[Add .5-inch when using N20 or FI.]
Backpressure and exhuast stream....
What engines need is LOW backpressure, but HIGH exhuast stream velocity. A fast-moving but free-flowing gas column in the exhuast helps creat rarefractoin or a negative back pressure wave behind the exhuast valve as it opens. This vacuum helps scavenge the cylinder of exhuast gas faster and more throughly with less pumping losses. An exhuast pipe that is to BIG in diameter has low backpressure but LOWER velocity. The low velocity reduces the effectiveness of this scavenging effect, which has the greatest impact on low-end torque.
How to get it....
Low backpressure and high exhaust stream velocity can be achieved by running straight-through free-flowing mufflers and small pipe diameters.
Some stock mufflers and exhuast systems have up to 18 psi of choking, power robbing backpressure. In direct contrast, a well-designed, high-performance street exhuast typically has about 2 to 6 psi of backpressure. For an intresting comparison, an un-muffled straight pipe on a real racecar usually has 1 to 3 psi of backpressure