Newbie
Seeing as how Flowmaster let the patent expire on their 40 series muffler, there are literally hundreds of great copies. I recommend a Thunderbolt all welded muffler. Great sound. We sell the hell out of them.
CF Veteran
I have a flowmaster 40 with 2.5 inch pipe and dual exits. Its probably the deepest sound you'll get out of the 4.0