You need to understand that exhaust when HOT has a large volume(area)... most SB can use a 3 1/2" collector but as you go down stream the exhaust cools and reduces in volume(area) and the cooler the exhaust is the easier it is to control the sound... the farther back you put the muffler the better off you are... if you notice that the new Hemi cars the mufflers are at the back bumper.. we did that for power, sound and the reduction of back pressure