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I was also wondering, performance wise how aftermarket mufflers compare to stock factory mufflers? I've heard not enough back pressure or to much back pressure hurts performance, do these aftermarket mufflers come with spec sheets to compare to stock factory mufflers? I see a lot of used diesel mufflers on Craigslist that are the same shape and size of my stock factory muffler, can I use a 4" diesel muffler reduced down to 3" pipe on my 454?