When looking for power, single-cylinder engines become less and less effective with increasing displacements. Longer stroke mean that you are limited to, comparatively, slow engine speeds due to material constraints. Also, single-cylinder engines are, naturally, less balanced than other configurations - which necessitates considerable counterbalance shafts in order to keep vibrations at a tolerable level. Compare the displacement per piston between a diffent bikes: 450 ccm motard = 450 ccm per piston, 650 ccm pro-twin = 375 ccm per piston, 1000 ccm superbike = 250 ccm per piston, 600 ccm supersports = 150 ccm per piston. Now correlate these figures with the specific power output (i.e. hps/ccm) of these bikes - data I haven't got at hand right now - and I think you'll see an obvious trend: larger piston == less specific power. And yes, you can vary the stoke-to-bore ratio and play with this - however, the sensible range of variation is somewhat limited if you want a well functioning engine.
As a result, increasing the engine displacement by 50% going from 450 ccm to 675 ccm is not going to give you 50% increase in power, staying with the same engine configuration.
Also, the lower capacity engine has two benefits: a) it's lighter - especially the dynamic mass - and as a result will be more lively and b) it will, generally speaking, be more smooth and will have a wider dynamic range - i.e. redline is higher.
At least that's my take on it.
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Originally Posted by NordieBoy
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