gas mileage and altitude

[barenekd]

mattsz wrote - Someday I'd love it if you'll tell me about why it is that sea-level mileage should be worse than high-altitude mileage.  Another time.

It's pretty simple really. The sea level barometric pressure averages 29.92 in/hg. The optimum fuel mixture 14:1 air to gas.
The barometic pressure decreases at a rate of about 1 in/hg per 1000' elevation increase. At 9000' the barometric pressure would be down to about 21.4".
In air pressure terms, the pressure at sea level is about 14.5 psi. At 9000' it's about 10.5 psi.   
As you gain altitude and the pressure decreases, the fuel flow needs to reduce to retain the 14:1 ratio. With a good fuel injection system that is taken care of automatically by the O₂ sensor. With a carb, as you gain altitude, the bike just runs richer until you need to change jets.
However,as you gain altitude, the lack of air starts taking a toll on your horsepower, so at 9000' you can only get about 75% of your sea level power. With the fuel injection, this will cut your gas flow considerably as the air pressure reduces. Even with a carb, since there's less air, it won't pull as much fuel though the jets as it does at lower elevations. Hence better fuel mileage at altitude.
Airplanes thrive on this as the higher they go even with the big power drop, the drag drops even more, and the planes go faster than they will at lower altitudes while burning less fuel.
Jets at 35,000 feet burn about 20% of the fuel than they do down low and can go 200 mph faster. That's why turboprops usually fly the short runs, because jets are very ineffecient at low altitudes. The propellor engines work better below 15,000 feet.
Hope this helps.
Bare
Bare

[Ducati Scotty]

Last year I went up to Timberline Lodge near Portland, OR.  It's at about 6,000 ft up.  Now the road was sloping on the way up but it wasn't anything horrible.  I couldn't get past 45mph even with the throttle wide open.  The slight incline and lack of oxygen do conspire against you!

Scott

[idk]

That is a good write up barenekd. I haven't ridden my G5 at altitude yet, but I imagine that lack of power at the top of Mt Evans (14,000') could be an issue. I got the best mileage yet in my RIII riding up there.

As far as carbs go, the old SU and Stromberg type carbs tended to compensate automatically for low air pressure because of their design. Less air pressure = less airflow = less lifting of the carb piston = less fuel. Simple but effective.

[mattsz]

Thanks Bare!

But - hmm.  You can't get something for nothing.  There's got to be a trade-off... is it the reduced horsepower and speed?  If I never drive faster than Scotty's high-altitude maximum of 45 mph, at sea level, I wonder what would my mileage be like?

I'll have to ponder this some more...

[gremlin]

..............If I never drive faster than Scotty's high-altitude maximum of 45 mph, at sea level, I wonder what would my mileage be like?...........

IIRC wind resistance is a log function  (squared for 2x speed)  so .....  staying below 45 MPH is going to do wonders for your economy.   ( and your exhaust chrome )

[barenekd]

If you ride at 45, you will break 80 mpg. High or low altitude. DOn't use more than half throttle at sea level, though.
Bare

[mattsz]

DOn't use more than half throttle at sea level, though.
Bare

Well, I would do that Bare, but a wise man I know of refuses to tiptoe through life only to arrive safely at death...   

[barenekd]

I found that out the first time I rode the bike and was breaking it in on the ride home! I got a bit over 80 mpg on those first few rides. I haven't accomplished it since. I have gotten over 75 on a few occasions up in the mountains when we were putting along dodging ice and rocks.
Bare

[gremlin]

Well, I would do that Bare, but a wise man I know of refuses to tiptoe through life only to arrive safely at death...   

 

[Royalista]

I found that out the first time I rode the bike and was breaking it in on the ride home! I got a bit over 80 mpg on those first few rides. I haven't accomplished it since. I have gotten over 75 on a few occasions up in the mountains when we were putting along dodging ice and rocks.
Bare

Wow, sincere congratulations.
The result for the full break-in period was 75mpg but I didn't make those highs again. The bike insists it would go bananas at that pace.
Nowadays annual average is 62mpg, neatly in the middle between summer (68mpg) and winter (56mpg) averages.
Summer average still beats Eurotest (66.4mpg).
Riding is stopNgo at sealevel.
I draw great comfort from the knowledge that I can pull out of the garage and ride all the way up to Khardang La without a care in the world. Mm, about carbs and jets e.a., that is.

[gremlin]

...................I draw great comfort from the knowledge that I can pull out of the garage and ride all the way up to Khardang La without a care in the world. Mm, about carbs and jets e.a., that is.

+1