Originally Posted by Ted B

Ok, time to get this cleaned up for the benefit of everyone.

First of all, the notion that a lack of backpressure reduces power in a NA engine is pure myth. Understanding NA exhaust system design reveals that backpressure never does anything but reduce power and torque 100% of the time. The end.

Now that we have that out of the way, an NA engine depends upon a negative pressure wave to enhance scavenging and increase VE (volumetric efficiency). If we have a correctly designed system, but the pipes are too large, this reduces torque (VE) because it reduces exhaust gas velocity. Backpressure has nothing to do with it, and arises from the opposite problem (pipes too small). Sizing and tuning pipes with an NA engine requires a balancing act between maximizing exhaust gas velocity while keeping exhaust backpressure in check. Too far in either direction reduces power. It's tricky to get it right, and it is rarely done to perfection with off-the-shelf performance parts.

A turbo motor does not depend upon exhaust scavenging to the extent that a NA motor does, and the turbo makes up for a multitude of exhaust inefficiencies. The most critical exhaust parts in a turbo engine are the manifold and the turbine housing. In a turbo motor, once the exhaust gas has 'spun the wheel', its job is done. It can do nothing else at this point but get in the way. This is why one can stick a 10" diameter downpipe on the back end of the turbo and it will lose no power. In our case, it probably won't make any more power than a 3" downpipe, but it won't cost power.