The proper exhaust pipe diameter is crucial to your engine’s operation. When dealing with pipe diameter, there are two things to consider: backpressure and flow velocity.
Backpressure is the amount of atmospheric pressure pushing back through the exhaust system. If the piping is too small, there will be too much backpressure in the exhaust system.
This will result in the exhaust gases not being able to escape the engine fast enough. Too much backpressure will cause the exhaust gases that are not able to escape fast enough to contaminate the cool air-fuel mixture entering the engine on the next cycle. This will reduce horsepower.
If you increase the pipe diameter, it will increase flow velocity. Flow velocity is how fast the exhaust flows out of the engine and through the pipe. By increasing flow velocity, it helps to pull the exhaust from the engine, as well as helping to pull the air-fuel mixture into the engine. This increases power and performance.
If you increase the pipe diameter too much, backpressure will be nearly eliminated, and flow velocity will be excessively increased. The result is the exhaust escaping too fast, and pulling some of the air-fuel mixture out through the exhaust. That creates extra pollution escaping the exhaust and reduced power.
The general rule for proper backpressure and flow velocity is 1 inch of exhaust tubing diameter per 100 horsepower. Therefore, if your engine is producing 300 horsepower, you would need 3 inches in diameter of exhaust pipe. The proper piping diameter can be obtained by a single 3-inch pipe, or dual 1 ½-inch pipes.