I just got a drag gen 3 turbo kit. I also have an 3" downpipe. The thing is the shop I went to (ace muffler) won't put on an exhaust system without a cat (i dont want to buy a new obd2 cat) So im thinking of running my 3" down pipe to the stock cat then up to a 2 1/2 or 3" cat back exhaust system. Will the stock cat cause back pressure or hurt my performance?

If it does im thinking of just having the shop make me a 2 1/2" or 3" cat back system but then myself just take out the catalic converter and put in a new pipe to connect from the downpipe to the rest of the system they put in. Also alot of people prefer 3" piping all the way out with turbo, but wont 3" not give it the pressure it needs? btw im planning on running around 9psi on my ls

 

The stock cat will kill the spool..
Get a 3" high flow cat from jegs for under $70..

And with a turbo you want free flow..Not like na where you would want backpressure for low end power..Turbo spools better with big straight through piping..