The cool thing about them is you can cruise along at say a steady 2,500-3,000 rpms and they are not bad at all, but when you romp on it WOW, its a deep roar.
I would not say they are quiet, and would now classify them as medium loud at cruising and very deep and loud at wide open throttle. Not for the faint of heart. Most importantly that annoying rapp papp papp papp sound is gone now, like going from Harley motorcycle slash cut pipes to dual Thunder Headers if you like that deep sound. I sure do. Much easier on my ears on longer drives at crusing speeds. Your cam size has something to do with it too though. Larger cams do get a bit louder with these pipes.
So far we have about 10 of us that have tried them. So far so good.
