Related; I've owned a number of inline 6 cylinder motors and currently have a flat 6 (boxster). I believe the math says the I6 is smoother than the boxer 6, but my perception is that the boxer is vastly smoother. I don't know how to account for that -- maybe because of where the motor is physically located, maybe it's due to the lack of a central driveshaft, or maybe it's something else entirely.