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I have not had a chance to compare a metro xfi muffler to a normal metro muffler, but I have been able to compare a crx hf muffler to a normal crx muffler and the higher mpg crc hf has smaller exhaust pipes all around, and that would appear that the people who designed it read the same types of books as I have on the shelf that say that just like air intake, you don't want to large around if you plan to run low rpm's, it's not restriction, it's a matter of maintaining air velocity as air velocity in a proper sized tube like a exhaust pipe is kind of like having the air equivalent of a fly wheel that is helping to pull that exhaust gas out of the engine.
the only way I've seen a cherry bomb muffler help with gas mileage is tha they are so loud people drive less and hey drive slower so they don't get a ticket.
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