i would have thought that the torque pulses when you get much over 1000cc would start to get in the way. ive a dr big and suzuki reccomends that you dont run round in the higher gears with the engine at under 3500 rpm. the jist being that the power pulses from that big slug will destroy the big end plain bearing in pretty short time.even with the balance shaft, its still a big slug bouncing up and down in there. the dr is an odd beast, even though its a single, it needs to be revved like a multi to let you get anywhere. so a huge 1000cc thumper would sound like more bother than its worth. after all, if a single is supposed to be simple and grunty, it defeats the object to first have to install balance shafts etc, and then have to rev the engine because plodding round at tickover will make it grenade. having had thumpers of all sizes, i have to say that the best engine ive had in any of them was my old sr500. the dr is just too big!
