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Avoiding the science, my expirience is going from a 1"7/8 crush bent pipe to a 2"1/4 mandrel bent pipe and didn't really see any noticeable improvement in FE. However, going from a CastIron log style exhaust header to a tuned, tubular, header did make an impact on FE, that is in addition to the larger exhaust pipe. Not a big impact on FE, mostly because of better torque down low has helped to decrease the amount of time in the Pulse phase of the Pulse and Coast routine. Consequently my throttle body is about 2"1/4, however the exit from the header is 2"1/2 back to the flex pipe.
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