The Stainless Works resonator that I have is 6" in diameter and, honestly, it's a bit large for our exhaust tunnel. It hangs down a small amount past the undercarriage. That, couple with the fact that I'm lowered on Gen 2 SSC's means that it doesn't like the large speed "humps" very much. While it hasn't been damaged and I've never had any problems, it's size is a bit of a concern. It'd probably be best to stick with a 5" round or smaller, but 6" does fit.