TURBOCHARGE or SUPERCHARGED - The $64 Question

[olddog]

It is often stated that the turbo is free power, since it doesn't have a belt robbing power from the crankshaft. This is not totally true. The turbo is a restriction in the exhaust pipe. It increases exhaust pressure on the engine. This increased pressure is pushing on the piston on the exhaust stroke. This is robbing power from the crankshaft. Some sources say boost pressure to exhaust pressure is close to 1:1. I do not know if this is always true.

On the other hand, the turbo does take advantage of the high cylinder pressure that is blown down, when the exhaust valve is opened early (before the end of the power stroke). Some of the power that drives the turbo would have been wasted, so some of it is free power, but not all.

When compared to a belt driven blower making exactly the same boost pressure on the same engine. the turbo will make more power. However because the turbo increases exhaust pressure, valve timing can be tuned to get a bit more power. So the best cam choice for a turbo verses a blower is a bit different. At mild boost pressure this can be ignored, but the higher the boost pressure the more important the cam choice becomes.