In the rocky mountain states, regular gas is sold at 85 octane while in the other states it is usually 87 octane. Does the thinner air at higher altitudes mean the 85 octane performs as well as 87 octane at lower altitude or do I just have to buy the mid-level 87/88 octane?
Yes, the lower density air at high altitudes does not need as high an octane rating as sea level air does. That’s why they sell 95 octane up in the mountains. Cars that need 87 octane gas at sea level do just fine with 85 octane gas up in the mountains.
The 85 octane rating is really antiquated. It helped older cars without computer controlled timing deal with the elevation. The fact is that modern vehicles compensate for elevation seamlessly and don’t need 85 octane to compensate for the elevation. Whether you use 85 or 87 won’t matter much because your engine will precisely retard or advance timing for you depending on your elevation. Western states should do away with 85 octane because it is pointless now.
True for most cars, but some can’t do much if any adjustment. You can tell by the owner’s manual. If it tells you that high octane is needed, then you better use it. if it recommends high octane, then likely between the reduced power and lower mileage, you likely would do well be following their recommendation as you would be giving up power and mileage, for ? What ?
The lower octane sold at high elevation is a profit center for the oil companies…They charge the same for 85 octane as they charge for 87 octane in low altitude areas…What was once 87 octane “regular” suddenly is worth .15 cents a gallon more and is sold as “mid-grade”…
Haven’t you noticed? There is now a .30 cent spread between Regular and Premium. That’s $4.50 a fill-up for most drivers…
If an engine’s compression ratio is low enough that it can burn 85 with a spark advance that is optimum for power and efficiency, there’s no point in using a higher octane fuel. It’s not like the more you advance the spark, the more power the engine can make, although a lot of shade tree mechanics seem to think so.
Over advanced ignition timing reduces both power and efficiency, even if there is no detonation.