Exhaust dimension vs engine displacement

[pauljones]

I have a book on motorsports engineering that has an example in it for a 2.0ltr with exhaust tunes for 7500k rpms.
Primaries are 1.75in ID
Lengths are 25.8in
Secondaries 2.4in ID

I used its example and had a rpm of 5k but with standard cam exhaust of 62?

Primaries increased to 38.8in and with a few more changes and different cams it seldom goes above 40in primary length.
So unless I've really messed up and completely miss understood then I would assume 40in primary length to be ok for road use.

Looking at the primary size. I copied the equation with the above info and had it at 400cc cyl volume for ease. Give or take i get 1.5in or 40mm.

We are off course limited by the sizes of pipe we can buy. So anywhere between 38mm and 40mm would be good.

Please correct me if I'm wrong, I'm expecting it.