My piping is 3.5" (I think, the car's kinda low for me to crawl under and measure) and my exhaust tip is 4.5". And it's still quiet. The trick is finding one that has the resonators installed in the right spot. If the distance betwen the two is right, then most of the noise is cancelled out, but flow is not reduced at all. And if it's made form titanium, THAT'S a quality part.
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