I'm in the same boat as you are. I'm currently running an X-pipe with flows. I also have heard that X-pipes are designed to work with straight through mufflers better. I'm going to be putting on a set of Dynomax Ultra Flo Welds in the next week or two. I've also talked to Flowmaster and they say that there mufflers are designed to work with H-pipes and not X-pipes. I have seen dyno runs where they compaired X-pipe w/ flows, H-pipe w/flows, X-pipe w/straight through, and H-pipe w/straight through. The diff between the X-pipe w/ flows and X-pipe w/ straigh through muffs was about 3-4 hp infavore of the straigh though mufflers. So hopefully I will find that extra 3-4 hp in a couple of weeks, if I don't find the extra hp then at least I hope for a better sound.
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2000 Mustang GT