Dobermansx said:
Cat's are nearly always the problem...cut it off and straight pipe it...if you can get away with it.
Forget cutting the cat off, Dallas is an emissions county. If you don't want to run a cat in Texas, get a 78 bronco. The 78's never had them.
If you are looking for a performance increase by simply changing a muffler, then you are barking up the wrong tree. The crimped and restrictive stock Bronco exhaust pipe is more an issue than the stock muffler. Putting a better muffler on the stock pipe is like putting a bubble in the end of a soda straw. It is not going to do much for flow, but it might make it loud. Putting a high flow cat in place of a properly operating stock cat will not help much either, same situation. The stock cat is not the retriction issue either. It is the pipe!
Forget about the scavenging effects that they used to sell you the chambered muffler. The catalyst and crimped pipe on a FSB act to dampen the pulsations a chambered muffler needs to improve scavenging. Add this effect to the length of pipe from cylinder to muffler, and the net scavenging is almost nill. On the bright side, most mufflers in the performance category flow well, chambered or not. So my advice is to buy a muffler based on sound preference and fitment.
FYI, an ideal system for a mild FSB would be 2.5" diameter pipe from the manifolds coming together into a good 3" single system all the way to the tailpipe. Use whatever mufflers you want and whatever catalyst the EPA will let you get away with. You could also do true 2.5" duals, but duals don't fit easily on FSB's.
