Choosing a performance cam is never easy because there are so many variables involved and so many cam grinds from which to choose. Finding the one that’s right for a given application requires some serious communication between you and your customer.
First and foremost, the cam must match the application. The cam determines the engine’s power curve and personality. More specifically, the valve timing created by the cam determines where the engine’s peak torque and horsepower will be developed along the rpm scale.
Ideally, an engine should develop its peak power output within the rpm range where it will spend most of its time working. For a street engine, this would be 1,500 to 4,000 rpm. For a competition engine, things do not start to get interesting until you are on the high side of 5,000 rpm.
To match cam to application, you have to ask your customer if he wants more top end power, mid-range torque, or low speed pulling power. Will it be an all-out competition engine, a combination street/strip performer, or a daily driver? Will the engine be turbocharged, supercharged or naturally aspirated? Will it be mated to a manual or automatic transmission? How about the final gear ratio? How much weight will the engine have to push?
Once these have been defined, there are two ways to go. One is to pick a cam and build the engine around it. With this approach, carburetion, compression ratio, cylinder heads and gearing are all matched to the cam to achieve the desired results.
The other way is to match the cam to an existing engine. In other words, given a certain combination of parts, a cam is selected that works with the stock or modified carburetion, compression, heads, and gearing.
Either way, the key is to end up with a combination of parts that work well together. This is where many a novice goes astray when picking a cam. They’re seduced by the "bigger is better" trap and insist on the hottest cam in the catalog. They often end up dissatisfied because they have too much cam and not enough engine.
As a rule of thumb, the longer the duration, the shorter and higher the useful power band of the engine. A radical drag strip grind that comes on strong above 5,000 rpm and requires lots of compression, carburetion and gearing is not going to work on the street because the power band is in the wrong range.
High duration cams have other serious drawbacks that make them impractical for the street: they reduce intake vacuum and idle quality (which can upset computerized engine control systems), and they increase emissions (which makes them technically illegal in areas requiring tailpipe emission inspections).
In a typical 300 cubic inch V-8, 215>1| (measured at 0.050" cam lift) is about all the duration the computer can handle before a recalibrated PROM chip becomes necessary. A 350 V-8 can handle up to 220 degrees of duration before intake vacuum is reduced enough to affect the computer.
There is more to picking a cam than comparing a list of duration and lift specs. Another consideration is the amount of spread or separation between cam lobes. This determines valve overlap and how the opening and closing of the valve relates to the pistons.
According to the experts, the intake valve should be at least halfway open by the time the piston reaches maximum acceleration away from top dead center. This is the point at which a piston generates the strongest pull on air in the intake port.
Choosing the right cam takes some thought. It does not have to be a "hit-or-miss" proposition. As long as you understand why the cam has to be matched to the application, you can usually pick the right cam by following the guidelines outlined in most catalogs, or by using one of the new "cam selection" computer programs or hotlines that are available. By entering the appropriate application data (engine displacement, compression ratio, rpm range, vehicle weight and gearing, tire size, etc.), the program will give you a specific cam recommendation by part number.
