Quote:
Originally Posted by dcb
given acceleration is largely a function of power to weight, you might want to do a *little* more analysis Ken  |
I gather that you misread my post. The OP is not asking about power-to-weight ratios, but instead about engine fuel efficiency, which is expressed (in the US) in pounds of fuel consumed per hp-hour.
Motorcycle engines (excluding Harley engines, which are better suited to lawn mower use) have been developed mainly for high specific power output (high hp per cc). Car engines, however (partly because of much tighter emission regulations and some market demand for fuel efficiency) have been developed for high specific efficiency (i.e., low brake specific fuel consumption, BSFC). Even motorcycle engines, however, are better than they were when two strokes were on the road. Many two strokes consumed .6 lb/hp-hour. Now, motorcycles are around .5, although, as Oldmechanic pointed out, some are around .45.
Expressed as efficiency in percent .5lb/hp-hr is 25%. Motorcycle engines and general aviation engines fall into this area. .4lb is about 31% efficient. Most current car engines are pretty close to this. .35lb/hp-hr is about 36% efficient, just a little lower efficiency than the Prius engine. (The Prius engine sacrifices specific power output and low speed torque to gain this efficiency level.)
All these are peak efficiencies. Because variable valve timing is common in cars and rare in motorcycles, the off-peak efficiency differences are even greater. Unless you have an actual BSFC maps in front of you, a simplified method of getting a feel for consumption vs rpm is to take the dyno torque curve and turn it upside down. Motorcycle engines tend to be peakier, with narrower torque curves. Their efficiencies also tend to fall off more quickly when they are away from peak torque.
If the engine is installed in something other than a series hybrid, then the off-peak efficiencies become especially important, because in non-hybrids, engines spend most of their time far away from the peak efficiency areas. Thus the often quoted 15% (or so) actual operating efficiency of the typical car engine.
As an old motorcycle racer, I definitely agree that motorcycles offer far more fun per dollar than cars.