Just for giggles, this is probably the ultimate in exhaust theory and application IMO....... It's a 180° exhaust headers/system on a Pantera. Commonly referred to as "a bundle of snakes".

It is based on the firing order where the next cylinder pushes the preceding exhaust pulse and then the next cylinder to be ignited pushes out that exhaust pulse, and so forth.... The primary lengths are calculated so they meet at the right time in the sequence... of course it is tuned for a specific RPM range, if not a very narrow band.

Anyhow, I think you get the principle...



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I bet they burned up two slide rules and a TI-30 figuring that pipe length and where the bends were to be done. Not to mention all that heat!

 

If I were a betting man, I would bet you could change the pipe diameter from 2 1/2" to 3" and NOT feel a bit of difference in performance on the street. Maybe on a dyno, but seat of the pants you won't know a thing. There is a big difference between driving it around and flat out at peak RPM.

This is a good watch:

My .02. Too many guys wayyy over think this stuff.

 

Not only that, but friction of the exhaust gas is greatest against the inside diameter sidewalls of the pipe. A single 4" pipe has less inside diameter sidewall surface area than dual 2" pipes... making the single 4" pipe less restrictive.

 

Not all backpressure is bad, some is needed to smooth out the pulsing flow caused by the gasses exiting different cylinders at different times. I can't do any flow calculations for you, but there are software packages that will help you figure out what you need.

The X or H crossover pipe is used to balance out both banks of the engine.