I wouldn't go over 2.5" at the outlet. The larger the diameter the slower the gasses move, the quicker they cool and become denser and thus you lose the benefits of scavenging and create back-pressure.... despite what you may have heard or read, back-pressure is NOT good.... Then there's the simple fact that too large of an exhaust and the absence of devices used to reduce turbulence and smooth exhaust flow just makes an N/A sound like shit and you look like a clueless idiot (varies by area :P ).
2.5" and a good helmholtz type resonator (placed as far forward as possible but after the cat and a straight flow muffler with a spun stainless steel insulator, not fiberglass (like Borla's) works very well for the 7M-GE.
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