The Header is metal plumbing that directs hot exhaust gases out of the engine to the exhaust. There are two specific design features to the plumbing. The Plumbing is called runner lengths that are designed in what is called the 4-1 header design, or the 4-2-1 header design. The purpose of header design is to manipulate the way power is used in the power band. Power is manipulated through pulses in a process known as scavaging, either for function of low to mid-end power use, or for the function of high-end power use. 4-2-1 is designed to function as a low to mid end power header. The stock header is restrictive and can be upgraded to a less restrictive unit for a slight increase in power, while the 4-1 header was made for high-RPM applications. Stock headers were created for maximum back pressure to regulate smog emissions, and in no way is the stock header a performance enhancer.