TECH: High altitude and octane
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This is a discussion on TECH: High altitude and octane within the High Altitude Tuning forums, part of the Engine Modifications category; Note: For those who frequent i-club , you have seen this before I recently moved out to Colorado Springs (elv. ...

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    Registered User EtchyLives's Avatar
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    TECH: High altitude and octane

    Note: For those who frequent i-club, you have seen this before

    I recently moved out to Colorado Springs (elv. 6053 ft) and noticed that the octane sold at the pump here is:

    regular 85
    better 87
    Premium 91

    Everywhere else I've ever driven is usually 87, 89, 91-94. I was wondering about altitude's effect on a vehicles aspiration--namely that since there is 2/3's as much oxygen here as at sea level that the chance of running a lean condition is less. Therefore requiring less of an octane rating.

    My counter-argument is that the gas stations here are getting away with (literal) highway robbery by selling cheaper gas at a more expensive price-point.

    Since I normally run 93 octane and have my Cobb AP tuned for 93 I am concerned about running 91. Granted, I've already noticed the power drop-off from driving in the midwest (elv. roughly 1,000-2,000 ft) with 93, but I'm curious how much of that is because of elevation change and how much is octane change.

    I'm off to Lowe's to get some Xylene and artificially boost my octane rating (1 gallon Xylene ot full tank of 91 octane gas should give roughly 94 Octane) and check on the results, but any input would be greatly appreciated.

    -Greg

    PS. I can now only get 0.9 bar when I formerly could get 1.1 bar. Looks like I may need a reflash or something

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    Zem
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    Yeah, paying the same price because it's "Premium" is crap when it's a different octane. But, then again, the price of gasoline is.. well... some magic number with no basis in any reality I'm aware of.

    The lower absolute pressure in the cylinder creates less heat when it's compressed, hence the lower octane rating works fine. Now, as with any car you can tune, the higher octane you can tune to, the better. I think most people use toluene for octane booster, though.

    For your question on boost pressure.... Something I posted before
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    Registered User EtchyLives's Avatar
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    Originally posted by Zem
    Yeah, paying the same price because it's "Premium" is crap when it's a different octane. But, then again, the price of gasoline is.. well... some magic number with no basis in any reality I'm aware of.

    The lower absolute pressure in the cylinder creates less heat when it's compressed, hence the lower octane rating works fine. Now, as with any car you can tune, the higher octane you can tune to, the better. I think most people use toluene for octane booster, though.

    For your question on boost pressure.... Something I posted before
    Outstanding info. You answered my questions that I had with what my basic knowledge of science told me.

    About the Xylene/Toulene: Toulene is harder to find that Xylene and has a lower (depending on mix, of course) octane rating. From the experiments I did in Hawaii I found no greater chance of stripping the oil from the cylinders or destroying fuel lines with Xylene than with Toulene. Of course, the highest ration I ever saw done was 2 gal Xylene to 7 gal 92 Octane. If you have questions about this, please refer to gpatmac over at i-club. He's not in the states right now, but he does check his PM's.

    A fix to the corrosive nature of Xylene has been flown. Somewhere over at NASIOC there is a link to the formula to make your own "safe" octane booster. Something like 1 gal Xylene, a couple ounces mineral spirits, and roughly a capful of ATF. Once again, I've never had issues with Xylene causing any problem, but if you see my car you may doubt my wisdom.

    Thanks again for the info. Hopefully I'll see you out at CHP next Sunday.

    -Greg

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    Registered User Capt. Ronin's Avatar
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    I've use quite a bit of Xylene and Toulene in the past with my Rex. I never encountered any side effects with the use. At the end, I just got tired of mixing my "cocktail" and just went with Sunoco 100. I never did the mineral oil mixing, just figured the gas I was mixing with had enough lubricants and cleaners already in it.

    As far as sourcing the stuff, I would get mine from either Sherman Williams or Kwal Paint. I would buy it in 5 gallon containers and mix it down into smaller units. A bit of a pain, but had some pretty good results.

    Another Hawaii transplant.
    Aloha
    "Knowing is not enough, you must apply. Willing is not enough, you must do."

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    Since you are in the Springs and if you didn't already know there is one gas station I know of (and used to live 4 blocks from =) that sells 93 octane. It's about $0.10 a gallon more but WORTH every penny IMHO.

    When I tuned my 02 WRX with a STG4+TMIC+headers and UTEC I could get run about 2psi more (20psi versus 18psi) on 93 versus Amoco 91 in the summer. In the winter it is less of an issue with the cooler air.

    Luckily I have found a gas station up here just south of Loveland that sells 93 octane on my way to work . Haven't tried it out yet though.
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    Registered User Ozymandias's Avatar
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    i am confused by this absolute preasure thing. i remember something from long ago about it, but..... also bear in mind, i am n/a.

    thanks in advance.
    -Mark Malsom

    rally: 2 down. up next: rally cross, rally sprint, hill climb, cog.

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    Registered User EtchyLives's Avatar
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    Originally posted by davidm_sh
    Since you are in the Springs and if you didn't already know there is one gas station I know of (and used to live 4 blocks from =) that sells 93 octane. It's about $0.10 a gallon more but WORTH every penny IMHO.

    When I tuned my 02 WRX with a STG4+TMIC+headers and UTEC I could get run about 2psi more (20psi versus 18psi) on 93 versus Amoco 91 in the summer. In the winter it is less of an issue with the cooler air.

    Luckily I have found a gas station up here just south of Loveland that sells 93 octane on my way to work . Haven't tried it out yet though.
    Where is this mythical font of high-octane goodness? Hopefully not too far. Also, is Sunoco 100 available nearby? Or do I have to go to Denver.

    Xylene is such an attractive option since Lowe's is right around the corner.

    -Greg

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    Originally posted by EtchyLives
    Where is this mythical font of high-octane goodness? Hopefully not too far. Also, is Sunoco 100 available nearby? Or do I have to go to Denver.

    Xylene is such an attractive option since Lowe's is right around the corner.

    -Greg
    DOH. I realized when I was reading that I did leave out that semi-important detail of location [heh]. It is at Farmcrest at the corner of Centennial and 30th St.
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    Registered User EtchyLives's Avatar
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    Originally posted by davidm_sh
    DOH. I realized when I was reading that I did leave out that semi-important detail of location [heh]. It is at Farmcrest at the corner of Centennial and 30th St.
    Thanks. Hopefully this helps out my currently DISMAL performance.

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    Registered User pace's Avatar
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    TTT

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    Hey I'm a newcomer and I've got a similar question pertaining to the octane difference and tuning. I'm from Texas and am used to 93 octane tuned to 93 with my AP. So now I live in Boulder, CO which is roughly 5,500ft in elevation and the highest octane is 91. I read that since the altitude is higher that 91 is sufficient, and also that 91 at my altitude is almost like 95 at sea level... So my main question is, what octane do I tune for? Right now I'm tuned to 91, but it seems like I could tune to 93 since 91 is more like 95? If anyone could shed some light on this I would deeply appreciate it.

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    Registered User SKI.WRX's Avatar
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    I live in Silverthorne but I am in Boulder middle of the week for school.
    Here are some answers to your questions although this thread is over 10 years old.

    Tune to 91 octane not 93. In many Rocky Mountain states we only get 91 octane how it has been my whole life I am not certain why.

    At higher altitude the atmospheric pressure is lower. This means that the density of the atmosphere is less and there for contains less Oxygen ( and all other air constituents) in a given volume.

    This is super simplified but you will get the idea, a turbo charger is controlled by a boost control system with specific targets for boost pressure at a given rpm and load. It does not care or know what the pressure on the intake side of the turbo is, it just spins until the air on the outlet side is at the target level. So, to compress less dense air to the same target density takes more work by the compressor and therefore more heat than at lower altitudes.

    The point is that your maximum cylinder pressures are the same with a turbo at any altitude therefore you still need the same octane as your tune called for and probably even more since the turbo is working harder and hotter so even more detonation resistance (octane rating) is needed. One option many people in CO use is E85. There is a station in Boulder that has it on 28th and Valmont SW corner, you will obviously need a re-tune and possibly injectors depending on what you currently have.

    If you need a tuner I would recommend Harvey at The Boost Creep in Denver. He has a very good reputation in Colorado.

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