Well area of a circle (cross-section of exhaust pipe) is pie (3.14etc) times radius squared. So:
2" pre-'75 exhaust = 3.14 sqin
2.25" 1975 on = 3.98 sqin
2.5" HD 292 truck = 4.91 sqin
dual 2" = 6.28 sqin
dual 2.25" = 7.96 sqin
The single 2.25" pipe at the Y is half the cross-section width of the 2.25" duals (obviously). I think for street application for the 250 it should be at least single 2.5" up to dual 2" pipes, but I'd like to see the chart on that too. I'm planning a HD 292 manifold for 250 with a 2.5" single exhaust hooked to a Z28 post-cat split setup (gets progressively larger from 2.5" to dual 2" ending with dual 2.25").
