i am curentlly runing svt header cat and flex. But i am still runing my stock cat back. the svt cat back is 2 1/2 inch. i think atleast that is what it is at the flex pipe. I went to the exaust shop today to see what my options are. they told me that if i put 2 1/2 inch pipe on my car it would rob the car of power. I had them cut out the stock muffler and put in a strait pipe and now only have a resignator. Its really loud and i dont like it. Didnt seem to help with power at all iether. any one have any ideas let me know.