HERE’S HOW IT WORKS:
The exhaust valve opens at the beginning of the exhaust stroke, and then the piston pushes the burnt gasses out of the cylinder. If there is any resistance that the piston has to push against to force the exhaust gasses out, power is wasted. This is known as backpressure.
Therefore, the goal is to get the burnt gasses out of the engine and exhaust system as efficiently as possible. These gasses travel through the exhaust port into the exhaust manifold, then travel through the exhaust pipe to the catalytic converter and muffler, and finally vent to the environment.
Any obstacles or severe “bends” in the system can affect backpressure, slowing the flow of exhaust gasses. Therefore, an exhaust system that reduces backpressure improves efficiency and can create additional power output.