Quote:
Originally Posted by Boss330
We have probably been talking past each other in these posts. To me the reasons why a 458/Ducati 848 engine is oversquare and a high torque low rev engine (such as the HD 883) is undersquare confirms my view on the difference and advantages of over- vs undersquare.
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I genuinely appreciate your belief, but the fact is that we still have no mathematical model that relates stroke length and bore size to torque at a fixed displacement.
What we do have are the equations I was sent offline and posted earlier which I think make for a good start. NISFAN suggests that while the formulas cover the mechanics of the rotating assembly, they ignore the combustion process which he contends is effected by the stroke length. So there is more to the picture. It may be too complex to boil down to a simple set of equations - I don't know.
Either way, absent of proof, I have to remain where I was before the discussion which is that I am not convinced one way or the other about how the bore dimensions effect torque. What I am fairly convinced about, however, is that a naturally aspirated engine approaching 90 ft-lb per liter has got to be coming close to what is physically possible or, by now - in the past 100+ years of evolution - we'd have seen that number go up and up and continue to as time goes on and engines have become more advanced. So, then if there were anything that could be done with the stroke to make a big advancement it would have occured by now even if it meant something seemingly ridiculous like using a 200mm stroke with a tiny 30mm bore. In other words, I think that in the end you reach a point of diminishing returns at a given displacement so rather than spending money on working out tiny gains at that displacement, an engineer will simply increase the displacement which could be done either by increasing stroke or bore (or both) since either will accomplish that.