My 99' GMC w/6.0 currently has a flowmaster single muffler 2 in-2 out setup with the pipes exiting at the rear. I have a pair of 2 1/2" glasspacks that are 28" long sitting in the garage. My muffler shop guy (Jason) told me that my current system is about as good as it can get and that I would lose alot of bottom end power with the glasspacks due to lack of backpressure but I would see a gain in the peak revs. I was going to have the glasspacks installed with a balance pipe a foot or so before until he told me that. I don't care for the Flowmaster drone but I don't want a loss of low end either. Any suggestions?

 

I would stick with the flowmaster.

 

Welp, I was running my stock intermediate pipe (2 3/4") with a 12" glasspack welded into it just behind the y-pipe to get rid of the drone from my dynomax muffler. but after I used my hpp3 to raise my shift points up to around 5900 rpm I noticed that it was nosing over around 5600. so I ordered and installed a 3" pipe from dynomax. Now it pulls hard all the way to 6000, but I can tell I lost a lot of torque down low. From what I understand the flowmasters are pretty restrictive so you'll probably lose a little torque if you switch to anything else.

 

I'm thinking maybe just get a couple of Hooker aerochambers to replace the Flowmaster. I don't know for sure but i think the Fmaster is a crossflow design. The Hooker is supposed to be engineered to keep the torque and give gains up top as well. Anyone wanna buy some unused glasspacks?