Here's a good thread on the exact subject.

http://www.donzi.net/cgi-bin/ultimat...=004809#000000

Humidity is the culprit not time.

An increase of absolute humidity of 1.0 g water/kg of dry air lowers the octane requirement of an engine by 0.25 - 0.32 MON [27,28,38].

What is "stale" fuel, and should I use it?

"Stale" fuel is caused by improper storage, and usually smells sour. The gasoline has been allowed to get warm, thus catalysing olefin decomposition reactions, and perhaps also losing volatile material in unsealed containers. Such fuel will tend to rapidly form gums, and will usually have a significant reduction in octane rating. The fuel can be used by blending with twice the volume of new gasoline, but the blended fuel should be used immediately, otherwise teh old fuel will catalyse rapid decomposition of the new, resulting in even larger quantities of stale fuel. Some stale fuels can drop several octane numbers, so be generous with the dilution.

Got MTBE?
 

Hey Nort- I'm not an expert, but I was told that the loss of the MTBE will lower the octane. Apparently, the MTBE (Cali mandated...that is now banned- go figure) will degrade at a faster rate than the other petroluem products....

If your going to Havasu, throw some 100 octane at Terrible's in there...it'll bring the octane back up...

See you out there!

Hey Nort,

Just do me a favor and have a drink for me!!! It looks like nothin' but rain here in Chicago this weekend:mad:

Have fun,
Dave

Octane rating on gas was related to its resistance to ping or knock. This is also called premature ignition, where the heat and pressure from the cylinder causes the mixture to burn before it is ignited by the spark plug. Think of it acting like a diesel engine. Occasional knocking is OK, dont get me wrong, but prolonged knocking can lead something called DETONATION. Thats why guys with blown engines need to run a high octane or avation grade gas. They dont want their engines to blow up. It is better to run with the grade designed for your engine, as said before you lose power.

Brad