of lobe separation and .218 duration at .050-inch lift should still give you a choppy idle while maintaining strong vacuum at idle.
High Compression Considerations
Cam companies have long preached the gospel of conservative cam specs for a true streetcar, meaning that you are always better off to go one step smaller on the cam instead of one step bigger. Most of the time that is true, but there are instances where a streetcar should run an aggressive cam, and sometimes bigger is indeed better.
Overlap (lots of duration and tight lobe-separation angles) decreases cylinder pressure, especially at low rpm, which allows an engine to run a higher compression ratio and still work on pump gas. High cylinder pressure, which is caused partly by a high compression ratio, is what makes an engine detonate on pump gas. Decreasing the cylinder pressure by adding duration is just like taking compression out of the engine, but mostly only at low rpm. A compression ratio of 11.5:1 running on pump gas is not unheard of when the cam has enough overlap and duration to bleed off the low-speed cylinder pressure.
Other Stuff You Will Need
Never reuse old lifters with a new cam, even if they have only been run for five minutes. Lifters and cam lobes develop a unique wear pattern almost immediately that will not be compatible with a new camshaft. Also make sure that the valve springs match the new cam, the pushrods are all straight and the rocker arms will handle any increased lift or stress the cam provides.
The Camshaft 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 is the opposite. That is because an internal combustion engine is just a controlled burn 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 will be using your engine in, and the intake-to-exhaust flow percentage. The first one easily explained; it does not matter what the heads flow at .800-inch lift if you are 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.
Finally
Picking the right camshaft involves much more than just matching valvetrain components and other engine parts. The entire sum of the vehicle is just as important. Most camshaft manufacturers will list the optimum rpm range on each camshaft. But as you are matching the rpm range you should also consider what your intentions are for the car. The type of transmission and torque converter, the vehicle weight, compression ratio, tire size, and rear-end gear ratio will all affect the rpm characteristics. So, it is important to know what kind of car you are planning to build from the start.
“Also, look at the recommendations given by the cam companies for our six sample vehicles at the end of this article.”
Where are the samples?