When I looked into upgrading the exhaust on my car, my original goal was simply to see if it made any difference. Would performance increase or decrease? It’s important to understand that the velocity at which your exhaust exits is one of the key elements to its performance. When your engine is at low RPM, the amount of exhaust gases leaving is low, so the velocity it exits the exhaust is low. You can increase this velocity by using a smaller pipe, but this will create a restriction for higher RPM.
Exhaust scavenging makes an exhaust system more efficient because as your exhaust gases pulse out of the engine (from each exhaust stroke of the engine), you have a high pressure area leading the exhaust pulse, followed by a low pressure area (with a transition). This low pressure area helps to pull the next exhaust pulse out, meaning the piston has less work to do when it’s forcing out the exhaust gases. Ultimately the goal is to have the fastest exhaust velocity with the least amount of restriction (which of course isn’t as simple as writing that sentence).
The whole idea is to increase your exhaust diameter as the amount of exhaust your engine creates increases. This reduces restriction and allows for more flow. If you have modified your engine, you’ll need to modify the exhaust as well to allow for more airflow.