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Old 12-30-2012, 09:17 PM   #86
because I can
Joined: Sep 2010
Location: San Francisco Bay area
Oddometer: 8,015
Originally Posted by motu View Post
I just calculated it all out, and then lost it all....should've written it down the hard way. Anyway, my final point was that bore/stroke is about piston speed, and you can't compare a 650 to a 1,000cc engine....better to compare the R65 to a Triumph 650. At 7,250 the 82mm stroke Triumph has a piston speed of 19.8 m/s, or 3900 ft/min. The 61.5mm R65 is doing 14.86 m/s or 2925 ft/min, at 8,000 the R65 is 16.4 m/s or 3228 ft/min, at 9,000 18.45 m/s or 3631 ft/min.

At 9,000rpm the piston speed of the R65 is lower than the Triumph at 7,000rpm - and the Triumph was considered to be a revver in it's day.
At 8400rpm a R100's piston speed matches a Triumph 650's average piston speed at 7250. My point is that the valve train which is almost identical between a R65 and a R100 is at its limit well before piston speed is on a R100, let alone a R65. The valve train is the limiting factor in both engines. Since piston speed is really not the issue with airheads, why not run as much stroke as you can for more torque. IF piston speed was an issue before the valve train, I would be looking to run a shorter stroke than 70.6mm but . . . .
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