I'm not sure what it is exactly. Is it a replacement for a catalytic converter? Or does it go behind the cat?

 

There are 3 major parts to the RX-8's exhaust system:
1) The header
2) The catalytic converter
3) The muffler

They connect together from the engine out to the tailpipes.



A "midpipe" is the "middle pipe" and replaces the catalytic converter in the line.

 

mid pipe is what we call the part that replaces the cat. they can be straight through, have one or two resonators and even a hi flow cat