There's a post by by me somewhere on the board about me going from twin Akra's to a 2 into 1 and the transition in how the motor behaved was extreme if you own a 660r and looking to go off-roading etc then it's the pipe for you because it turns the bike into a real old fashioned thumper,but if you looking for quick revs and and max HP on your X give one a miss,I didn't ride more than a couple of miles got home and took it off.