Fuel octane at high altitude

General Discussion
1

So a few weeks ago on the show a caller from Denver asked about using a lower fuel octane at altitude and the answer was there is no harm since the compression in the chamber is lower with the thinner air you can use a lower octane. So that got me to thinking if using a higher octane fuel than recommended is actually worse for the car. You usually read that there is no benefit to using a higher octane, but it is actually worse? If the compression in the chamber isn’t high enough are you just not exploding all the gas and getting worse mileage? I live in Denver as well and my car recommends 91, since the next lowest is 87 I have just kept putting 91 in the car but after the call got to thinking that I may actually be losing performance by using 91 since I am at altitude and my cars compression is effectively lower. Could this be the case?

2

There is no down side to running with higher than necessary octane, other than wasting money.

3

Well, the theory is that since higher octanes take longer to combust fully, using a higher than necessary octane might cause the fuel not to burn up fully and cause problems with emissions equipment. But it seems like if this were really an issue, you’d hear about it more because newer cars are pretty sensitive to emissions problems and there’s a lot of people out there who use higher octane gas than they need to.

I will also mention that if your car is turbocharged, the altitude effect on octane requirements is less pronounced, which is why the premium is still 91 octane at high altitudes.

4

You should be using 89 octane here in CO if your car recommends 91 Octane.
Octane loses 2 points in the transition from sea level to our 5200+ feet here in the Denver area.

So 87 octane in NYC is 85 octane here in CO.
89 Octane turns into 87 at high altitude.
91 octane becomes 89.
93 becomes 91.

BC.

5

+1

6

+2

7

= 3

8

I don’t think I’ve ever seen 89 octane premium. I live right near the edge of where they make the change over in Montana. Where I live it’s 87 regular, 89 midgrade, and 91 premium. But if I drive about 80 miles east, it’s 85 or 85.5 regular, 87 midgrade, but usually still 91 premium.

I always assumed this was because on a car with a turbocharger, the turbo can compensate for the altitude.