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Octane vs Altitude

Discussion in 'Gen 4 Prius Fuel Economy' started by MrMischief, May 25, 2016.

  1. MrMischief

    MrMischief Active Member

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    I'm going to regret asking this question, I'm pretty sure it's a bannable offense on most forums along with asking about oil......

    What octane of gas should I be putting in the Prius? Now before you tear into me with "read the manual" or "87 regular, DUH!!" please realize that I live in Colorado where "regular" is 85 octane. This is my first non-Chrysler product that I've bought and I'm used to those saying "regular" or "premium" and when pushed Chrysler has told me "whatever the low grade is for your area is good." The Prius says 87 octane. Seems specific. I sent an email to Toyota asking if that means it really needs 87 octane at all elevations or if I can get away with using regular here. I got back a chart that shows Toyota's entire line up requires 87 octane. It just feels like I'm talking to a lawyer rather than an engineer. (side note, does Toyota ever do engineer chats on the forums like Chrysler does?)

    So the reason Colorado and other mountain states have 85-ish octane (Montana is 85.5) is that as elevation increases the fuel is less likely to pre-detonate. I'm sure there's something about starting issues and complete fuel burn that is used for the justification of 85 octane. On turbo or supercharged engines elevation has less of an impact so you run what the motor is made for, but on an NA motor that has not been the case for me historically. The biggest reason I bought the Prius over some other options is to save money on gas. Having to step up to mid-grade has a considerable impact on that. Can (should?) I get away with running 85 octane as long as I'm staying up here and not heading down to sea level.
     
  2. bwilson4web

    bwilson4web BMW i3 and Model 3

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    The Prius has a knock sensor and uses it to tune for the grade of gasoline. I've long had a desire to run 85 octane at high altitudes for some Prius mileage tricks. . . . Perhaps another day.

    Bob Wilson
     
  3. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    OK let start at the beginning.

    When the piston goes from the bottom of the cylinder to the top, it compresses the air in the cylinder 5 to 15 times as much as it was when the piston was at the bottom. In general the higher the compression, the higher the octane needs to be to resist spontaneous combustion or Knock.

    By the time you are at 4000 feet, over 15% of the air is below you so your air trapped in the cylinder is 15% less and your compression ratio act like it is 15% less.* Because this altitude induced Knock resistance will remain while you are at 4000 feet you need lower octane gas to run well. (If you buy gas at 4000 feet and then commuted to sea level, you would have some Knock. Get better gas when you are about to leave CO)

    In theory, you would have the opposite issue 4000 below sea level, but hardly anyone drives gas engined vehicles 4000 feet under sea level.

    *turbochared engines do not work like this, as the turbo just keep on pumping air until they get the same density as sea level, do not buy low octane gas for a turbo
     
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  4. alfon

    alfon Senior Member

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    Ran a tank of 85 octane in Reno Nevada in our 2010 Prius. No problems and the same mpg's as 87 octane, 55+ mpg calculated, with 87 octane or 85 octane.
     
  5. MrMischief

    MrMischief Active Member

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    Perfect! that's what I was looking for! Any other car I could figure this out myself rather quickly as I'd burn 4 - 6 tanks of gas in a month easily. Seems like the way this tank is going it's going to be winter gas season before I gathered any data myself.
     
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  6. TomNat

    TomNat Junior Member

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    We just drove from FL to Colorado Springs. Regular gas was 87 octane from FL to maybe Ft. Worth TX altitude 610ft. At Childress TX altitude 1900ft, gas switched to 86 octane. It also was 86 octane in Clayton NM at 5000ft, and was 85 octane in Colorado Springs at 6000ft.

    We do not know at exactly what altitudes the octane changes, as we only bought gas at Buffalo TX 300ft about 120 miles southeast of Ft. Worth, at Childress, Clayton & Colorado Springs. We did not notice any change in mpg that we could blame on lower octane.
     
  7. wjtracy

    wjtracy Senior Member

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    Many threads...Prius is fine with 85 Octane in those high altitude areas where 85 is same as 87 due to altitude. We must have officially from Toyota somewhere...not sure where.
     
  8. CR94

    CR94 Senior Member

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    I agree with most of the above responses. Don't worry about the 85 octane rating if you aren't hearing knock, and aren't about to take that fuel down to a much lower altitude. That's the same as for any other car designed for "regular" unleaded.