85 Octane Gas in Mountain States - Fiberglass RV


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Old 06-27-2015, 12:24 PM   #1
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85 Octane Gas in Mountain States

Just to mention something I encountered in several of the mountain states this past month:

Many stations sell 85 octane fuel as Regular and 87/88 Octane as mid-range. The 85 Octane is always the lowest price on the board and I always bought the mid-grade as my owners manual calls for a minimum of 87 octane fuel.

In past years that was also called "Farm Gas", I don't know who uses it in over-the -road vehicles, but I would avoid it, especially in mountain areas.

I know that modern engines will sense knock and retard spark as required, but I think that the loss of power will also result in an increase in fuel usage.
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Old 06-27-2015, 02:33 PM   #2
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In https://en.wikipedia.org/wiki/Octane_rating it says:
"... United States: in the US octane rating is displayed in AKI. In most areas, the standard grades are 87, 89-90 and 91-94 AKI.[41] 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.[42] 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. A disadvantage to this strategy is that most turbocharged vehicles are unable to produce full power, even when using the "premium" 91 AKI fuel. In some east coast states, up to 94 AKI (98 RON) is available.[43] As of January, 2011, over 40 states and a total of over 2500 stations offer ethanol-based E-85 fuel with 105 AKI.[44] Often, filling stations near US racing tracks will offer higher octane levels such as 100 AKI .[citation needed] ..."
Hope this helps.
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Old 06-27-2015, 03:03 PM   #3
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WOW... That is great information, I wondered why the lower Octane was being offered. There's no mention of the octane v. altitude consideration in my manual.
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Old 06-27-2015, 03:19 PM   #4
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If I understand correctly the difference between 85 at altitude vs. 87 at sea level is an artifact. I believe that it is the same stuff but when measured at a different pressure (as a function of altitude) the octane measurement is different.

Water boils at 200F. where I live. It is just different.
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Old 06-27-2015, 04:01 PM   #5
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I found those states with the farm gas also offered premium gas for about .20/gal more. On a recent 7,000 mile cross country trip I average 14.5 mpg using premium on the return side of the trip.
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Old 06-27-2015, 06:33 PM   #6
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We never changed grades of gas in our Honda CRV as we towed around the country. AS a result I can't say what benefits there might be however I can say I never saw a general difference in gas mileage as we went her and there.
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Old 06-28-2015, 06:55 AM   #7
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I use the 85 and it works just fine in my truck.
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Old 06-28-2015, 08:01 AM   #8
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85 Octane Gas in Mountain States

What happens if you are passing through, fill your tank with the lower octane gas, and continue on to a lower elevation? I have sometimes paid the extra price for mid-grade just to be safe.

I am hearing two different things here. One, it's two different things, but the lower octane compensates for the lower oxygen density at higher elevations. Two, it's the same thing, but measures differently at higher altitude.

Hmmm...?
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Old 06-28-2015, 09:44 AM   #9
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Me thinks that most modern engines can compensated for the lower octane if required, but am with Jon and would just buy the higher grade, especially because the area in which it is sold if often fraught with long grades.


Besides that, compared to California gas process, even premium is cheeeep.
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Old 06-28-2015, 09:44 AM   #10
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My motto: "Better safe than sorry". I'll opt for no change in octane to protect my engine. I do wish this country would give up that ethanol addition to our gasoline...it reduces gas mileage and can harm your engine overtime by not supplying proper lubricants to your cylinders...bet most folks don't know that correctly blended fuel does lubricate your engine and the addition of ethanol to gasoline can cancel that lubrication.
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Old 06-28-2015, 09:53 AM   #11
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Quote:
Originally Posted by Jon in AZ View Post
...
I am hearing two different things here. One, it's two different things, but the lower octane compensates for the lower oxygen density at higher elevations. Two, it's the same thing, but measures differently at higher altitude.

Hmmm...?
I don't know which it really is but I believe that they are the same. I just buy low-test.


There is another altitude issue not often discussed. I haven't gone thru the Eisenhower Tunnel on I70 in CO during the summer in a long time. I do recall on hotter days that there were lots of overheated cars approaching the tunnel which is at about 11,000 feet above sea level. My theory is that it takes a lot of energy to go uphill creating lots of heat and the the density of the air is so low that there is much less air mass to carry away the heat. If your cooling sysem was marginal you'd get caught.
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Old 06-28-2015, 10:03 AM   #12
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Overheating at Altitude

Quote:
Originally Posted by minke View Post
I don't know which it really is but I believe that they are the same. I just buy low-test.


There is another altitude issue not often discussed. I haven't gone thru the Eisenhower Tunnel on I70 in CO during the summer in a long time. I do recall on hotter days that there were lots of overheated cars approaching the tunnel which is at about 11,000 feet above sea level. My theory is that it takes a lot of energy to go uphill creating lots of heat and the the density of the air is so low that there is much less air mass to carry away the heat. If your cooling sysem was marginal you'd get caught.
My experience is that most of the overheated vehicles stopped along side the road are the same ones that passed me earlier going 75MPH up the hill with the windows all closed and the a/c going full blast.

Waaaay back in my Super Connie daze (EC-121k radar planes) we didn't have to compensate for engine cooling due to altitude changes, only outside air temp changes. Now temp/altitude concerns for lift is a different story.
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Old 06-28-2015, 10:09 AM   #13
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I live in Florida, but am in the Colorado Rockies now. I will spend the extra on 87 octane, rather than take a chance since my truck is new. Good insurance!
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Old 06-28-2015, 10:55 AM   #14
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Quote:
Originally Posted by Bob Miller View Post
My experience is that most of the overheated vehicles stopped along side the road are the same ones that passed me earlier going 75MPH up the hill with the windows all closed and the a/c going full bla nst.

Waaaay back in my Super Connie daze (EC-121k radar planes) we didn't have to compensate for engine cooling due to altitude changes, only outside air temp changes. Now temp/altitude concerns for lift is a different story.
It is well established that autos with open windows at >50 or 60 MPH have more drag from turbulence than the energy cost of air conditioning.

Thanks for stirring me up to read about Lockheed Constellations. I read a little about the Wright R-3350 engine too. One thing I've never understood to my satisfaction about air cooled engines is how much is air cooled, how much is oil cooled, and is there any temperature regulation like the thermostat in our water cooled engines. Getting back to the Eisenhower Tunnel high altitude cooling issue, My thought is that if you had adequate cooling you made it and if your cooling was inadequate you had some picnic time.

BTW the Wright R-3350 engine didn't always have adequate cooling. There is a very confusing discussion (confusing to me at least) of overheating at https://en.wikipedia.org/wiki/Wright...Duplex-Cyclone

If I've dragged us too far off topic I apologize. Just tell me and I'll quit.
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