If you won't belive me maybe some of these people can help.
Got thies from PY.

(Will)

A properly sized header will do something similar to what bastero is talking about with a 2-cycle engine. In a 4-cycle engine the idea is to use the dimensions of the header to work with the cam and intake tract to produce a scavenging effect on the cylinder. What happens with this scavenging effect is that the exhaust pulse heading down the headpipe creates suction on the cylinder behind it so that when the intake valve starts to open during the overlap period the fresh intake charge gets pulled into the cylinder by the suction created by the exhaust pulse.

In general terms, shortening or enlarging the diameter of the head pipes will increase the RPM range at which effective scavenging begins, and the opposite is also true.


Mr. Pebody
Bruce nailed it. The ONLY purpose for blocking the crossover without isolating each port, is to keep the intake cooler. This may offer a little gain, but not much. By isolating each cylinder from the others, the scavenging effect, as Bruce noted, is improved. _The sound wave and vacuum signal are also much less disrupted. This can increase power at a specific point as much as 20-25 horsepower.

(Tom Vaught)

Tuning of ports works on the helmholtz principal. You get a wave form from the exhaust
traveling down the header pipe to the collector discharge. If the header discharge is designed correctly (with the pointed triangle at the center of the 4 connecting pipes and the right sized collector diameter) you will get a strong reverse wave back up the pipe acting on the exhaust valve. You time the pipe length/ resonance wave correctly you get a mild form of scavenging