I stumbled across this article by David Vizard on another site and while I haven't finished reading it, found it a bloody interesting intro into some of the basic physics behind exhaust systems. Obviously the info in that article might be specific to the types of engines and exhaust systems David was working with and you can't trust everything you read on the Internet but this was first published in a print mag so it might be legit, right?
http://www.superchevy.com/technical/eng ... ewall.html