I'm dealing with some of those same issues currently. I just finished a solid roller Cam Motion 383 with AFR 220's and running through a 3" intermediate and Borla XR-1. Power is way down currently and the car isn't making power past 6k rpm. We are going to drop the y-pipe and see what effect is has on power. I'll report back but I expect some pretty big gains (hoping). I think on larger cams with a significant amount of overlap, that the exhaust back pressure makes a big difference. We'll see when the car is dyno'd again by the end of this week and I'll let you guys know what we find when the guys at Thunder get back to me.

 

Just as an extra tidbit. . . my last 550 rwhp combo gained only 5 rwhp when dropping the single 3" Hooker Aerochamber.

 

Just as further support of the oppinions already listed:

On my car the difference between full exhaust (Ghetto 3in y pipe, stock I pipe and flowmaster muffler) and open cutouts was 31rwhp. We couldn't figure out why the car wasn't making what we thought it would, then we opened the cutouts and it all made sense. 31rwhp gain for the car N/A.

Flowmaster =