Variation in Octane Rating
Thu, 10/11/2012 - 5:18pm
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 15 years
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Over the last few months I made a couple of long trips through the western states. I noted that the Octane Rating for Regular gas was 85.5 at almost all gas stations. However, the Octane Rating for Regular gas in Alberta and British Columbia was 87.0.
I did some checking and found that the Octane Rating is calculated the same in Canada and the U.S. Does anyone have an explanation why the ratings for the same standard of fuel would be different??
altitude
Does anyone have an explanation why the ratings for the same standard of fuel would be different??
I believe the lower ratings are due to higher altitudes in use areas.
Ron
I'm thinking because of higher higher altitude
At higher altitudes, the air is thinner. The octane can be lower to get the same performance w/o knocking.
Octane and Elevation
Thanks for the responses, but I'm not sure that elevation is the entire reason.
Most gas stations in Las Vegas were offering Regular fuel with an Octane Rating of 85.5. Las Vegas has an elevation of 2,000 - 2100 feet. But my hometown has an elevation of 2,200 feet and all the gas stations here offer Regula fuel with an Octane Rating of 87.0. So the elevation is pretty well the same, but the Octane is different.
I'm not disagreeing with your comments, just wondering
Wiki says elevation
From http://en.wikipedia.org/wiki/Octane_rating
"United States: in the US octane rating is displayed in AKI. In the Rocky Mountain (high elevation) states, 85 AKI (90 RON) is the minimum octane, and 91 AKI (95 RON) is the maximum octane available in fuel[citation needed]. The reason for this is that in higher-elevation areas, a typical naturally aspirated engine draws in less air mass per cycle because of 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 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."
-Quest, Nuvi 1390T
I always wondered about
I always wondered about that. In California the regular gas is always 87 octane but, when I travel eastward, I often encounter 85 octane gas.
I always figured it was due to all the emission standards that California has since there are soooooooooooooo many freaking cars in parts of the state.
GPSMAP 76CSx - nüvi 760 - nüvi 200 - GPSMAP 78S
Washington State
Thr octane rating I have seen is always 87 for regular..
Bobby....Garmin 2450LM
excellent thread
great information contained here. Thanks
I drive, therefore I am happy. Rodeo, wildlife and nature photography rodeophoto.ca
Second That, Sort Of
At higher altitudes, the air is thinner. The octane can be lower to get the same performance w/o knocking.
At altitude, there is less air, so there is no way the car will have the same performance, it will lose power with altitude.
But with less air, the total cylinder pressure will be less, reducing the possibility for knocking. Therefore, fuel can have a lower knock rating for operation at altitude.
Knocking is when the remaining air/fuel mixture spontaneously ignites and burns all at once. Normal combustion is that of a flame front emanating from the spark plug and moving away from the plug.
When you are dead, you don’t know that you are dead. It is only difficult for the others. It is the same when you are stupid.
..
Knocking is when the remaining air/fuel mixture spontaneously ignites and burns all at once.
Sometimes referred to as dieseling.
If you don't know where you are going, you might wind up someplace else. - Yogi Berra
Ethanol??
Many states require the addition of ethanol to be added into gas for cleaner burning gas. While pure ethanol has an octane number of 99.5(R+M)/2. The percentage of ethanol added to the gas could alter the octane number.
Carburetor
From http://en.wikipedia.org/wiki/Octane_rating
"United States: in the US octane rating is displayed in AKI. In the Rocky Mountain (high elevation) states, 85 AKI (90 RON) is the minimum octane, and 91 AKI (95 RON) is the maximum octane available in fuel[citation needed]. The reason for this is that in higher-elevation areas, a typical naturally aspirated engine draws in less air mass per cycle because of 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 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." [/quote
Who still drives a carbureted car?
Interesting
This thread has inspired me to read a bit more about octane levels for different applications and different altitudes. I had never heard of different octane levels like that beyond what I am used to seeing for regular, mid, and premium where I have lived (and always saw the same numbers)
I will bet that the octane
I will bet that the octane rating will even vary from that advertised. I see differences in MPG from one fill up to the next when traveling long distances.
This may also be due to the gas-ahol additive.
romanviking
High Altitude
My gasoline powered van did not like the 85 octane regular when I was in the Rockies. I had to use 87 octane "mid-grade" to avoid knocking and pinging, so the thinner air factor doesn't necessarily work for all vehicles.
My electric truck would run fine at any altitude. It should even work with no air at all on the moon...