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Thread: Benefits of High Octane at High Altitude?

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    Registered Member StewieCO's Avatar

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    Question Benefits of High Octane at High Altitude?

    Hello Everyone!

    I live in Evergreen, CO and I'm at 8,500 feet above sea level. I have a 2014 CX-5 (GT AWD) and would like to know if there is any benefit on using high octane gas at high altitude, knowing that the CX-5 engine has a high compression ratio?? I currently use regular gas (85 octane) but was wondering if I switch to premium gas (91 octane) I would get any benefit in terms of millage and/or horse power; or is just a waste of money.

    Thank you.

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    MoMo MikeM.'s Avatar

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    I wouldn't run 85 octane at any elevation (the specified minimum octane is regular 87 MON). There will be no benefit to running anything higher than this (assuming that the 87 octane is actually 87 octane). Sometimes it's not.

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    Quote Originally Posted by MikeM. View Post
    I wouldn't run 85 octane at any elevation (the specified minimum octane is regular 87 MON). There will be no benefit to running anything higher than this (assuming that the 87 octane is actually 87 octane). Sometimes it's not.
    85 octane is the natural decrease in octane needs when at high altitude.
    It's perfectly acceptable to run on 85 here in thin air land, any car that was designed to run on 87 at seal level.

    Just don't take a tank of it down to sea level, and start hooning it up.

    BC.

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    85 octane is perfectly fine in a place like Evergreen, or anywhere else in CO for that matter.

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    MoMo MikeM.'s Avatar

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    Quote Originally Posted by rideandslide View Post
    85 octane is perfectly fine in a place like Evergreen, or anywhere else in CO for that matter.
    Not true, while Colorado is home to over 50 peaks over 14,000 feet, roughly 2/5ths of the state is flat land with elevations as low as 3300 feet. That is not high enough to run 85 octane without potentially serious issues and, if you experience detonation, and Mazda knows you were running fuel 2 octane below the minimum, they do not have to pay for the resulting engine damage under warranty. A certain owner responsibility to follow basic manufacturers fuel requirements (among other things) is required to maintain warranty protection.

    Personally, I would follow the manufacturer's minimum octane rating regardless of how high I was. Double meaning intended.
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    It seems that you are more concerned with being right than factual understanding of how higher elevations make this whole argument ridiculous. There is so little of this State that sits at 3300-3500 so if this is the technicality that you need to call all I can say is "oh well." My statement still applies to the significant population centers of the State, including the high plains, front range, high country, and the western slope. For the 20,000 people that this does not apply to my apologies.

    Point being it isn't and won't ever be a warranty issue, because less oxygen means lower compression which means 85 Octane runs just fine. If you want to bury your head in the sand from science, I'm not here to save you.

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    Also assuming the ASTM scientific study is accurate people running 87 at lower elevations are closer to knocking than someone in say Denver running 85 octane. Even if you take the midpoint of all studies you get 1.06 ON per 1000 feet. So that leaves even the folks at 3300 safe so for you 20,000 people I rescind my apology.

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    MoMo MikeM.'s Avatar

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    Quote Originally Posted by rideandslide View Post
    Even if you take the midpoint of all studies you get 1.06 ON per 1000 feet. So that leaves even the folks at 3300 safe so for you 20,000 people I rescind my apology.
    Detonation is not a simple function of altitude and compression. Humidity, temperature, and barometric pressure all play significant roles.

    But my point is that if you don't want to jeopardize your warranty coverage you will follow the manufactures minimum octane. Cars have wheels, my CX-5 running on regular 87 has a range of over 500 miles, more than enough to get into trouble, even from Independence Pass on the Continental Divide. And the humidity in the Rockies is capable of becoming extremely low. Because humidity is a very significant factor in terms of the detonation threshold, this is worth considering. Moisture in the air slows down the burn rate, something that can be counted on in coastal areas.

    But if you want to save a few cents playing with fuel that doesn't meet the manufacturers minimum standards, that's your prerogative. I'm just saying Mazda does not have an exception to their minimum octane requirements based on altitude.
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    Humidity, temp, and barometric pressure vary everywhere all the time and if it was as significant as you felt people would be making regular decisions about ON based on current and forecasted variances. It's more than a few cents, but even if it was just one cent and science said "no brainer" I would still make the same choice because my family and charity will make better use of that penny than my Mazda. I own 3 vehicles now that all require 87 octane in the manual, and none of them get it. My truck has 150k, my car has 140k, and my wives CX5 now has 503. None have had a problem with 85 and none ever will. Scientifically proven way to keep more money in my pocket? No brainer.
    Last edited by rideandslide; 08-07-2013 at 12:07 AM.

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    Quote Originally Posted by MikeM. View Post
    Detonation is not a simple function of altitude and compression. Humidity, temperature, and barometric pressure all play significant roles.

    But my point is that if you don't want to jeopardize your warranty coverage you will follow the manufactures minimum octane. Cars have wheels, my CX-5 running on regular 87 has a range of over 500 miles, more than enough to get into trouble, even from Independence Pass on the Continental Divide. And the humidity in the Rockies is capable of becoming extremely low. Because humidity is a very significant factor in terms of the detonation threshold, this is worth considering. Moisture in the air slows down the burn rate, something that can be counted on in coastal areas.

    But if you want to save a few cents playing with fuel that doesn't meet the manufacturers minimum standards, that's your prerogative. I'm just saying Mazda does not have an exception to their minimum octane requirements based on altitude.
    Okay Mike, it's time for you to stop pretending that you know everything about autos.

    Colorado, and every surrounding state that have the Rocky Mountains and High Plains have been running 85 octane fuel for decades, without a single recorded octane related failure of a car, probably in the same amount of time. There are many many thousands of cars sold here every single year, and there have been many millions of various vehicles and fuel injection systems, including carbs and high revving motorcycles, and none of these vehicles have failed due to octane related issues using 85 in a vehicle that recommends 87 at sea level.

    Every year, thousands of cars visit their dealers here at high altitude, and the manufacturers don't void people's warranties for using 85 octane, as it's what is recommended for high altitude.

    Millions of vehicles, and thousands of warranty claims a year, Mike.
    No denied claims.
    No low octane related issues.

    Now, give it up, you don't know everything.

    BC.

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    Registered Member IHeartGroceries's Avatar

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    Agreed - don't fret running regular, 85. In fact, I'd suggest you just stick with it, in high elevation.
    Just as, IMO, you won't see benefit from running super through a CX-5 at sea level, you won't see any from running a plus blend at elevation.

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    Registered Member StewieCO's Avatar

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    Thanks everyone for your input. I was getting dizzy with all the technical info above lol.

    I'm not really concerned about running 85 up here; that's what I typically use. Also, I asked the dealer about that and they say it is totally ok and won't void the warranty to use 85 here in CO. My question was more directed to the use of higher octane (91 in our case) with the CX-5 and being up here; if there is a benefit in regards of fuel economy, horse power, etc... Have anyone used premium gas and compared the performance vs regular gas at altitude? Or it's just the same and a waste of money to use 91 vs 85?

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    Quote Originally Posted by StewieCO View Post
    Thanks everyone for your input. I was getting dizzy with all the technical info above lol.

    I'm not really concerned about running 85 up here; that's what I typically use. Also, I asked the dealer about that and they say it is totally ok and won't void the warranty to use 85 here in CO. My question was more directed to the use of higher octane (91 in our case) with the CX-5 and being up here; if there is a benefit in regards of fuel economy, horse power, etc... Have anyone used premium gas and compared the performance vs regular gas at altitude? Or it's just the same and a waste of money to use 91 vs 85?
    It will only come into play if you are planning on driving down to lower altitude on a full tank, or if you are working the car really, really hard.
    Other than that, there is no difference in fuel economy or power, that I have noticed with my CX-5 here in Arvada, when I experimented with different octanes.

    BC.

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    Registered Member StewieCO's Avatar

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    Quote Originally Posted by Bladecutter View Post
    It will only come into play if you are planning on driving down to lower altitude on a full tank, or if you are working the car really, really hard.
    Other than that, there is no difference in fuel economy or power, that I have noticed with my CX-5 here in Arvada, when I experimented with different octanes.

    BC.
    Thanks BC.

    Since I open this thread I decided to experiment myself using 91. I typically get an average of 28.2mpg with 85. On 91 I'm getting 29.3mpg so far and only two fuel-ups. Please note I drive up and down the foothills everyday (live up, work down). I will update the numbers here after the next one.

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