Does anybody have experience with the QP muffler? They claim money back if you don't gain mpg. Are they quiet? TIA

I'm also wondering what the cfm would be on the qp's ... friend of mine had a set that slipped down inside his pipes .... Im gonna haft to call him to verify his fuel mpg results, but i do remember him sayin turbo spooled quicker?... he sold that truck and moved west , gives me a good reason to call..On GR's web site i noticed that over a certin horsepower they recomend 2 mufflers and size reqiurements ... maybe because of cfm?.. have to call them also.... the donaldsons work very well had a few of those... also tried walker mega flows once they had a really nice sound and flowed well... the mf's were a bit more money than donaldsons dont think ya go wrong with either... just outa curiosity would like more info on qp's