[Rev]

Thunderbolt is right on. Back pressure "per se" never increases anything except loss. An exhaust velocitity in the headers in the right amount will help in removing exhaust from the chambers and cramming more charge into the chambers, but only at a certain RPM range. Scavenging is the name of the game here.

Balancing the headers with the torque range of the engine is absolutely necessary for good performance.

Rev