Fuel Consumption Suddenly Decreases

[Boxer]

I have the F650GS twin which is the 800cc Rotax engine detuned to lower HP. I've been casually watching the fuel mileage and it seemed to be getting an average of somewhere around 51-53 mpg. Now, with over 5,000 miles, it suddenly began getting in the 60-63 mpg range. The riding weather has changed somewhat during that time from very cold to quite warm, and the higher mileage figures seem to coincide with twisties riding and less highway miles. You would think it would be the opposite, however, on the highway I run pretty constantly around 70 mph, where on the back roads I keep the rpms up there but am running more like in the 50 mph range most of the time.

Any observations on this phenomenon concerning the 800cc Rotax engine?

[CycleRob]

I noticed the same differences. As long as the throttle openings are small and smooth, it will get great fuel economy. It has everything to do with wind resistance. Wind force (or wind power available) increases as the cube (3rd power) of the wind speed. The difference between 70 and 55 MPH is (343000/166375)=2.06 times the wind drag. That means 70 requires almost 2x the cruising engine power of 55 (MPH or kph).

Just going down the road steady speed in 4th, 5th or 6th at 55 MPH the MPG's don't change that much because of the excellent engine management system. Specifically, the wide-ranging responsive ignition advance curve and the refined EFI mapping at part throttle cruise in any gear. You notice this when you ride the bike in the lower gears and use very tiny throttle openings. It responds! It noticeably contrasts to the OilHead's comparatively lazy response to small throttle openings because of it's essentially "centrifugal advance only" type ignition curve. The Rotax motor also adds the load sensitive throttle opening "Vacuum advance" component, logically recreated from the throttle position + RPM. I have seen this occur on cars with an intake manifold operated vacuum advance where the timing light will show well over 55 degrees advance during no or low load high RPM conditions. The greater lead time is needed because the mixture is lean and partially filled cylinder dynamic compression pressures are low. That hyper ignition advance greatly improves the MPG's without pinging.

THAT is the primary reason that makes the newest generation F, K and R model BMW's so much fun to ride.

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