Jerry Eichenberger
Serious Thumper
Offline
2006 S40. OEM windshield, saddle bags, Sportster
Posts: 2919
Columbus, Ohio
Gender:
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Understanding fuel consumption characteristics is beyond a simple posting.
Engines burn a mixture of air and fuel. So, how much fuel you burn to produce a given horsepower is a function of how much air/fuel is going thru the engine, the mixture setting (rich or lean) and the rpm of the engine.
For instance, in piston airplanes with constant speed props, you can increase power production by either increasing rpm, or what's called "manifold pressure", which is the amount for fuel/air being shoved thru the intake manifold, or by increasing both rpm and manifold pressure. Throttle controls manifold pressure in such an airplane.
Our bike and car engines don't have a way to control rpm independent of manifold pressure. So the only things we can do relate to throttle setting and mixture. We already know that the mixture on a Savage/S40 is too lean to begin with.
Hence, since throttle setting controls rpm, the only practical way to increase mileage is to reduce rpm. Remember that the engine is burning fuel each time the piston goes down on the power stroke, so we need to reduce the number of power strokes, or rpm, per mile.
You could do that by running at a slower engine speed by running at a slower vehicle speed. That's why the best mileage is obtained at the minimum speed to run in 5th gear effortlessly, without lugging. But better, you could reduce rpm by going to higher gearing, so you get more rpm of the rear wheel, and hence more distance traveled, for a given engine rpm.
However, we all know that higher gearing results in poorer acceleration, so gearing ratios are a engineering design compromise (as every machine is) between the desire for acceleration, and efficient engine rpm at cruising speed. You can't have both at once.
To improve the mileage of any piston powered vehicle, just gear it up (assuming engine displacement remains constant). Since there is no free lunch, you'll pay for that with slower acceleration.
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