Octane and altitude

[stock500]

Does anyone know what altitude does to octane.  I am at 6,600 feet, and I ride from about 4,000 to 7,500.  Any info would be great.  I am running 85 octane pump gas with no issues in my stock 500.  My buddies are always giving me hell about it because they all run race fuel. 

[bigbellybob]

as you go up in altitude your octane requirement come down. 85 octane sounds scary but if your not experiencing detonation i would not worry

[don46]

have to agree with Bob, took a fully modded KX 125 to mammoth mountain mx, we always ran straight race gas in it. During practice  day I spent all day trying to get out a low end bog, couldn't get it done, no matter what I tried it still bogged. Mixed my race gas with pump gas 50/50 bog gone. This was before the oxyengenated fuels were popular, now I'd run the U4.2 or comparable for a race bike, a trail bike I'd at least run 91 octane pump gas, 85 octane is for the car, wait a minute, I don't run that in the car either, 87 octane.

[Kev]

In reality altitude does nothing to octane. Octane is merely a numerical representation of resistance to detonation. Detonation is typically a result of heat and/or pressure causing end gasses to combust in front of the flame front after ignition. At 6000 ft. you have lost approximately 20% of your atmospheric pressure as compared to sea level. Your chances of detonation have been greatly reduced. But remember detonation is usually caused by heat and or pressure so you can still have detonation, it's just that your combustion pressures have been greatly reduced which in turn results in lower top cylinder and head temps. Since our bikes are water cooled you are not going to suffer from lost cooling due to lower absolute air pressures. You will most likely not experience any problems, but you should do what you are comfortable with. I am at 6200 ft. and I run 91. If I could afford racing fuel I would burn it because I love the smell. 
Kev

[Texas Champ.]

A friend of mine I race with on the vintage circut begain using Sunoco racing fuel at my suggestion to start running race fuel ...( I use VP)...He started having trouble with bogging but ran great when the rpm's were up , he switched back to pump gas and the bogging went away. At a race later I had him run the VP I had and he had no problems what so ever...Since then I have heard of several people speak of "bogging" with race gas only to hear that they are running Sunoco....If you have the VP product in your area try that and see if the bogging goes away. My friend is a professor at a major university...so of course he compared the two fuels only to find very slight differences...on thing he found out by trail and error....Sunoco = bogging VP= no bogging......km