Exh manifolds in an Early A...

Cuda416 said

We modded the crossmember for duals, so we're half way there. Just need to get from the exhaust manifold flanges down to the jegs 2.5" kit. I realize if we stick with the 273 manifolds, we are going from 1-7/8 to 2.5 so we'll be playing step up games.

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You will be going from 1 5/8" to 2.5, the resistance to flow is proportional to one over the radius squared, so if the radius (or diameter) is cut in half, the velocity of the fluid will decrease by a factor of 2^2 or a factor of 4. So in a pipe 1/2 as large, 2.5" down to 1.25", flow will decrease 4 times. This is true for fluids in a pipe I'm not sure about air flow but I do know it decreases dramatically.