At what point would this (3.5 in. single ) be too restrictive? I will be running more than 600 rwhp. I am more concerned with detonation than horsepower loss. 383 F1

 

you'll probably be a very mild restriction....

may as well step up to the mufflex 4" setup if you're already planning the 3.5"

 

I installed a Mufflex 3 1/2" system. 97SS 383ci - D1 MM6. I thought I had ordered the 4" but messed up. Anyway it fit fine. The 4" will be tight over the axle. The SpinTech 3 1/2" muffler is restrictive to flow (not straight thru). I would advise a different muffler option. So, I added a 3 1/2" QTP cut-out. Helped alot. I also run slp shorts and a Randon dual cat Y. The Random cats & Y flow well. I had to make alot of fit and adjustments to the Mufflex system due to the pipe pressure blowing the slip joints apart, and, the band clamps really don't work that well (before QTP). Turns out I really like the QTP cut-out and lets the motor rev free thru redline.