that 7rwhp was probably attributed to many other things. ambient temp, oil temp, coolant temp, etc... you wont even see 7 going from stock to a dynomax.
2. It wasn't a muffler swap, it was a catback swap- I went from one catback to another- I didnt just weld in some mufflers-
3. I did a muffler swap on my 67 from 3 chambers to Magnaflows and my idle went up about 150 RPM because the car was breathing that much better-
4. www.mach1review.com If a Mag catback can gain close to that over another straight through muffler on a Mach 1- why can't I have gotten about the same gain over a chambered muffler on a bolt on 5.0?
5. Flowmasters sound great, but there is a reason why guys lost hp on 05 GTs after welding in 40 series-
6. Hot ROdding Engine Masters Challenge- the winner gained 28 hp by switching from Flows to Mags- I would be more then happy to give you the name and contact info for the winner if you would like to talk to him yourself