Ideally, we need to eliminate all of the burned gasses from the cylinder when the piston gets to top dead center (TDC) of the exhaust stroke. The more backpressure there is in the exhaust system (headers, catalytic converter, and mufflers), the higher the pressure will be inside the cylinder, lessening the exhaust gas that is expelled.

While not wanting to “over simplify” things, an engine is just a big air pump - more air in and out typically results in more usable power. As such, we don’t want spent gasses inside the cylinder, so we want the backpressure to be as low as possible.

There are several other factors, i.e., cam and valve timing that the pros use to tune and tweak just when the spent gases are released and new, fresh air/fuel charges are drawn into the cylinder. There are several such pros on this site who will, I'm certain, chime in with their expertise.

Also remember that atmospheric pressure has an effect on backpressure, especially when tuning for the last little bit of flow.

Headers are one of the best and easiest bolt-on accessories you can use to improve an engine's performance. The primary goal of headers is to make it easier for the engine to expel the spent exhaust gases out of the cylinders. Exhaust headers eliminate some of the back pressure created by the stock manifolds by providing each cylinder with its own exhaust pipe. Typically, the individual pipes are cut and bent so that each one is the same length. These individual pipes come together in a larger pipe (collector). The equal length allows each cylinder's exhaust gases to arrive in the collector in a manner that eliminates any back pressure generated by the cylinders sharing the collector.

Think dragsters and funny cars – short very open exhaust!

G’day and Happy New Year,
Glenn