Home Forums Gear Hunting Trucks and Four-Wheel Drives Octane Ratings

Question.

Around here (Missouri/Kansas), stations typically sell 87 and 89 octane, sometimes 91 and maybe even a 93 if I remember right (not the station I go to).

I've noticed in the mountains 86 and 85 octane. Why the difference? I assume something about the elevation doesn't require anything higher. End results?

Lower ignition fuel because of lower air mass in the atmosphere, thus lower pressure in the cylinder.

From Wikipedia:

* United States: in the Rocky Mountain (high altitude) states, 85 AKI is the minimum octane, and 91 AKI is the maximum octane available in fuel[citation needed]. The reason for this is that in higher-altitude areas, a typical naturally-aspirated engine draws in less air mass per cycle due to the reduced density of the atmosphere. This directly translates to less fuel and reduced absolute compression in the cylinder, therefore deterring knock. It is safe to fill up a carbureted car that normally takes 87 AKI fuel at sea level with 85 AKI fuel in the mountains, but at sea level the fuel may cause damage to the engine. A disadvantage to this strategy is that most turbocharged vehicles are unable to produce full power, even when using the "premium" 91 AKI fuel.

http://en.wikipedia.org/wiki/Octane_rating

My seat of the pants dyno says that my truck makes more power with the higher octane gas up in the mountains, so if the price isn't too high I'll run plus or premium.

I do the same with my Ford F-350 and the motorhome! The motorhome will not run on anything less than 90-Octane. I'll run 89 octane in the turk if the price is only a few cents more per gallon!