NATURALLY ASPIRATED vs FORCED INDUCTION

There are many differences in profile design between cams intended for naturally aspirated engines and those packing forced induction, but the two most important contrasts are duration and overlap.

With a naturally aspirated engine, you want to encourage the flow of as much intake air as possible. Longer duration and the resulting increased overlap help to achieve this. With forced induction, the opposite is true — you don’t need to worry so much about encouraging air into the cylinder on a boosted engine due to the fact air is already being forced in by the accompanying turbocharger. Even so, you don’t want any of the exhaust gases to slow the speed of intake air as a consequence of escape through the inlet tract, which is why most turbo camshafts feature a shorter duration and a resulting shorter overlap than naturally aspirated equivalent parts.

Forced induction cams are designed to work in conjunction with a turbocharger when full boost is reached — there’s no point in having a set of cams designed to rev at 9,000rpm and make peak power at 8,500rpm if your car’s turbo runs out of puff at 5,000rpm. Conversely, there’s no point in having a set of cams that will make peak power and peak torque early on in the rev range if you’ve installed a massive turbo which doesn’t start producing boost until 4,500rpm.