I have a stock engine FD with a PFS catback and want to install a downpipe to reduce heat/back pressure. Will the sound level increase much, as it has with the PFS catback. Also what happens to the air pump if the cat is removed?

 

I dont think youll hear much if any differance with just a downpipe, and the airpump is usless if you have no cat, so you can either leave it sitting there or take it off.