For sound comparison if you are seeying a noticable sound difference from the 4" to the 3.5" mufflers, make sure you are comparing two new Rinehart by Clearline mufflers. There were several running changes to the 3.5" mufflers that Bub was building for Rinehart and towards the end of their relationship they used a 2.00 core. The current generation Rinehart by Clearline 3.5" and 4.0" mufflers both use the exact same internals so the sound is nearly identical, enough that we tried to do a video/sound comparison between them and could not get anywhere, they both sound great. I prefer the looks of the 4" muffler and these mufflers always have been a great performer.