How do exhaust headers work to improve engine performance?

When you look at the four-stroke cycle of an engine, you can see that the engine produces all of its power during the power stroke. The gasoline in the cylinder burns and expands during this stroke, generating power.

The other three strokes are necessary evils required to make the power stroke possible. If these three strokes consume power, they are a drain on the engine.

Back Pressure

During the exhaust stroke, a good way for an engine to lose power is through back pressure. The exhaust valve opens at the beginning of the exhaust stroke, and then the piston pushes the exhaust gases out of the cylinder.

If there is any amount of resistance that the piston has to push against to force the exhaust gases out, power is wasted. Using two exhaust valves rather than one improves the flow by making the hole that the exhaust gases travel through larger.