What is the theory behind header design?
“Headers function in different ways. Lets go with the ‘scavenge’ effect first. On a four stroke, four cylinder engine, you have a cylinder firing every 180 degrees of crankshaft rotation. That means each of the 4 cylinders is at a different stage in the four stroke cycle. Because of valve overlap, the exhaust valve on a cylinder is still open for a very short period of time while the piston is starting down on its intake stroke, thus pulling vacuum on the exhaust system. Now, that small amount of vacuum can be used to help pull the exhaust from another cylinder, or ‘scavenge’. The length of the tubes from the head to the collector determine the rpm where maximum scavenging occurs. The stock manifold is short, thus scavenging occurs low in the rpm band; the Pacesetter header which is longer scavenges most in the mid range; and some other brands which are a long tube design are more for the upper rpm range. What this scavenging does is increase the torque in the area of maximum scavengin
