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In order to adjust for the effects at altitude, it is normal for older engines (especially with carburetors) to run better with 85 octane when they would ordinarily use 87 octane at sea level.

However, most modern engines with fuel/direct injection and an Electronic Control Unit (ECU) to control the engine functions, the adjustment at altitude will be made by the engine and it work fine with 87 octane. So I personally would use 87 or 89 octane at altitude with a modern engine, because otherwise you will be losing up to 5-10% HP at altitude, and that horsepower is often appreciated when driving in the mountains or in hilly areas.

But I can not guarantee that any specific engine will prefer 87/89 octane over 85 octane at altitude, so try both and see which one performs best in your vehicle.

One way to get 87 octane if only 85 and 89 are offered, is to fill half with 85 and half with 89. They will mix together better if you don't fill up past 7/8 full.
 

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What changes at altitude?
Why would 85 be better at altitude?
Mainly because barometric pressure is normally lower at altitude, so vehicles can perform as well with 85 octane as they would with 87 at lower altitudes, thereby saving money. In the US, you may see this above 4000 ft altitude. But running a little higher octane does not hurt.

Back when I lived in Denver, I had to take my lawnmower to get the engine (mainly carburetor) adjusted to run well at the higher altitude. Modern automobiles with fuel or direct injection, computer controlled ignition, etc, can make the adjustment automatically.
 

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When I was passing through Nevada (or maybe Utah) in my '00 I4 Camry (recommended minimum 87) I tried 85 for shits and giggles. I filled up nearly a full tank (18.3 out of 18.5 capacity) because I almost ran out in the hills. Oh my god the car became a dog. Sluggish, and acceleration got cut in half (approximate).
 

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Mainly because barometric pressure is normally lower at altitude, so vehicles can perform as well with 85 octane as they would with 87 at lower altitudes, thereby saving money. In the US, you may see this above 4000 ft altitude. But running a little higher octane does not hurt.

Back when I lived in Denver, I had to take my lawnmower to get the engine (mainly carburetor) adjusted to run well at the higher altitude. Modern automobiles with fuel or direct injection, computer controlled ignition, etc, can make the adjustment automatically.
Don't get air-density mixed up with octane-rating. With less air, you want to adjust to have less fuel injected. On modern cars, altitude detection is built into ECU a they'll reduce fuel as necessary. Also uses O2-sensor feedback to auto-adjust as well. With carbs, you'll have to adjust jetting.

However, octane-rating has nothing to do with any of this. Without altitude adjustment, both 85 and 87-oct will run too rich at altitude.
 

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Don't get air-density mixed up with octane-rating. With less air, you want to adjust to have less fuel injected. On modern cars, altitude detection is built into ECU a they'll reduce fuel as necessary. Also uses O2-sensor feedback to auto-adjust as well. With carbs, you'll have to adjust jetting.

However, octane-rating has nothing to do with any of this. Without altitude adjustment, both 85 and 87-oct will run too rich at altitude.

This is correct, it has nothing to do with mixture, it's all about knock. At high elevation, the manifold pressure will be lower, reducing the BMEP which reduces the tendency for the engine to knock, and you can safely run a lower octane fuel and save some money. Notice how your car feels much less powerful at higher elevations? Same thing.. lower manifold pressure, lower BMEP, less power. It's not the grade of fuel that makes the car feel slow, it's the lower air pressure.



On a vanilla street car you'll be fine. Most modern engines automatically correct for knock (and mixture) anyway. You're not going to hurt it and you are not going to get more power by running more octane unless the engine is knocking (which it also would be at lower elevations too, but even worse). A turbocharged engine can compensate for the lower air pressure up to a point, and may still benefit from buying the higher octane fuel if that's what it requires at sea level.
 

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Discussion Starter #10
My other car uses 87, so the dilemma was 85,89,or 91.

Being 4-5k above sea level created a confusion for me. The attendant told me that 85 is the same as 87. Took it with a grain of salt and decided on 89.

Car ran fine with no issues.

Thanks for the additional info. Never realized that it was more involved than just on the surface.
 
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