The Q4 is the quietest, then the PowerCore4 is next then Its the speed and then 4.1 being the loudest. I couldn't tell a difference in power between the Q4 and Powercore but the PowerCore is noticeably louder than the Q
I was just wondering. They all look exactly the same and the descriptions on the FMF site are very vague. I wish there was somewhere to hear the different exhausts on the 700. exriders.com has an entire thread with just that. But it's just for the 400ex.