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post #1 of 10 Old Aug 17th, 2010, 7:49 pm Thread Starter
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Improved mileage!?

Heading back from the western mountains (Washington, Oregon, Montana, Wyoming and Utah) and I checked my mpg. 54-56 mpg! This is riding two up at speeds between 45 and 65 mph. Back home I normally get 45-50 mpg riding solo. I've compared mpg back home using two different fuels - 100% gas and gas with 10% ethanol. 100% gas gave me about a 10% boost in mpg. Average elevation back home is between 350 and 750 feet above sea level.

We've spent the past three weeks above 5000 feet. According to the fuel pumps, some of our gas was pure and some had the 10% ethanol. It didn't seem to make any difference as to what fuel we used. (Octane was always 91 or better).So, does the higher elevation cause improved gas mileage? If so, why?

In advance, thanks.

Chris

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Chris

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post #2 of 10 Old Aug 17th, 2010, 8:09 pm
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Re: Improved mileage!?

All computer controlled engines get better mileage at high elevation due to less oxygen being available therefore it leans the mixture to compensate.I might add that the engine also produce less power.It is very obvious on my R1200RT and my car.
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post #3 of 10 Old Aug 17th, 2010, 10:31 pm
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Re: Improved mileage!?

I ride regularly in the Sierra Nevada Mountains, at high altitude. Mileage always improves in the mountains, especially in the summer when the air is even thinner.

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post #4 of 10 Old Aug 17th, 2010, 11:01 pm
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Re: Improved mileage!?

Yup, mixture mapping by the motronic ECU leaning out the mixture across the rpm range as air density decreases. This can be effected by temperature ( minor but there) or elevation ( Quite dramatic)
As some other west coast riders have eluded too, with a fair amount of long distance 5000 to 7000 foot routes within the state and some 8000 & 9000 foot pass routes of lesser distance, our LT's get upper 30's down low in winter and mid 50's or better up high during the summer time.

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post #5 of 10 Old Aug 18th, 2010, 1:06 pm
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Re: Improved mileage!?

A motorcycle is a high aerodynamic drag vehicle with a comparatively powerful engine compared to other forms of transportation. You have to multiply all these factors together to compare fuel economy under different conditions:

Aerodynamic drag is reduced at higher elevations due to the reduced air density. It is the reason for higher temperatures giving lower drag too as air at the same elevation at say 100 degF is about 87% of the air density at say 30 deg F. But even the same temperature air density at 5000 ft elevation is about 84% of the density at sea level.

The pumping loss in the engine is proportional to (local atmospheric pressure minus the engine intake manifold pressure) X rpm. The local absolute pressure is reduced at higher elevations. Keeping the rpm down by reducing the use of performance gearing helps, too
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post #6 of 10 Old Aug 18th, 2010, 2:29 pm
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Re: Improved mileage!?

Nice discussion. By the way, Chris, you might be led to think that your engine is running "lean" because the computer "leans out" the mixture at high altitude. Not true. The computer senses less oxygen available at altitude, so it feeds less fuel. The result is the same stoichiometrically-correct air/fuel ratio at altitude, as you have near sea level. The engine runs fine, but because the computer is feeding less gas, your mileage is up and your power is down.

I once encountered a /2 Beemer at the top of Wolf Creek Pass in CO. My computer-controlled, fuel-injected Beemer was running fine. On his carbureted bike, he had less air and the same fuel. Result: rich mixture. He told me he barely made it to the top of the pass - his /2 was running lousy at altitude.

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post #7 of 10 Old Aug 18th, 2010, 5:36 pm
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Re: Improved mileage!?

Quote:
Originally Posted by niel_petersen
A motorcycle is a high aerodynamic drag vehicle with a comparatively powerful engine compared to other forms of transportation. You have to multiply all these factors together to compare fuel economy under different conditions:

Aerodynamic drag is reduced at higher elevations due to the reduced air density. It is the reason for higher temperatures giving lower drag too as air at the same elevation at say 100 degF is about 87% of the air density at say 30 deg F. But even the same temperature air density at 5000 ft elevation is about 84% of the density at sea level.

The pumping loss in the engine is proportional to (local atmospheric pressure minus the engine intake manifold pressure) X rpm. The local absolute pressure is reduced at higher elevations. Keeping the rpm down by reducing the use of performance gearing helps, too
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post #8 of 10 Old Aug 18th, 2010, 8:06 pm Thread Starter
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Re: Improved mileage!?

Thanks for all the answers. I think I have the picture unless there's going to be a "Pop" quiz on this. I did note that there was a reduction in "oomph" when I hit the throttle but attributed that effect to my riding 2 up. I usually ride solo.

So if I have this right, there is to be proper ratio between oxygen and fuel. If there isn't enough O2 then the amount of fuel needs to be reduced. So with the carb bike his engine was being flooded?

I also understand the air density factor. From my years of bicycling, I recall that at 12 mph, 80% of my energy was spent pushing wind. Negating this wind resistance is what let John Howard hit 150+mph back in 1985 on a bicycle that was drafting a vehicle.

One more question, there was a notice at our campsite in Wyoming that advised motorists to adequately warm their engines before departing. This, according to the sign, was due to the elevation. Am I correct in assuming that the warmer engine allows for a faster vaporization of the fuel and thereby increases the efficiency of the combustion/explosion?

Again, my sincere thanks for your answers.

Chris

Take care,
Chris

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post #9 of 10 Old Aug 19th, 2010, 5:42 pm
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Re: Improved mileage!?

... "So with the carb bike his engine was being flooded?"...

Yup. "Running rich" might be a better term than "flooded".

- Bob

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post #10 of 10 Old Aug 19th, 2010, 8:01 pm
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Re: Improved mileage!?

Quote:
Originally Posted by Tracus

One more question, there was a notice at our campsite in Wyoming that advised motorists to adequately warm their engines before departing. This, according to the sign, was due to the elevation. Am I correct in assuming that the warmer engine allows for a faster vaporization of the fuel and thereby increases the efficiency of the combustion/explosion
No. This came from some bureaucrat I'm sure. Yes there is a vaporization effect but the quickest way to warm up a carbureted engine is to get it generating power by driving it.

The fixed venturi carb relies on the air density remaining fixed for proper mixture - which of course it won't.

Fuel injection and processor controlled engines have eliminated the need to worry about higher altitudes on mixture. Although even the old carbs that had a variable venturi like the BMW airheads etc had some inherent compensation for altitude.
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