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Thread: Octane choice at high altitude

  1. #31
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    wow..and was thinking Arizona is cheap. about 2.70 where I am for 87.
    3.20+ when I filled last for 91 at shell.

  2. #32
    Structural Member shadonoz's Avatar
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    It's higher here 365/yr. We get Cali blends PLUS no self-service.
    I'd expect CA and WA to be similar, but dunno.
    Maybe 20 cents cheaper at grocery store.

  3. #33
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    Quote Originally Posted by cz5gt View Post
    wow..and was thinking Arizona is cheap. about 2.70 where I am for 87.
    3.20+ when I filled last for 91 at shell.
    Premium (93) was $2.27-2.39 here this evening, and $2.15 at Costco.

    I sure do miss the days of .10 increments between 87, 89, and 93...

  4. #34
    Structural Member shadonoz's Avatar
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    Quote Originally Posted by NelsonLewis View Post
    I sure do miss the days of .10 increments between 87, 89, and 93...
    Yeah, what's up with that?
    Nice extra profit.

  5. #35
    Resident barbarian ColoradoDriver's Avatar
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    Quote Originally Posted by NelsonLewis View Post
    Premium (93) was $2.27-2.39 here this evening, and $2.15 at Costco.

    I sure do miss the days of .10 increments between 87, 89, and 93...
    Even I remember those increments.
    2014 CX-5 Touring AWD 2.5L | Jet Black Mica - 83k miles
    01/2013 build | 4/20/2013 delivery

  6. #36
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    Back to why 85 is sold in high-altitude areas, as someone mentioned, for a normally-aspirated engine (i.e., no turbo or supercharger), the engine is essentially derated to 85% of so of its maximum power at 5K feet MSL simply by virtue of the thinner air at altitude. Since engine knock potential is directly related to specific power output per displacement, the potential for knock at 85 octane at 5K feel is essentially the same as 87 octane at sea level. So if the mfg says 87 is adequate at sea level, then 85 should be adequate at 5K feet.

    In reality, there are tens of factors that affect an engine's propensity to knock so its all a game of getting "acceptable" knock margins for average conditions. Using what the mfgs recommend is not an ironclad guarantee of the engine never knocking under any and all condition, but it gives you fairly high confidence that you're good for most conditions and in the corners of the envelope most engines have protective knock sensors nowadays. And all engines can tolerate some knock now and then. I'm dating myself here, but forty years or so ago - before FI and knock sensors - lots of engines knocked so mfgs and fuel makers got together and were advertising that a little light engine knock now and then was the "sound of peak performance."

    Now with turbocharging, things are different. I don't know what type of turbocharging system Mazda uses, but if it increases the amount of boost above atmospheric pressure to compensate for the power loss at altitude (so that the engine can make sea level peak power at altitude), then this derating of engine power with altitude is no longer happening and you really should be sticking to the octane numbers the mfg recommends, even if it means going up a grade at the pump. (This all has to do with whether then turbocharging system has an absolute pressure controller or whether it just bumps pressure a fixed amount above atmospheric).

    If I owned a new CX-5 turbo in Denver, I'd be feeding it 87 as a matter of course and would go higher if I knew I was going to be leaning on the engine. But for a normally-aspirated CX5, I'd go with the 85.

    - Mark
    Last edited by markjenn; 01-11-2019 at 12:50 AM.

  7. #37
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    The dealerships will use whatever is cheapest.

  8. #38
    Registered Member Lbear's Avatar

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    The engine will be more prone to detonation with low octane fuel in these conditions:

    Climbing mountain grades
    Driving in extreme hot weather
    Pulling a trailer
    Fully loaded vehicle with passengers

    The computer will pull timing and reduce power output if it senses detonation.

  9. #39
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    Quote Originally Posted by markjenn View Post
    before FI and knock sensors - lots of engines knocked so mfgs and fuel makers got together and were advertising that a little light engine knock now and then was the "sound of peak performance."
    - Mark
    All knock is damaging to an engine. Its an uncontrolled explosion! Geeeeez! I would like to see where this statement above in your response was ever posted???
    You want me to date myself LOL here is what one after market Performance manufacture offered to combat engine KNOCK!!!
    Edlebock, Holley, Clifford and few other had other products as well.
    NIB (new in box) Vintage 1980 or older?

    I may even have this exact part in a picture in my
    Perfoamnce shop in the early 80's?

    oh engine makers and gasoline refiners NEVER got together.
    Last edited by Grendel65; 01-12-2019 at 01:51 PM.

  10. #40
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    Quote Originally Posted by Lbear View Post
    The engine will be more prone to detonation with low octane fuel in these conditions:

    Climbing mountain grades
    Driving in extreme hot weather
    Pulling a trailer
    Fully loaded vehicle with passengers

    The computer will pull timing and reduce power output if it senses detonation.
    NICE basic information! ^^^^^^
    The ECU in Mazda's from about 2010 and newer does about 20 more adjustments besides the basic ones you mentioned.

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