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Discussion Starter #1
It's interesting (to me) to speculate on how the various producers of parallel twin motorcycle engines arrive at their bore and stroke dimensions.

Obviously, they decide on a displacement, then choose a bore and stroke combination that would result in it.

The older 650 parallel twins went with small bore and long stroke while the more modern motors go with a shorter stroke and a larger bore.

For example, the old, air cooled Triumph 650's had a 71mm bore and an 82mm stroke. The modern Kawasaki Versys 650 has an 83mm bore and a 60mm stroke. The 650 Burgman engine more or less splits the difference between the two by using a 75.5mm bore combined with a 71.3mm stroke.

In fact, most modern parallel twins go the big bore, short stroke route. The 790cc Triumph Bonneville motor uses a 86mm bore and a 68mm stroke-- a 790cc motor with a stroke that's 3.3mm shorter than that of the 650 Burgman motor.

It makes me wonder why Suzuki chose to give it's 650 motor a long stroke with a relatively small bore (by modern standards) to achieve its displacement.
 

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It makes me wonder why Suzuki chose to give it's 650 motor a long stroke with a relatively small bore (by modern standards) to achieve its displacement.

Sitting here studying a cutaway of the engine unit, I think I've answered my own question.

I was thinking in terms of torque and RPM,..and the simple answer to the question is, Suzuki engineers chose the bore of the engine simply because of overall size constraints. To keep the engine assembly narrow enough to fit in the compartment they planned for, they had to keep the bore small and compensate with increased stroke to achieve the desired 650cc displacement.

 

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It may have to do with torque as well. A longer stroke engine typically has more torque and this plays well for the CVT, which typically keeps an engine in its largest torque band.

Greg
 

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Being a HotRodder for 50 years now, IF I could hotrod a Burgman 650 engine I would sleeve the cylinder down in bore size and offset grind the crank for a longer stroke. Make it a 550 to 600CC with lots of lowend torque and up the gear ratio. :D
 
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