The engine is merely an air pump that takes up and down motion and transforms it into rotary motion to turn the vehicle’s wheels. In order to create more horsepower and torque, we need to get more air in and more air out and at a faster rate. There are many ways to do this. On the intake side, we can add more carburetion, turbocharging, or supercharging. We can then add more radical camshaft timing. Finally, on the exhaust side, we need to get rid of the spent gases quickly.
We can PUSH more gases away from an engine by opening up the tubing diameter. As a rule of thumb, a stock engine can usually be improved by a 1/4” diameter enlargement and a mildly-modified street engine can benefit usually by a ½” increase. Any larger tubing than this tends to have horsepower and torque drop off at the same RPM levels. This is because we have failed to also PULL more gases through the system. This additional pull-through action on a V6 or V8 engine is achieved by scavenging the exhaust by utilizing a slightly smaller H or X pipe junction between the left hand and the right hand exhaust pipes.
The high pressure wave coming through one exhaust pipe creates a low pressure wave immediately behind it at the smaller junction which tends to accelerate the incoming pulse from the opposite side exhaust pipe as it makes its way through the system. ‘Bigger is better… up to a certain point’ and properly located H/X pipes will enhance performance and improve sound.