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Thread: Octane choice at high altitude

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    Octane choice at high altitude

    So, here in Colorado (and a couple other states) the octane choices are 85/87/91. Some areas (not Colorado) have a mandate to put a warning sticker on the 85 pump that says use at your own risk-could damage your engine.

    The manual says use 87. The gas cap says 87. The manufacture says explicitly use 87. Yet, most of us use 85. This is not a debate on using premium versus regular. I think I'm going to start using 87. The reasons 85 was allowed goes back to carburetor cars and the fuel industry has fought to keep it this way to avoid costly upgrades. There is no conclusive evidence one-way-or-the-other on of damage has occurred from using 85.

    Does anyone in Colorado know what the dealerships use? I know they don't use premium (unless it's tuned). Do they use 85 even though the manufacture "forbids" it? Does anyone know for a fact?
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    I would not use anything lower than 87 personally

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    Quote Originally Posted by Colorado_2018 View Post
    So, here in Colorado (and a couple other states) the octane choices are 85/87/91. Some areas (not Colorado) have a mandate to put a warning sticker on the 85 pump that says use at your own risk-could damage your engine.

    The manual says use 87. The gas cap says 87. The manufacture says explicitly use 87. Yet, most of us use 85. This is not a debate on using premium versus regular. I think I'm going to start using 87. The reasons 85 was allowed goes back to carburetor cars and the fuel industry has fought to keep it this way to avoid costly upgrades. There is no conclusive evidence one-way-or-the-other on of damage has occurred from using 85.

    Does anyone in Colorado know what the dealerships use? I know they don't use premium (unless it's tuned). Do they use 85 even though the manufacture "forbids" it? Does anyone know for a fact?
    I don't know, but manual says use 87, I use 87. Why chance it? Spend a couple extra bucks. If nothing else, suppose you do ever have a busted engine, using the "wrong octane" is one less thing they could throw at you.

    Edit: Could you be ok running 85? Probably, and I see a lot of CX-5's here now, and I'm sure plenty are running 85 because 85 is what most people use here, agreed. But considering the high compression ratio of the Skyactiv engine and it's design to use 87, I feel better running 87.
    Last edited by ColoradoDriver; 01-10-2019 at 12:33 AM.
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    Quote Originally Posted by Colorado_2018 View Post
    So, here in Colorado (and a couple other states) the octane choices are 85/87/91. Some areas (not Colorado) have a mandate to put a warning sticker on the 85 pump that says use at your own risk-could damage your engine.

    The manual says use 87. The gas cap says 87. The manufacture says explicitly use 87. Yet, most of us use 85. This is not a debate on using premium versus regular. I think I'm going to start using 87. The reasons 85 was allowed goes back to carburetor cars and the fuel industry has fought to keep it this way to avoid costly upgrades. There is no conclusive evidence one-way-or-the-other on of damage has occurred from using 85.

    Does anyone in Colorado know what the dealerships use? I know they don't use premium (unless it's tuned). Do they use 85 even though the manufacture "forbids" it? Does anyone know for a fact?
    At altitude, the volume of air in each piston stroke is the same but the weight/content of that volume is reduced due to the lower barometric pressure at altitude and lower density of the air - less air weight/content has expanded to the same piston volume. Even through the compression ratio is the same at altitude, the actual compression pressure in the cylinder is lower as less air got in the cylinder, allowing lower octane fuel to operate similar to higher octane fuel at sea level. Modern engines have antiknock sensors that the computer uses to reduce the spark timing and fuel mix to protect the engine from harmful knocking if for instance you got some 85 octane in at sea level in error etc. Everybody is using it, nobody is getting damaged but do as you wish, higher octane won't hurt it, it just burns extra money.

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    Yeah I've been using 85 in my vehicles the last 21 years if 87 was recommended due to being at higher elevation. The more research I do the more I'm rethinking it.

    Trying to find something official is touch but came across this from https://www.fueleconomy.gov

    "What is 85 octane, and is it safe to use in my vehicle?
    The sale of 85 octane fuel was originally allowed in high-elevation regions—where the barometric pressure is lower—because it was cheaper and because most carbureted engines tolerated it fairly well. This is not true for modern gasoline engines. So, unless you have an older vehicle with a carbureted engine, you should use the manufacturer-recommended fuel for your vehicle, even where 85 octane fuel is available."

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    Quote Originally Posted by GAXIBM View Post
    At altitude, the volume of air in each piston stroke is the same but the weight/content of that volume is reduced due to the lower barometric pressure at altitude and lower density of the air - less air weight/content has expanded to the same piston volume. Even through the compression ratio is the same at altitude, the actual compression pressure in the cylinder is lower as less air got in the cylinder, allowing lower octane fuel to operate similar to higher octane fuel at sea level. Modern engines have antiknock sensors that the computer uses to reduce the spark timing and fuel mix to protect the engine from harmful knocking if for instance you got some 85 octane in at sea level in error etc. Everybody is using it, nobody is getting damaged but do as you wish, higher octane won't hurt it, it just burns extra money.
    This is how it works. I used 85 for 20 years with "modern" FI engines at elevation, and had no issues. Upgrade to 87 if it makes you feel better, but there's no need; also no harm, except to your wallet. It is analogous to using "premium" vs "regular".

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    Quote Originally Posted by Colorado_2018 View Post
    Yeah I've been using 85 in my vehicles the last 21 years if 87 was recommended due to being at higher elevation. The more research I do the more I'm rethinking it.

    Trying to find something official is touch but came across this from https://www.fueleconomy.gov

    "What is 85 octane, and is it safe to use in my vehicle?
    The sale of 85 octane fuel was originally allowed in high-elevation regions*where the barometric pressure is lower*because it was cheaper and because most carbureted engines tolerated it fairly well. This is not true for modern gasoline engines. So, unless you have an older vehicle with a carbureted engine, you should use the manufacturer-recommended fuel for your vehicle, even where 85 octane fuel is available."
    This is back-asswards. Carbureted engines couldn't adjust their timing, so they wouldn't "tolerate" lower octane than necessary-- they'd just knock. FI engines do adjust.

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    GAXIBM's explanation is correct. To boil it down into something simpler: the reason you see 85 octane in high altitude areas (I have it in my area as well) is because for us it is the equivalent of using 87. So there is no harm. Mazda's ECU and knock/harmonic sensors do adjust timing for whatever octane you are using as well.

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    The barometric pressure being lower due to altitude is true. However I might leave the high altitude so I wouldn’t use it myself.

    If the motor would adjust that far I don’t know.

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    MAZDA's at least with Skyactauve engines should for best overall engine operation to at least use 87 displayed at the pump or 85 MON certified fuel. If only RON or RON/MON then 87 minimum should be the first choice with an occasional use of 85 because other fuels are not available at the time of getting gasoline.

    Anyone can get a simple and inexpensive OBDII direct access device, I prefer ULTRA GAUGE and set the page for ignition ,o2,timing,knock, fueling, etc., and compare testing different levels fuel 85,87,93,97,100 and what is going using them.





    I live at basically the bottom of the Serra Mountains Ca. We frequently drive over to RENO Nevada about a 2 hour drive, the peak is Elevation: 14,505′. My 2018 MAZDA NA runs just fine with the ECU compensating for those operating condition as I have viewed watching my UG.

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    Octane choice at high altitude

    I certainly wasn’t saying it wouldn’t run fine at the higher altitude. Rather if you fill with 85 octane then drive to a lower altitude will it? Considering it’s designed for 87 minimum. I don’t know as I have never tried it. Just saying I wouldn’t.
    Last edited by jthj; 01-10-2019 at 03:31 PM.

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    Quote Originally Posted by jthj View Post
    I certainly wasn’t saying it wouldn’t run fine at the higher altitude. Rather if you fill will 85 octane then drive to a lower altitude will it? Considering it’s designed for 87 minimum. I don’t know as I have never tried it. Just saying I wouldn’t.
    More or less my attitude.

    I ran 85 in my previous car just fine. But when I bought this one new, and taking impromptu road trips, decided it was easier to just throw 87 in and not worry about it.
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    Quote Originally Posted by jthj View Post
    The barometric pressure being lower due to altitude is true. However I might leave the high altitude so I wouldn*t use it myself.

    If the motor would adjust that far I don*t know.
    By the time you've driven down to lower elevation, you will have refilled the tank with higher octane, since that would be available at lower elevations.
    85 is not and never has been a problem at higher elevations. That's why it's sold there.

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    Quote Originally Posted by shadonoz View Post
    85 is not and never has been a problem ...... That's why it's sold here.
    Lets see OREGON very semi wet state lots of growth and nice lawns......Lawn mower use....85 octane fuel hahahahahaha!



    pulling your chain ya know!

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    Octane choice at high altitude

    Quote Originally Posted by shadonoz View Post
    By the time you've driven down to lower elevation, you will have refilled the tank with higher octane, since that would be available at lower elevations.
    85 is not and never has been a problem at higher elevations. That's why it's sold there.
    I can make it to lower elevation and quite a bit further without using all the fuel in my tank. You won’t use much gas going downhill.

    I get amazing gas mileage coming home from Colorado!

    But if you won’t leave that elevation I guess it might be fine. I just won’t do it is all I’m saying.
    Last edited by jthj; 01-10-2019 at 03:50 PM.

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