I have a 91' Yamaha Banshee with Wiseco pistons and a cool head with 19cc domes. My elevation is around 5000ft. Does any one know if I should use straight race gas, AVgas, or half pump/half race gas? All help would be appreciated.

 

I live in michigan so elev. is different but any way my firend has a highly modded shee with 19cc domes and he runs staright race gas. Not a problem. Its better than AV gas.

 

The main reason for running trick or AV gas is to keep the engine cooler. See the higher the octane the faster the burn(hence cooler). Also the higher the octane the easier it ignites. If you don't have a problem with spark ping or cooling then your alright. But if you run into any of these issues then go up a couple of octanes. Just my experiences[img]i/expressions/face-icon-small-cool.gif[/img]

 

For each thousand foot of elevation gain, octane requirements dropn early one full point. So at 5000' foot you will require 5 points less octane. The compression at 5K' is going to only be around 130-140 psi. You'll be fine with pump gas and good spot-on jetting at 5000' or above, if you drop down to 2000' or lower I would start thinking about running at least 50-50 race-pump. Oil mix ratio will influence octane as well, oil reduces the octane rating of the fuel.

Octane refers to the fuel's ability to resist pressure or heat induced self-ignition. Octane doesn't refer to flame speeds or evaporative cooling properties. That's a whole different ball game of the "chemical" soup of gasoline.

Here's a pretty decent tech article on fuel, written by Bell, who has probably forgotten more about 2 strokes than many of us will ever know.

http://www.teleport.com/~mpracing/fuel/

Of special interest is the last paragraph...

 

Octane rating is the fuels stability, Or Anti-knock index. It does not determine burn rate or heat, which are not related to each other. Burn rate and heat do affect the octane rating, but are not measured by the octane scale.

BackCountry, Are you sure oil decreases octane? I have been searching long and hard for any information on premix oils affect on a given fuel. It would seem to me that oil would INCREASE octane rating. If you know of any good sources of information on premix oil characteristics and their affects on fuels I would greatly appreciated if you would share.