I don't understand why the high altitude areas have lower octane gas. I seen as low as 84 or 85 with mid grade being 86-87 but the premium always ended up at least 91 (which is the best you can get anywhere in Cali). WTx??
New Mexico, Utah, Colorado, parts of AZ have the lower octane fuels that I know of.

Is it based on higher altitude = less air with same amount of gas thereby making the engine run richer? If not, I just dont get it. My SD manual recommends 87 and not using lower grade fuels at altitude.

Any engineers out there that can explain this?

 

You were right about the less air part. Less air equals less compresion so less octane is needed for the engine to run without pinging or having the timing pulled way back.

 

It's cheaper for the gas companies to produce lower octane if it's not needed.

 

It's also cheaper for the consumer to buy lower octane when it's not needed. One gas comapny figured this out and the others followed suit to stay competitive.

Mmaxed is correct. Lower atmospheric pressure puts less air in the cylinder and is the same effect as having a lower compression ratio. Turbo- or supercharged gasoline engines will still require the higher octane fuel.

 

Here is a post I put together a while ago about altitude and power...hope you find it helpful...

joe.

http://www.coastresorts.com/cforum/i...461046/p/1.cfm