I hear ya on the 800... which, on the surface, seems like a No-brainer.
Just like the new "Quick drive" low-inertia drive has less "gyro" effect on the sled and accelerates faster...
The 64mm stroke of the 600 (compared to the 70mm stroke of the 800) just plain feels more nimble than the larger 800 and has a snappier throttle. The smaller rotating/recip mass of the 600 imparts less of this "gyro" effect and allows the sled to be thrown around easier. Very evident with smaller riders. Plus a carls 660 would be pretty hard to keep up with on a stock 800.
For me, in a PRO RMK chassis, if I were to turbo one... I would choose the 600 since I do not need to have the highest hp sled on the snow (that is what 12+ lb boost race-gas turbo 800's and Nytros are for). Riding a modern, well sorted, 600 turbo PRO will be one those things that make you go HMMM.
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