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Discussion Starter · #1 ·
While in Colorado last week, I noticed that the gasoline available had octane ratings of 85, 87, and 89. In California, we typically have 87, 89 and 91. Are the ratings different because of the altitude (the gas stations I was visiting were all above 8000 feet), or is the gas really different? Which grade should I have been using (I stuck with the 87)?

Thanks!

Keith
 

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Freaky. I thought there were national standards on octane ratings. I couldn't drive my vette there for example. Most places have 87, 89, and 91-93. Sure you weren't at Sunoco which has 5 different octane ratings up to 94? :confused:
 

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Campfamily said:
While in Colorado last week, I noticed that the gasoline available had octane ratings of 85, 87, and 89. In California, we typically have 87, 89 and 91. Are the ratings different because of the altitude (the gas stations I was visiting were all above 8000 feet), or is the gas really different? Which grade should I have been using (I stuck with the 87)?

Thanks!

Keith
I think you are on it with the altitude. Also, there are literally, dare I say hundreds? Well at least 100 different mixtures of gas used throughout the country due to geographic requirements, they all vary slightly based altitude, season, metro area etc. Some require minimum % of alchohol during winter months in certain area etc etc. That is another reason for supply issues. Gas that might be OK in Denver is not the right mix for Salt Lake, or LA etc. Of course this is all a bunch of beaurocratic BS, a national set of standards would be more flexible for distribution. At least that's my story and I'm stickin to it. (for now!)

This has alot of info on gas, if you're bored..... there are 4 parts.
http://www.faqs.org/faqs/autos/gasoline-faq/part2/
 

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Discussion Starter · #4 ·
92TripleBlack said:
Freaky. I thought there were national standards on octane ratings. I couldn't drive my vette there for example. Most places have 87, 89, and 91-93. Sure you weren't at Sunoco which has 5 different octane ratings up to 94? :confused:
I never saw anything higher than 89 while in Colorado. This was common to every station I visited, irrespective of brand. I noticed most other people in the station pumping 85 into their cars, but didn't ask.

Keith
 

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Discussion Starter · #5 ·
armoody said:
I think you are on it with the altitude.
I would think that octane requirements would go up with altitude, since the oxygen content of the air decreases with altitude, so more octane is required to produce the same amount of energy. But, I didn't do all that well in Chemistry in college (that's why I became an aerospace engineer!), so I should probably go back and look at it.
 

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Campfamily said:
I would think that octane requirements would go up with altitude, since the oxygen content of the air decreases with altitude, so more octane is required to produce the same amount of energy. But, I didn't do all that well in Chemistry in college (that's why I became an aerospace engineer!), so I should probably go back and look at it.
Winter in Colorado.....both reduce octain requirements...
Here is an excerpt from the earlier link:

7.10 What is the effect of air temperature?

An increase in ambient air temperature of 5.6C increases the octane
requirement of an engine by 0.44 - 0.54 MON [27,38]. When the combined effects
of air temperature and humidity are considered, it is often possible to use
one octane grade in summer, and use a lower octane rating in winter. The
Motor octane rating has a higher charge temperature, and increasing charge
temperature increases the tendency to knock, so fuels with low Sensitivity
( the difference between RON and MON numbers ) are less affected by air
temperature.

7.11 What is the effect of altitude?

The effect of increasing altitude may be nonlinear, with one study reporting
a decrease of the octane requirement of 1.4 RON/300m from sea level to 1800m
and 2.5 RON/300m from 1800m to 3600m [27]. Other studies report the octane
number requirement decreased by 1.0 - 1.9 RON/300m without specifying
altitude [38]. Modern engine management systems can accommodate this
adjustment, and in some recent studies, the octane number requirement was
reduced by 0.2 - 0.5 (R+M)/2 per 300m increase in altitude.
The larger reduction on older engines was due to:-
- reduced air density provides lower combustion temperature and pressure.
- fuel is metered according to air volume, consequently as density decreases
the stoichiometry moves to rich, with a lower octane number requirement.
- manifold vacuum controlled spark advance, and reduced manifold vacuum
results in less spark advance.
 

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Many people don't realize that 91 octane fuel is actually LESS flammable than 87.

That's why high compression engines need higher octane gas - the lower octane fuel ignites too early. Before the days of knock sensors, this pre-ignition caused the pinging that many of us old folks so fondly remember.

Nowadays, most cars have knock sensors that will detect pre-ignition, and will retard or delay the ignition timing (spark) when too low of an octane fuel is used. The problem with delayed timing is that the piston has moved past the "sweet spot" for generating maximum power, and the lost power can be quite noticeable.
 

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Discussion Starter · #8 ·
armoody said:
Winter in Colorado.....both reduce octain requirements...
Here is an excerpt from the earlier link:

7.10 What is the effect of air temperature?

An increase in ambient air temperature of 5.6C increases the octane
requirement of an engine by 0.44 - 0.54 MON [27,38]. When the combined effects
of air temperature and humidity are considered, it is often possible to use
one octane grade in summer, and use a lower octane rating in winter. The
Motor octane rating has a higher charge temperature, and increasing charge
temperature increases the tendency to knock, so fuels with low Sensitivity
( the difference between RON and MON numbers ) are less affected by air
temperature.

7.11 What is the effect of altitude?

The effect of increasing altitude may be nonlinear, with one study reporting
a decrease of the octane requirement of 1.4 RON/300m from sea level to 1800m
and 2.5 RON/300m from 1800m to 3600m [27]. Other studies report the octane
number requirement decreased by 1.0 - 1.9 RON/300m without specifying
altitude [38]. Modern engine management systems can accommodate this
adjustment, and in some recent studies, the octane number requirement was
reduced by 0.2 - 0.5 (R+M)/2 per 300m increase in altitude.
The larger reduction on older engines was due to:-
- reduced air density provides lower combustion temperature and pressure.
- fuel is metered according to air volume, consequently as density decreases
the stoichiometry moves to rich, with a lower octane number requirement.
- manifold vacuum controlled spark advance, and reduced manifold vacuum
results in less spark advance.
Bottom line, it sounds like it is okay to use 85 at higher altitudes in the winter. Correct?

Keith
 

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Campfamily said:
Bottom line, it sounds like it is okay to use 85 at higher altitudes in the winter. Correct?

Keith
Looks that way to me. Computer will adjust timing.
 
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