The header is one of the easiest bolt-on accessories you can use to improve your engines performance. The main purpose of the header is to make it easier for the engine to push exhaust gases out of the cylinders. Most headers are constructed of either stainless steel or heavy gauge mild steel that is ceramic coated for appearance and long life. The ceramic coating may aid catalytic light off under cold start conditions, and the stainless steel will generally have a longer lifespan.
When you look at the four stroke cycle, you can see that the engine produces all of its power during the power stroke.
The gasoline in the cylinder burns and expands during the combustion stroke, generating power. The other three strokes are necessary to make the power stroke possible. If these three strokes consume power, they are a drain on the engine.
During the exhaust stroke, a good way for an engine to lose power is through back
pressure. The exhaust
valve(s) open 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.
In a normal engine, once the exhaust gases exit the cylinder they end up in the exhaust manifold. In a 4-cylinder
or 8-cylinder engine there are 4 cylinders using the same manifold. From the manifold the exhaust gases flow into
one pipe toward the catalytic converter and the muffler. It turns out that the manifold can be an important source of
back pressure because exhaust gases from one cylinder build 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 pipes 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
A typical header is good for between 8-15 wheel hp, with highly modified cars getting more power and stock cars getting more toward the 8 hp side of the scale. Some people claim that headers reduce bottom end torque but for most, the difference is negligible.