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Search on this forum for exhaust sizing and you will find it talked about 100's of times along with the math to back up sizing your exhaust for the size and RPM of your engine.
The question about the cam, the cam is the big difference between the cars that get 50+mpg and the cars that get 30mpg, of course you it helps if you have gearing that takes advantage of the high torque at low RPM's.
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