I've been looking at exhaust manifold design, specifically for the V6.

All the formula stuff seems to be generic for engines with pulses separated by 180 degrees. which is cool for 4 cylinders and v8's, but shite for V6's. I've decided to assume that the primary lengths generated by formula for the 180 engines need to be 33% longer for the V6, assuming constant gas speed. This is because the exhaust pulses are 33% closer together (120 degrees of the crank turning as opposed 180).

This is all fine and dandy, but should I use the same formula's for the 180 degree inside diameter?