I don't generally respond to muffler questions on the internet, just because "sound" is just too subjective. I personally don't care for Flowmasters, but they sell thousands of them, so what do I know? lol. I've had a few sets of oval mufflers, different brands, on my 428 powered Shelby that I primarily drag race over the past 30 years, they've all sounded fine to me. But all of the aluminized ones eventually rusted out, and even when they had a lifetime warranty it was a pain replacing them. Eventually I decided to try a stainless steel muffler. There isn't a lot of room under a Mustang to have a muffler tucked up into the floor recess space under the rear seat and still have room for the exhaust pipe to turn up after the muffler to go above the rear axle without it having a very tight bend, so the length of muffler was one of the primary concerns for me over anything else. At the time I picked Edelbrock stainless mufflers that were a little shorter than most. Although the mufflers didn't rust out, eventually they ended up cracking badly, probably due to vibration. When I went to replace them a few years ago Edelbrock no longer made those mufflers and there were few choices in a relatively short oval stainless muffler. I found Magnaflow's and Borla's and I ended up picking the Magnaflows just because I think they were maybe a little cheaper, best I can remember. So far the Magnaflows are holding up fine. As far as sound I'm happy with them, I don't think they are loud, they do have a bit of a "rap" at full throttle. I run 2 1/2" pipe.
