Choosing Performance Camshafts

Americans are very enthusiastic about their automobiles. Therefore, when they purchase, rebuild or even imagine their dream cars, a tremendous effort is put forth to make sure it is exactly what they want. Because the engine is arguably the most significant element in the performance of the car, it is very important that the internal components be properly selected so that the engine, and the entire vehicle, can achieve the desired results.

The camshaft is the main component that will help enhance the performance characteristics of the engine, but it is important to understand what role the cam plays in the engine and how the cam specs can be changed to optimize the performance. The cam causes the valves to open and close, and that regulates how much air and fuel enters and exits the combustion chamber. As is the case with most decisions, there are clear tradeoffs that must be made with cam selection. It is critical to select the proper cam for the size engine, the components in the engine, and ultimately what the expectations of the vehicle will be. Chances are that if we are even considering a high performance camshaft, the car will be used for some type of recreation, and it must be fun to drive. It’s best to take a good look at things before making any decisions.

There are two basic types of camshafts used most commonly in most V8, pushrod style, mild performance street engines. The older and most traditional designs used a flat faced hydraulic lifter, while most newer designs utilize a hydraulic roller follower. While it is commonly thought that the decrease in friction is the main advantage of the roller cam, the benefit is actually mostly that the roller design allows the valve to be opened and closed at a much faster rate, resulting in more area with shorter seat timing.

It is very important to understand how the cams are rated prior to comparing a stock cam to an aftermarket cam, and before selecting a cam. The very beginning and end of the lift curve on a stock cam is usually such that the valve opens slowly and sits down very gently on the seat. This is particularly important for noise and seat recession, as well as overall durability. It is the area in which the performance aftermarket cam companies push the envelope to increase performance.

Getting the valve off and on the seat more quickly sets the stage for either less seat timing and more area, or the more common goal of similar seat timing and much more area. With any valvetrain, there are limits on the maximum velocity of the valve, either due to the tappet design or how much velocity (and energy) the valve spring can control. In either case, getting the valve off the seat quickly allows the valve to reach maximum velocity sooner, gives it more time to turn around, and results in increased area. It is somewhat analogous to the effect of 0 to 60 mph time on quarter-mile time. If a Top Fuel dragster took 6 seconds to get from 0 to 60 mph, like a respectable passenger car, it would be impossible for it to run a 4.5-second quarter-mile time. Similarly, the cam profile design cannot take all day to get off the seat and still expect to have enough area under the curve to allow the airflow needed to optimized high rpm power.

Figure 1 shows the difference between a popular factory performance grind and an aggressive aftermarket design. By accelerating the valve off the seat faster, the modern aftermarket camshaft is able to reduce the seat timing by 18 degrees while increasing overall area, especially in the high lift region where the cylinder heads are most effective.

By focusing on performance, aftermarket camshafts can greatly improve power output without sacrificing drivability and responsiveness. The key for engine builders is to select a camshaft sized appropriately for the application.

While larger camshafts typically make more peak power, the largest camshaft available is likely not the best choice in more than 99.9 percent of all applications. Camshafts that are too large will rob the vacuum for power brakes and accessories as well as cause a very rough idle.

Even a camshaft that is just a little too large will hurt responsiveness such that the vehicle will not respond quickly to throttle changes. The best camshaft should be very responsive at the low end of the operating range, make peak torque near where the engine spends most of its time, and be able to provide enough air to hold on to the maximum rpm.

In automatic transmission applications, one must take particular care to consider the stall rpm of the torque converter when choosing a camshaft. Stock converters typically will not allow the engine over 1,800 rpm before rotating the tires. Hence the cam that can operate at or below that limit must be selected. When a looser converter is chosen, the operating range of the camshaft can be selected to match the flash rpm. If too large of a cam is selected, the engine will bog or stumble until it reaches the effective operating range, then clear up and accelerate properly.

A common problem is that a customer may want the engine to operate from 1,500 to 6,500 rpm. Unfortunately, even with the best technology we have available today, a camshaft’s effective operating range is typically less than 4,000 rpm. Hence, if a cam that makes good torque at 1,500 rpm is selected, it is basically out of its effective range by 5,500 rpm. Some of the latest engines, such as the Honda VTEC, use variable valve timing to extend this range, but this type of system is not available for most applications.

In real word applications, it is best to size the camshaft by focusing on the lower rpm, being certain the camshaft will work well there, and hope that the cylinder head, manifold and exhaust systems will help the engine not fall off too steeply at high rpm.

The best choice in this case is to narrow the operating range to a 3,500 rpm window, and select all the components to work best in that limited range. Hence, in the case of the person who wants the engine to operate in the 500-6,500 rpm range, either move the bottom up to 3,000 rpm, the top down to 5,000 rpm or find some other compromise.

The main considerations when sizing a camshaft are operating range, engine displacement, cylinder head flow and compression. We have discussed operating range, but it must be stated that with increased engine displacement within an engine family, larger camshafts are needed. A 400 cid small block Chevrolet will need far more camshaft to make power at 6,000 rpm than a 283 cid small block will require.

The opposite goes for cylinder heads; the larger the port, the smaller camshaft that is required. Hence, if a customer chooses a very good set of aftermarket cylinder heads, the required camshaft durations may actually go down to reach the same rpm levels as with stock cylinder heads.

Figure 2 shows how to select the approximate duration of camshaft needed for the given operating range. Realize that these are based on a typical 300 to 380 cid, pushrod actuated, OHV V8 engine with 1.5:1 to 1.7:1 rocker ratios, and cylinder heads and compression properly selected for the operating range.

Fuel injected applications follow the same principles, except each system has its own requirements as to vacuum at idle. Also, most mass air systems will "count" the air each time it passes the flow meter and add fuel accordingly, so cams with some reversion may run very rich at idle because of overcompensation for the air that has passed through several times during overlap. With these considerations, it is best to keep durations lower and lobe separations wider in these applications to ensure adequate vacuum and minimal reversion.

Typically, it is best to keep duration below 210 and lobe separation above 112 unless either the computer can be modified or someone with experience with that specific application can provide information about any necessary modifications for the increased duration. Most aftermarket camshaft manufacturers have experience with popular applications and can help with the specifics.

When the proper camshaft is selected to match the operating range of the vehicle and the components used in the engine, the results can be outstanding. The engine can idle at a reasonable rpm with a distinct sound pleasing to most enthusiasts, without affecting power accessories. The responsiveness can be better than original, and the driving performance will be extremely enhanced.

Figure 3 indicates the results of replacing a stock GM L79 camshaft with a modern aggressive aftermarket design with shorter seat timing and increased area. Even greater gains in peak power can be obtained with increased duration, however, seeing gains of 40 horsepower or more while improving torque throughout the range and improving vacuum is remarkable.

Clearly, the faster the cam accelerates the valve off the seat the better the engine will perform. The most aggressive camshafts should only be used with matched components, but there are many steps between stock and the fastest designs.

Many manufacturers with performance reputations can help you to select a combination that provides both excellent performance and value. By coupling the basic principles in this article with the recommendations of the manufacturer, any engine builder should be able to select a camshaft to exceed his customer’s expectations.

There is one last rule of thumb. If you are ever in doubt when choosing a cam, choose the smaller one. The customer will almost never complain about the engine making too much bottom end power, but will almost certainly be unhappy if the engine is lazy at low rpm. What you will sacrifice at the higher rpm will always be offset by the gains down low.