Headers are one of the most popular bolt-on accessories you can purchase to improve an engine's performance. The primary goal of a performance headers is to make it easier for the engine to push exhaust gases out of the cylinders. 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 the piston pushes the exhaust gases out of the cylinder. If there is any amount of resistance, power is wasted.
In a normal engine, once the exhaust gases exit the cylinder, they end up in the exhaust manifold. In a four-cylinder, there are four cylinders using the same manifold. From the manifold, the exhaust gases flow into one pipe toward the catalytic converter and muffler. It turns out that the manifold can be an important source of back pressure because exhaust gases from one cylinder builds up pressure in the manifold that affects the next cylinder that uses the manifold.
The idea behind an exhaust header is to eliminate the manifold's back pressure. Instead of a common manifold that all of the cylinders share, each cylinder gets its own exhaust pipe. These pipes come together in a larger pipe called the collector. The individual pipe's are cut and bent so that each one is the same length as the others. By making them the same length, it guarantees that each cylinder's exhaust gases arrive in the collector spaced out equally so there is no back pressure generated by the cylinders sharing the collector.