A common method of increasing the power output of an engine is the use of upgraded headers. The increased power output is often due to a result of a larger cross-section area of the pipes (reducing the resistance on the exhaust gasses) and/or designing the pipe lengths so that the pressure wave assists in exhaust scavenging. For inline-four engines and V8 engines, exhaust manifolds are usually either a 4-2-1 design (where the four pipes merge into two, followed by a separate merge of these two pipes into one) or a 4-1 design (where the four pipes directly merge into one).