Choosing a camshaft is highly dependent on the type of driving you do, the type of car and the other components of the engine and drive train. Selecting one that is too big or too small can have disappointing and possibly damaging results, and it is easy to select a cam that is more of a problem than a solution. Finding a design that is well suited to the entire combination makes the final results worthwhile.
Determine the car weight, rear axle ratio and transmission stall speed. With respect to the engine, calculate or identify the compression ratio, cylinder head and induction system airflow, and how restrictive the exhaust system is. Identify the type of operation the engine will see during normal usage. These factors figure into exactly what valve event timing is needed for a particular camshaft design. It is important to be completely honest about the service the car and engine will be used for. Selecting a camshaft designed for a racing engine that is built to operate only at full throttle between 5200 and 7500 RPMs and installing it in a stock muscle car engine will have poor–perhaps catastrophic–results.
Decide how the car will be driven. For example, if your car will be driven on the street and cruise on the highway at 2500 RPMs, the cam needs to be designed to provide peak torque at that level. Target the engine speed the engine will be driven at almost all of the time and modify the entire combination first so that it provides the best environment for the cam to operate in. If the engine’s rotating assembly and valve train are reliably limited to 5800 RPMs, the camshaft should have a power band that stops at about the same area. (Few camshafts have “optimal” powerband wider than 3500 RPMs.) Change the gear ratio in the rear differential, if needed, to a gear that cruises at the targeted engine speed most used in high gear (i.e., 2500 RPM at 60 MPH would take a gear ratio of approximately 3.42:1 with a 27-inch diameter tire and no overdrive). If the transmission is an automatic, the torque converter should have a stall speed slightly below that highway RPM.
Perform modifications to the car's transmission, gear ratio, exhaust system and engine first. This will narrow down choices and zero in on a cam that will be more suitable. Make the camshaft one of the last considerations until the car is set up for its intended function.
Choose the camshaft manufacturer/grinder that you feel confident will accurately recommend and offer a design that is ideal for the application. With the car and drive train correctly set up, a very exact selection can be made to give optimum performance.
