Dynomax makes bullets with a 12" shell and 18" shell.
Whats the deference they make sound wise?

 

Its the simple balance of noise characteristics. The smaller, the louder it will be. The larger, the more tamed it will be.

 

Just installed, replaced Flowmaster that rusted off rear of muffler after 6 yrs.
Quieter than flowmaster now but breaking in. Much better clamps, 2 double hangers. Had to exit straight back. Welder & mod to parts required 3hr install on lift.

2002 Supercrew 5.4.