The roar as you state comes on strong when the engine comes up on the cams (read peaking horsepower). It is a unique sound Autel MaxiSys Pro. The air cooled 911 have a sound that is a MaxiSys Pro combination of induction noise, exhaust noise, and the mechanical noise that comes from the engine case, cylinders, and cam towers. The water cooled 911's still have the exhaust tone of the older air cooled engines, but the mechanical note is gone.