Thread: Gas(Octane question)

14* timing.

You're right, the reason we have 85 is that our air pressure is roughly 85% of what it is at sea level. Thereby, we need an even more flammable gas to get reliable combustion.

Atmospheric pressure is 14.7 at sea level, but here in utah it is about 13.0. Which means that generally a 91 octane will act like a 93 or a 94 octane. On a turbo motor you will need 2psi more boost to make the same power as sea level, it is slightly more complicated than that, but you should be ok.

I ran my DET on 87 on 18psi on a disco all winter last year. Not a single issue. Now the VET I would not run that much because I am scared of detonation. But I will run 20psi on 91 in the summer, not a single issue with that.

Backwards. Because you have lower ambient air pressure, the fuel is less likely to pre-ignite, so you can get away with the lower octane. If you're only at 14 degrees BTDC you should be fine with it I would think No way you need 91 to run that!

Not backwards. the lower the octane the less resistant to ignition the fuel is. I said that the first time.

Higher octane has a higher flash point, and also burns slower unless additives are added.

Lol we are on the same page here man, I meant that the lower air pressure means you can use the lower octane without the worry of detonation.

So the answer to the original questions is:
1. The reason for the lower gas mileage is the Winter fuel mix
2. Running 89 octane is fine

I believe that:
Running lower octane winter mix fuel is probably fine for most cars for all elevations.
Denser cold air & quicker lower octane fuel ignition.
But I havent proved this. Just my belief from experience and elementary testing with multiple cars.