I have a 12 inch Cherrybomb glasspack on my jeep. Its got a nice rumble at idle and its loud driving down the road. Gave me more power. I have never been a fan of flowmaster. Its more for sound than performance. glasspacks are strait through and if you look at a pic of a flowmaster it has alot of obstructions to block flow.
Black '98 Wrangler, 5 speed, 4.0