Just got back from a 10 day Colorado ride, and I've got a couple of questions regarding altitude and how it affects our bikes. I live in East Texas, ride a 2012 UC, my altitude is about 200', premium gas is 93 Octane, and I get about 35mpg on average, riding 2up most of the time.

I noticed that about Amarillo, TX the octane of premium gas dropped to 91 octane. Amarillo is about 3,600'. It seems to me that I saw a 90 octane somewhere on the trip, but 91 was common. My bike didn't really like it. I got a little clatter on acceleration.
Question is: Why did the octane get lower at higher altitudes? Why the clatter?
My other question is about mileage. As stated, I normally get +/-35mpg. On this trip, I averaged 42, and my highest mpg was 52. I would think that with the air being thinner that there would be less oxygen, and that this would require more fuel for the same amount of "go". That should make the mileage less. Why did the mileage increase at altitude?

Thanks, Ride Safe

David

 

mileage should be about same under same conditions ( but for altitude) as controlled by the ECM.

variable could be less wind resistance, due to thinner air ( marginal), steady throttle hand, steady cruising speed(s) lower speeds than when running around home territory( wind resistance).
It takes about twice as much power to push the bike to 80 MPH as it does at 55 MPH

higher octane fuels do not "pack more power" or put a tiger in your tank. octane is a rating of the ability of the fuel to resist compression ignition ( like a diesel)

in fact a number of TX chains have been fined for selling 87 as premium...so who knows what you've been getting at home.

also ethanol content is a factor 10 to 15% ethanol gas gives reduced MPG by about 5%
maybe you have 15% ethanol at home, and the stations you stopped at didn't...pumps should be marked as to percentage of ethanol

so maybe some or all of the above.


unlikely: when the ecm senses pinging, it will retard ignition timing, which reduces MPG and power...could it be possible that the fuel you use at home has the ignition timing not as advanced as might be ?

mike

 

It's an altitude thing, the air's less dense, so you use less fuel to reach the target AFR/Lambda. The BMW I just traded in reliably went 360 miles on a tank....But when I was out in Colorado last summer, I regularly saw 500 mile tanks - and that wasn't because I was going slowly looking at the scenery.

I have no idea about the octane thing. I never had an issue finding hi-test when I was there.

 

I too spent 5 days in the Rockies recently and normally ride at about 600' here in Minnesota. I think part of the high mileage is that there is so much downhill grades. Everyone on the trip had great mileage in the mountians. Just a thought.

I have an 88" carb which ran great on the 91 octane. But my friends newer 103 hated it also. So bad he made a call to one of the HD dealerhships in Denver. They suggested an octane buster additive which seemed to solve the problems immediatly.

 

We took our trip the first week in June. Nine days 3300 miles and I saw the same thing. Bike was loaded to the hilt & got 53mpg on one tank. Mine seem to run fine on the 91. Beautiful country except where the pine beetle has killed so many trees.

 

The AFR does not change. Less air = Less fuel to maintain the same ratio. Higher altitude allows for lower Octane so most of our premium is 91. Typically retard the ignition at higher rpm range to prevent pinging. I average mid 40 mpg in town and low 50 mgp highway. Nice place to live and ride, but weekend travel sucks!

 

Hey Dave... I had some interesting issues in the Colorado Rockies last Summer... What mods have you made to your bike... What ECM tuner are you using... Has the bike been dyno'd... Tell me more about this 'clatter'...

 

"I too spent 5 days in the Rockies recently and normally ride at about 600' here in Minnesota. I think part of the high mileage is that there is so much downhill grades. Everyone on the trip had great mileage in the mountians. Just a thought"

BINGO, there's the answer. The Science Museum in Pierre, SD, has or had a great exhibit a few years back about driving in the mountains and why gas mileage was so good.

They had two troughs about ten feet long. One was even, a steady decline of perhaps 30 degrees off horizontal. The second started at the same spot, but was down and up all the way down. The "ups" were at the same level of the steady incline at that point.

All my life I believed the shortest distance between two points is a straight line, but this exhibit proved that even though the straight trough was shortest, it was also the slowest. Two marbles rolling down the troughs, released at the same time, showed this to be true. The marble on the hilly trough, traveling a further distance, arrived at the bottom much quicker than the one on the steady incline, even though its distance was shorter.

The hypothesis was that that downward pull of gravity was greater than the energy needed to overcome it. Hence, a vehicle driving up and down hills got better mileage than one driving on a steady decline or horizontal surface.

We have a Chevy Impala that has the trip computer. Ever since seeing this exhibit at Pierre, I have watched what happens on the mileage function. On the level, it shows us getting around 30 mpg at a steady 60 mph. Going up a hill, it will show the mileage dropping, but never more than down to around 20 mpg. However, going down the same hill, I can take my foot completely off the gas, maintain the speed (or maybe even need to brake), and the mileage jumps to 70-80 mpg, as the engine is just idling and gravity is maintaining the speed.

I personally found this exhibit in Pierre one of the most fascinating science exhibits I have ever seen.

 

To the OP,

I had the same results you had when I did a 10 day trip to Colorado last year. I normally get about 42 mpg at home in the Baltimore, MD area. But out in Colorado I was getting 52 mpg all the time. I dropped back to the 42 mpg when I got home. I didn't notice any issues with the fuel and octane out west. As a matter of fact, I thought my bike actually was performing better out there. I'm pretty much stock other than a FL-XiED. I also chalk up the extra long distances I was riding as to better mpg. I was making 460 mile rides.

 

The fuel economy has been covered: less air, less fuel added, and the marble thing. Never heard of that before.

As stated, octane is a rating of the ability to resist ignition from heat generated by compression. In the mountains with the lower amount of air, there is less compression pressure and less heat generated during the compression stroke. With less heat from compression, a lower octane fuel SHOULD work as well as a higher octane at a lower elevation. If you are pinging, either the fuel companies are cheating their octane numbers or a slightly higher octane is needed at that elevation. If you are just by chance right at the threshold of pinging with high pressure in the area, having a low pressure system move in could put you below that threshold. I also wonder if a delivery company might have added some left over fuel for a higher elevation into a load for a lower elevation and lowered the octane rating enough to allow pinging in sensitive engines. As consumers, we will never know.