I have an 09 cobalt ss i wanted to put a downpipe and pipeing all the way back but leave stock exhaust want to leave it quiet what size piping should i go for and will it be way louder. only planning on putting cai and a tune. Thanks for any input.

 

Just put a 3" DP in and leave the rest if you want it quiet. The stock catback won't hurt performance...

 

This

 

I agree with them.