Why does a knackered exhaust reduce engine power? Is it something to do with the air pressure through the system?
Yep, counter intuitively, back pressure can actually help the engine produce more power. Imagine a slug of gas moving along a pipe at very high speed, now close the valve behind it, The 'pressure' (vacuum) moves along the pipe at approximately the speed of sound. If you get the lengths right you can use this vacuum to suck the exhaust out of the next cylinder and close the valve after it.
Also you don't want the cylinder completely empty of the exhaust when your not at full throttle, leaving a small amount of exhaust gas in there reduces engine temperatures and fuel consumption. This is normally done with an exhaust gas recirculation valve, which feeds the exhaust back into the inlet manifold.
That's why most big motorbikes and super cars often have a 'power valve' (or other trade name) which keeps pressure in the exhaust below a certain set of conditions (usually something like full throttle and 75% engine max RPM).
