[vin63]

Originally Posted by SactoJim

You do confuse me on the "H" pipe maybe being a restriction.....I have dual 2.5" exhaust all the way to the back with a 2.5" crossover pipe (H pipe) about 12" beyond header collectors.....don't understand how that could be any kind of restriction. Appreciate the info......Jim.

I used to work closely with the engineering group that was responsible for the exhaust system designs and testing at Gale Banks Engineering - we did both CFD modeling, flowbench testing, and chassis dyno flow analysis on every exhaust system imaginable. Basically, H-pipes create turbulence in the flow, which creates backpressure. This is OK and in certain applications purposely leveraged on a street car system that operates at lower RPMs the vast majority of the time...it reduces a certain frequency of exhaust noise. At the upper RPMs it will probably restrict more flow than what a larger set of primary headers will provide in flow gain.

The best way I can describe it is...it is commonally accepted that if you are driving at high speeds, a vehicle with its side windows down/open is less aerodynamically efficient than with its side windows up/closed. It's the same principle. Which is why you might see more van trailers on the road with deflectors between the kigpin and the rear wheels to reduce the wind turbulence and help improve fuel mileage for the tractor pulling it.