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As many of you know, in Colorado regular gasoline is 85 octane; in most states, it's 87 octane. The topic of what octane to use at higher altitude has been hashed out here before, and I'd thought the matter was settled: Even john1701a - who I trust implicitly - says 85 octane is okay at high altitude.

However, I recently ran across a couple of local news articles that claim 85 octane is only better at high altitude for cars with carburetors, which the Prius, along with just about every other car, is not:

http://www.koaa.com/news/view.asp?ID=251

http://www.koaa.com/news/view.asp?ID=250

Of course, the Prius manual calls for 87 octane, without any provision for altitude (which would have been easy to include). And our salesman recommended 87 although, of course, he's a salesman, not a technician.

Does anyone know a definitive source for information on this? Meanwhile, I'm going to keep using 87 - I can make up the cost difference by slowing down a little.

In my manuals it says at least 95 octane. And lesss than 95 is not even for sale here

I think there are different methods for estimating octane, in different countries. This is why recommended octane values for the same car differs by countries, and cannot be compared.

2001 Prius Manual says:
"Fuel type: UNLEADED gasoline, Octane Rating 87 (Research Octane Number 91) or higher."

Depending on where you live (say, outside the US), will depend upon what number is listed as the octane rating on the pump.

from a quick search on google:

http://chemistry.about.com/library/weekly/aa070401a.htm

"Gasoline pumps typically post octane numbers as an average of two different
values. Often you may see the octane rating quoted as (R+M)/2. One value is
the research octane number (RON), which is determined with a test engine
running at a low speed of 600 rpm. The other value is the motor octane
number (MON), which is determined with a test engine running at a higher
speed of 900 rpm. If, for example, a gasoline has an RON of 98 and a MON of
90, then the posted octane number would be the average of the two values or
94. "

http://www.encyclopedia.com/articles/09474.html

"At present three systems of octane rating are used in the United States.
Two of these, the research octane and motor octane numbers, are determined
by burning the gasoline in an engine under different, but specified,
conditions. Usually the motor octane number is lower than the research
octane. The third octane rating, which federal regulations require on
commercial gasoline pumps, is an average of research octane and motor
octane. Under this system a regular grade gasoline has an octane number of
about 87 and a premium grade of about 93."

http://www.shell.ca/code/motoring/encyc ... ctane.html

"Technically there are three different "octane numbers" associated with
every gasoline. The Research Octane Number, or RON, is measured under fairly
easy test conditions. The Motor Octane Number, or MON, is a tougher test
measured at higher engine speed and temperature.

The value that relates most closely to actual driving conditions is the
average of these two values: Road Octane Number = (RON + MON)/ 2.

Occasionally, less scrupulous gasoline outlets will use the confusion of
these different octane measurements to exaggerate their octane rating
claims, by advertising their fuel's Research Octane Number - which will be
higher than the Road Octane Number. Motorists should always be sure that the
octane number a vendor advertises is its Road Octane value, not its RON."


[note: those URLs I copied from an email I sent back in March 2002, so the links may not work or the text may be slightly modified now...]

I have driven quite a bit in the Colorado mountains on vacations (beautiful country); and octane doesn't seem to make any noticeable difference. I always buy the cheapest available, and can't tell any difference.

If it makes you feel better to buy 87 octane, you're only talking about maybe 50 cents a tank. You can't buy a soft drink at MacDonalds for that. That's why I don't shop around for the cheapest gas while on vacation.

That's the way to do it, take the lowest octane that gives the best performance. I.E. no pinging, though probably detecting ping might be hard to do on a Prius.

In locations with higher altitudes there is less available oxygen in the air. So for a given compressed combustion chamber, the total oxygen content being less limits the amount of fuel that can be burned. This is controlled by a fuel flow decrease as determined by O2 sensor feedback and can be further controlled by limiting octane to that which can be effectively burned.

Think about this in reverse performance car terms. Higher compression means higher octane is required. Higher altitude means lower barametric pressure which lowers the effective compression ratio achieved. You therefore need less octane.

85 octane is more than adequate for Colorado Springs. I used to do powertrain development in that area, and some engineers debated the octane requirements. Try using 89 octane (premium in that area) before going up Pikes Peak. As you pass 8500 feet, the poor engine is doing everything it can to burn the mixture of low atmospheric pressure, high octane, and lower oxygen content. The engines ran much better with fuel of an octane that could be effectively fired by the spark plug, which was lower octane fuel.

Octane is the measure of resistance to self ignition. When there is reduced effective compression and lower oxygen content, the ability to self ignite is reduced. The 85 octane is what you want to use in that area. Anything more results in inefficient combustion.

Carburetors had more issues because while fuel flow was reduced as a result of lower engine vacuum at altitude, the amount of reduced fuel flow was not controlled as a function of the combustion process. With closed loop fuel control (fuel injection and an O2 sensor) the amount of fuel used can be more closely controlled to that which can be consumed during the combustion process.

Thanks to everyone for the feedback. It will be 85 octane from now on - unless we're heading to lower altitude, since the Prius can make it to eastern Kansas on a single tank!

Been up Pikes Peak, too -- Cheap 85 octane gas runs fine at the summit, 14,410 feet (I think).

Yah, cause it's all down hill! Try making it BACK on a single tank!