What is the best Cat-back exhaust system for a 2004 Pontiac Grand am V6 3.4L DOHC

Even though you seemed to have swapped out your 2004 engine for the mid '90s 3.4 DOHC, replacing the exhaust from the cat back is the same as if you had not changed the engine. As the DOHC has more power and torque than the stock 3.4 OHV, you will probably want the highest flowing cat back available. Look for one that has the largest diameter piping with mandrel bends. That is the one to go with.

borla is the only one that makes a cat-back for the grand am. slp used to make one but not any more nnngood luck