I have a 71 chevelle with a 350sb pulling about 425 hp. The exhaust is headers ( not sure of pipe size) and 21/4 pipe with standard turbo mufflers. I am going to change over to Flowmaster system. Should I go to 21/2 or 3 inch pipes? Also should I go with 30 or 40 series flowmaster mufflers? If I change to this system should I also change the headers to bigger tubes? Is anyone running the H pipe and can you feel a difference?

I'd go minimum 2.5" exhaust pipe with at least an H pipe if not an X pipe...I have no experience with flowmasters, but I love my Dynomax Ultraflo welded ovals graemlins/thumbsup.gif

2.5" flowmaster 40 series worked on a 454.
The 40 series sound absolutely wonderful, not to loud, not to quiet...just perfect :D
Chris

2.5" tube, no larger...pick a good free flowing muffler like a Dynomax Ultraflow or a magnaflow...straight through design.

some people have said that they dont like the flowmaster and that they create a resonant droaning sound at certain rpms. I supose you could use the flowmaster pipes and any other muffler.