I have an '85 Camaro Iroc-z with a 305 Tpi. I am getting a flowmaster muffler put on but im not sure what size pipe from the cat back would work best for my car. I was thinking 2 1/2" would provide enough back pressure but some people are telling me go with the 3". The car is mostly all stock if that helps at all. thanks

 

you should get a 3 in. madrel bent catback.
back pressure=bad performace
free flowing=good performance