Quote:
Originally Posted by stillsearching
Most of all I would LOVE to have SFC figures for modern bikes. For instance on cars it's pretty much a given that they'll be the most efficient fuel use, per HP used, maybe around 2000rpm or so. At 1000rpm it will suck. At 5000rpm it will suck. But bikes have such an incredibly wide range, like the Ninja 250 with what, 13,000rpm redline?? I don't know where the ideal part of the curve is for such an engine. I would like to have something with enough power and gear, to put it in that sweet spot of the curve for 55-75mph range approximately. If a 500 is more efficient putting out a 15-20hp load than a 250 is (being run too hard) the 500 will probably get better mileage under those conditions - this i've finally understood.
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Bikes have no variable valve timing, so there is considerable (undesirable) EGR at lower loads for quite a chunk of the rev range, and this is probably the main thing that robs efficiency. In car engines the low rpm combustion characteristics are better, but the proportion of friction power increases a lot faster. Looking at a dyno chart of a CBR600RR, the torque seems to pick up a lot at 3000, going to 30 lb-ft or ~45Nm by 4000rpm, and 75Nm/L is around what "Atkinson cycle" engines with a similar intake valve closing point pump out, which is a good sign. So maybe like 4000-7000 is a decently efficient range for applying heavy throttle and you could get away with cruising at 3000 efficiently. For other bikes maybe try to observe where the torque has a similar steep ascent, that probably indicates combustion efficiency picking up. I would guess that it scales mostly with the stroke, since bike engines tend to be designed with the highest possible rev limit and are given cams to suit, though obviously an FZR600 that's basically a detuned R6 probably will be efficient at a considerably lower rpm.