"One of the biggest obstacles to better performance is back pressure, which is the unwanted result of the system's inability to remove sufficient spent gas," says Anderson. "Back pressure causes some of the restriction in exhaust gas flow, as does the design of the stock manifold itself. Typically, a properly designed header is an arrangement of individual pipes that lead from the exhaust ports into a common passage, which eventually enters a muffler. Exact pipe size and length not only help determine how much but where in an engine's rpm range power improvements are provided. Obviously, for trucks it needs to be in the lower range, below 4,500 rpm, where sportsmen need it most."