[niles]

guys:
This is a very complex subject that I can only start to describe the physics going on; few tech tidbit for the soup:
Normally some backpressure is good; on decent exhausts, small pipes dumping into a collector; you need back pressure to set up the standing and reflected waves. These reflected acoustical waves go backwards up the pipe and provide negative pressure when another exh. value opens; hence more flow out. This typically is what is referred to a scavaging.

The top fueler example isnt applicable; because they have such hugh overlaps they just use extreme blower pressure to blast the exhaust out; alone with hugh amounts of excess fuel; hence the roman candel effect on TF exhausts.

The savageing effect is easily demonstrated on a hot two stroke cycle; the effect of coming on the pipe is with this savaging effect actually occurs at the exhaust port.

Cross overs are excellent; just hard to execute with V-8's with the cyclinders seperated by a trannie and drive shaft. Look at all the aftermarket mustang system; crossovers, X-pipes, etc. This is a poor man solution with some benefit.

The real solution is demonstrated on rear enginge F-1's, Indy cars. Nothing to get in the way of the tube benders. You will note that "long tangle of pipes" jointing at diffferent spots, lengths with no apparent reason. What is actually going on is designed savaging at certain rpm's; where the pipe #3 joins pipe #7 such that 3's reflected wave happens to go up 7's pipe at just the right time.
Told you, it was more complicated than mortals want to know and I only know enough to be dangerous.

See Jay Torborgs web page on engine design; he goes over the basics of exhaust and cam design theory.

I really wish some smart people would work this item, Cobra exhaust by and large are terrible and noisey. I've seen tech papers that show a well designed exhaust only give up 1 or 2 %, not the 60 - 100 hp some have seen on dyno's.

The most advanced exhausts are probably formula race cycles; about 4 hp per CI and relatively quiet large diameter collectors. What's inside them are big Honda secrets but they sure work!

gn

gn