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  1. #1
    MM-FXSVHO's Avatar
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    AFR on e85 vs 93 Octane

    To the best of my knowledge, I have always heard that optimal AFR on 93 octane gas should be around 11.8 - 12.4 (depending on tuner/modifications. My question is, what should the optimal AFR be on e85 for the same mods? As far as my mods, I am running your typical stage 3/4 with 28lbs of boost and spinning close to 10,000 RPM. This is on a 21 FX, but also building a 21 GP with same mods. Thanks!

  2. #2
    raiderx72's Avatar
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    What cam?

  3. #3
    MM-FXSVHO's Avatar
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    Quote Originally Posted by raiderx72 View Post
    What cam?
    N/A 1.8 VXR cams.

  4. #4
    WaterDR's Avatar
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    I know on a car it’s a lot lower…7.5 to 8.5 if I still recall.

    But someone with more experience should respond.

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    WaterDR's Avatar
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    Stoich on gas is 14.7 and only 9.7 on e85….so the answer would be certainly lower than stoich.

  6. #6
    MM-FXSVHO's Avatar
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    Quote Originally Posted by WaterDR View Post
    Stoich on gas is 14.7 and only 9.7 on e85….so the answer would be certainly lower than stoich.
    Thank you!

  7. #7
    Tropical Hoon Astrofizix's Avatar
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    Yeah with cars I don't think anyone runs above 9:1.

    But one thing to be careful with is the consistency. They can mix anything from 15% to 30% fossil gas into the ethanol and still call it E85 (instead of E70 etc). Colder areas and times of year tend to get lower ethanol content because of hard starting etc. And of course if you tune to the 85% stuff and then get a tank full of 70%, you will run rich, but also have a lower knock resistance so then boost could become an issue.

    However I am not sure of the E85 situation where you are.

  8. #8
    Quote Originally Posted by Astrofizix View Post
    Yeah with cars I don't think anyone runs above 9:1.

    But one thing to be careful with is the consistency. They can mix anything from 15% to 30% fossil gas into the ethanol and still call it E85 (instead of E70 etc). Colder areas and times of year tend to get lower ethanol content because of hard starting etc. And of course if you tune to the 85% stuff and then get a tank full of 70%, you will run rich, but also have a lower knock resistance so then boost could become an issue.

    However I am not sure of the E85 situation where you are.
    For what it’s worth, I have dealt with tuning Audi/vw between E85 and E70. In my experience there’s virtually no difference in knock resistance in those applications. But you still have to battle running a bit rich.

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    Tropical Hoon Astrofizix's Avatar
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    Quote Originally Posted by hypoxia View Post
    For what it’s worth, I have dealt with tuning Audi/vw between E85 and E70. In my experience there’s virtually no difference in knock resistance in those applications. But you still have to battle running a bit rich.
    Interesting, I would have thought the difference in those two blends would have been more than enough to show up when pushing boost / ignition timing to knock thresholds. I am lucky that I live in a hot climate so there is no need for blending below 85% Ethanol. In fact straight alcohol would really be no issue here, even in 'winter'.

    Did you happen to test the alcohol content in the two blends by any chance, to determine that they were actually as labelled? I suppose if you had the mixture issues then it shows they were certainly different. I wonder if maybe they even blend a higher octane petrol (gasoline) into the E70 to mitigate the octane loss from the mix ratio, such that the stoichiometric A/F ratio shows a larger variance than the knock resistance? It's things like this that make me wish pure alcohol fuel was road legal....

  10. #10
    mittens's Avatar
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    I was told that the fact these skis can not tell the change in the Mix, makes going to E85 a pain. as its cant sit for too long, and the pump e85 is inconsistent. On cars though its game changer, also being the fact you can just pull over and get more fuel.


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