91 or 100 or 110 octane

[spencerhenry]

i work on offroad race cars that we run 100 to 114 octane in. the fuel is leaded. i have been told my motorcylce guys that i should mix 100 octane 50/50 with pump gas(91). i was reading a thread about alcohol, and wondered about the pump gas that i get here in the mountains. i think it has some additives. i usually run my saws at high elevation, never lower than 5700' but sometimes above 11,000' i check my max rpm when i change altitude. i run stihl mix oil at 40:1
does leaded fuel help a 2 stroke?
does 91 octane pump gas have additives that are bad for my saws?
will 100 octane fuel help my saws?
any thoughts anyone
sam

 

[bwalker]

Use leaded, high octane fuel at altitude in a stock saw will do nothing but lighten your wallet and add lead fouling to your muffler and cylinder. Remember as altitude rises octane requirment goes down.

 

[tony marks]

ben is this because the oxygen level is lower at higher altitudes.

 

[Lumberjack]

Originally posted by tony marks
ben is this because the oxygen level is lower at higher altitudes.

Yes, and no.

As you go up, the barametric pressure goes down, so there is less of a "push" for the air to fill the cylinder (14.7 lbs psi at sea level). Hence the cylinder is filled with less air per each stroke, hence less oxygen.

There is the same amount of oxygen available (the ratio 21% I think) the thing is that there is less air in a given space than at lower elevations. Another way is sayin that the air is less dense.

I think you lose around 3% of your hp for every 1000 foot of elevation gained. That excludes force induction motors.