... the tail section of a F1 exhaust does not have any megaphone and that the flow at the exit of the merge collector is but a few sizes larger than the primary tubes themselves. You will also notice that after the pipes collect there isn't a very long tail section. This brings up a difference that occurs when you try to “merge” five tubes on one side of a V-10 F1 Engine as opposed to two primary tubes on a Harley. In the case of the F1 exhaust designer he has to, fairly abruptly, end the collector to minimize collector volumes to keep the velocity and wave scavenging at a high level. He can do this by sharply angling the tubes into one another (Ford) or by bringing the tubes parallel and turning them at the collector (most others). It is a question of packaging.
http://en.wikipedia.org/wiki/Exhaust_pipeHeaders are generally circular steel tubing with bends and folds calculated to make the paths from each cylinder's exhaust port to the common outlet all equal length, and joined at narrow angles to encourage pressure waves to flow through the outlet , and not back towards other cylinders.
I've read that Stepped Primaries are to prevent 'reversion', which is when the exhaust reverses flow. The step prevents this by causing drag in the expanding exhaust gases, thereby discouraging reversion (imagine the flow in the reverse direction, the gas will have to 'step up').I only noticed this the other day, that F1 exhausts tend to have a small 'step' increase in diameter on the primaries, after say 5" of pipe or so. Any idea why?
Reversion: at the beginning of the intake stroke during cam overlap, exaust gas in the header is under high pressure (negative delta P) and is pushed back into the cylinder, diluting the new air/fuel charge.