Same rule applies as you are still talking about air flow; e.g. exhaust gasses vs. intake gasses (ambient air).
Scavenging of exhaust gasses is a theory that has resulted in several different design philosophies that include primarily tubing configurations of the exhaust system itself, starting at the cylinder heads & continuing onto the tail pipes.
Such considerations as the diameter or varying diameters (stepped headers), length, bends, location of cross-over (H or Y pipes), internal baffling of mufflers & other similar areas have all been addressed in different ways to improve the efficiency of a particular engine's ability to effectively rid itself of exhaust gasses.
Once again this involves the balancing of laminar vs. turbulent flow which brucer touched on in his original reply.
Perhaps the most extreme example I can think of off the top of my head regarding exhaust system theory application is the elaborately overlapping "bundle of snakes" utilized by Ford on the GT-40 & early Lotus IndyCar programs in the 1960's.
Similar systems continue to be utilized by prototype endurance sports cars & F1 machines to this day.
1999 SVT Contour, #2140 of 2760, Tropic Green - Medium Prairie Tan
Koni Sport struts; TSW Blade wheels; Nakamichi, a/d/s/, Boston Acoustics, Infinity I.C.E.
1989 Saleen Mustang, #89-0408, too many mods to list here
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