exhaust manifold design question . . .

[mikedc]

Why do OEM cast iron manifolds neck down so small, so quickly after leaving the cylinder heads?



Let's say a hypothetical car's main (dual) exhaust pipes are 2.5" diameter. Why not shape the iron manifolds to have a 3" collector opening at the bottom, and then reduce the exhaust pipe size from 3" down to 2.5" a bit farther down (perhaps 1 foot)? It seems like that would give the exhaust system a far less brutal bottleneck. It would mimic the overall stepping-down effect of a set of headers better.

Granted, what I'm proposing would cost the OEMs a couple more dollars per-car. But uncorking the exhaust manifold area is very low-hanging fruit for a production drivetrain. Wouldn't that be worth doing, at least on hi-po models?


I know, there is a penalty for using an exhaust pipe any larger than necessary. It kills the gases' velocity and hurts the engine's low/mid torque. But there's just one problem with that argument here - the hot rod world is full of cars running 3" exhaust systems the entire length of the car. It's pretty clear that going 1 pipe size bigger than the OEM system is not enough to cause a practical drawback.