x1glider wrote:
BTW, regarding your long post about rod ratios and different accelerations either BTDC or ATDC has more to do with cylinder offset. It doesn't matter what length con rod you have when the cylinder axis is directly over the crank axis, the piston acceleration will always be the same, you just may have undesirable thrust loads on the piston side walls.
When you offset the cylinder axis away from the crank axis, that's when you see differences in acceleration. Forward (usually 2-4 mm), the compression stroke slows down more and has more dwell just BTDC allowing for more combustion heat to accumulate and allowing retarded injection and firing timing. As such, the piston has a better angle to push down on at the big end journal, allowing for more leverage on the crank, giving more torque and it also accelerates faster on the way down.
Easy to engineer in with an inline or with offset journals on vees. Can't really do it on a twin with a shared rod journal. Move one axis how you want and the other moves in a direction you don't want. You'd have to change the angle of the faces of the barrels in relation to the crank axis to simulate an offset. Our Buells cylinder axis is pointing directly at the axis of the crank so that at TDC, the piston is always pushing directly down on that axis where there is no leverage.
I think you are referring to 'desaxe' engines like the MT-09 and 07 which have offset cylinders.
I also read once that US versions of the XS650 had shorter rods than UK models!?
I rode a KTM 950, 990 and 1190 V, but the 990 SMT? was the best in my view. My XB9 as it is now is still more fun overall even if the 990 is faster and more comfortable long distance.
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