Originally Posted by Troux
I've heard that any low-end torque lost due to an unrestricted (to any degree) exhaust is overcompensated for in the higher end, so if streetability isn't an issue, the widest EG exit is the optimum.
You can have en exhaust that's too large. If the widest EG exit is optimal, Civics would be running around with 4" exhausts. Even at higher RPMs their little engines still wouldn't flow enough air to push all the exhaust out of thesystem in a timely fasion.
The optimal size of the exhaust depends on the engine and how it's used. An autox car that needs pulling power out of tight corners might opt for a smaller exhaust diameter to retain low-end torque. On the other hand, a drag car that's only in the lower RPMs once, may opt for a larger exhaust to gain top end.