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I'm running them in mine, BTW.. luv 'em, and they are designed to fit inside 4" side pipes.. They use freeze plugs (1 1/2" cap)to block off the center straight passage. If you want x-tra flow/sound you just drill the approriate hole in the block-off freeze cap. I drilled a 3/4" hole and I love it! The trouble is.. I don't think the local authorities is going to share the same fondness for the side pipes... I'm going to reduce the hole from 3/4" to 3/8" and try it.. Jus' gives you an idea on where you might want to be, between quite to riot..

BTW... Great price and a quality product..