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I've always been under the impression that high-flow exhaust could only add benefit (mpg gain) over a certain RPM/speed. When I sourced out the exhaust for my car, the guy doing it put all of my info into a calculator to decide what I needed (stuff like header diameter, # of cylinders, rpm threshold, HP, etc).
I could very easily be wrong, but I've always thought that reducing "back pressure" would hurt on the bottom end more then it would help on the top end (for economy anyway).
Would love to see some actuall verifiable results either way though.
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