Question brought up by "Better size header"

[MiracleMax]

Back pressure in any form is a power killer, thats a force the engine has to work against in order to produce power. There are no positive benefits to back pressure what so ever.

That said....

Primary pipe dia. seems to be dependant on the volume of exhaust that the cylinder is capable of expelling during the exhaust cycle.

Here is a formula for determing the needed dia.

(CID / # of cyl) x RPM /Pi x ^2 + (pipe wall thickness x 2) = desired primary pipe O.D

Notice that its all volume related.

You can plug in whatever rpm your looking to work with, be it peak torque, or peak power or whatever. Also note that an engine reaches peak volumetric efficiency at peak torque (100% or better), but this isn't nessecarily the highest volume of gas that then cylinder is trying to expel (anybody with a dyno sim could monkey with this and determine the best possible pipe O.D over the widest range of operating rpm by pluging in the observed VE in the first part of the formula then plug in the rpm where it occurs).

Now onto header length. I haven't the foggiest on how to determine proper primary tube length, however I bet the more accurate formulas use some info from the cam timing events (or not??) since a header design is governed by the same princples used to tune an intake manifold.

Then again primary pipe length could also be tied with pipe diameter as it relates to cylinder volume? (but not in the resonant tuning capacity which is length dependant)

Anywhosit, I assume the formula is tuned (no pun intended) for the proper dia needed to maintain proper gas velocity at the desired RPM.