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Thread: Pump vs E85 on turbo map

  1. #1
    wants Sloan clone Lonx's Avatar
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    Pump vs E85 on turbo map

    Simple one here and I could be completely stupid but happy to be told so...

    Suppose we're looking at a compressor map and considering the airflow of say 40lb/min when using whatever pump fuel at stoic. If the fuel was then switched to E85 and therefore changing the optimum stoic (richer, so less air per cycle), would this effectively reduce the airflow required, maybe 30lb/min in the same conditions and output?

    I'm sure this might seem like 101 but have been asking a few friends from here and I might be explaining poorly but interested in your thoughts. Thankyou
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    Quote Originally Posted by klampykixx
    if a motor has big enough injectors to run say, 7psi but only just starts to max out right on rev limit, will it run more boost lower in the revs without maxing out if you lower the rev limit?

  2. #2
    BLING BLING PLAYA's Avatar
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    Gut feel is no. For a given hp you need same air, fuel volume will vary depending on energy contained.
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  3. #3
    \_(ツ)_/ burn is weird's Avatar
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    you have this backwards I think. it won't be less air, but more FUEL.

    your engine is an air pump, you add as much fuel as you need to hit a target lambda for the air that is ingested. at WOT you don't have control over the air ingested (other than target boost (or pressure ratio on the compressor map) at a specific operating WOT operating point (100% TPS, x RPM, y Boost) you motor takes in the same air, you however will need more FUEL to match this due to the stoich ratio of combustion of the fuels.

    at lambda 1.0:
    for gasoline, this is 14.7:1 by mass.
    for e85 it is 9.8:1 by mass.

    ethanol is about 0.785 and 98 is about 0.745 specific gravity at 20 degrees C.

    so to hit the same lambda target with the different fuels you need 1.5x more fuel by weight, or ~1.43 by volume.

    you can usually get away with a leaner mixture on ethanol due to the lower cylinder temps and slower burn. so that reduces fuel requirement further.

    but if you're looking at compressor maps, ignore the fuel. e85 means you can just operate higher in the table (more BOOOOST)
    Last edited by burn is weird; 21-05-19 at 04:25 PM.

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  4. #4
    wants Sloan clone Lonx's Avatar
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    Quote Originally Posted by burn is weird View Post
    you have this backwards I think. it won't be less air, but more FUEL.
    your engine is an air pump
    QFT!! Sorry, that was basically the brain fart that made me waste half a dozen hours Really appreciate the clear response and I'm sure it'll help others as well mate. I'll be running one of the gen2 garrett's so will love just a touch of boost and efficiency to run with it! Thanks so much.

    PLAYA, you got there first and didn't even know you were still around! Cheers mate.
    Last edited by Lonx; 29-05-19 at 07:48 PM.
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    Quote Originally Posted by klampykixx
    if a motor has big enough injectors to run say, 7psi but only just starts to max out right on rev limit, will it run more boost lower in the revs without maxing out if you lower the rev limit?

  5. #5
    Desert Nigga vet 180's Avatar
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    Quote Originally Posted by Lonx View Post
    Simple one here and I could be completely stupid but happy to be told so...

    Suppose we're looking at a compressor map and considering the airflow of say 40lb/min when using whatever pump fuel at stoic. If the fuel was then switched to E85 and therefore changing the optimum stoic (richer, so less air per cycle), would this effectively reduce the airflow required, maybe 30lb/min in the same conditions and output?

    I'm sure this might seem like 101 but have been asking a few friends from here and I might be explaining poorly but interested in your thoughts. Thankyou
    The turbo airflow is the turbo airflow. It doesn't change depending on the fuel used. Fuel volume will change depending on the airflow and type of fuel used.

    What you might be thinking though is different fuels can change the VE of the engine, therefore meaning you can make more power for a given airflow? In turn a smaller turbo can make more power due to a higher VE.
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  6. #6
    Registered User GSRman's Avatar
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    Or a bigger turbo can spool faster because of increased exhaust gas.
    This is a post i wrote by mistake, which is nice...

  7. #7
    Registered User MWP's Avatar
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    Quote Originally Posted by vet 180 View Post
    What you might be thinking though is different fuels can change the VE of the engine, therefore meaning you can make more power for a given airflow?
    How does fuel type (or fuel at all) change VE?
    The only thing I can think of is fuel volume displaces air volume, so the more fuel you inject, the lower the VE.

  8. #8
    \_(ツ)_/ burn is weird's Avatar
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    yes, this is one factor, but the opposing factor is the removal of latent heat of vaporisation energy from the inlet charge as it changes phase, which cools it down so it's density increases, and you fit more in. if you have an ecu that has a charge temp offset table (or charge air cooling coefficient) the combination of these effects is negated in the fuel model, and because high ethanol mixtures require more fuel to hit a lambda target, the cooling effect is stronger than the air displacement effect.

    for an example of density difference, Liquid ethanol is 785kg/m3 and vapourized it is about 1.59kg/m3 at one atmosphere.

    in my ecu gasoline has an offset of 12 degrees at 1.0 lambda, and ethanol is offset by 18. this is effectively a 2.5% charge density change due to fuel type.
    Last edited by burn is weird; 05-07-19 at 02:44 PM.

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  9. #9
    Desert Nigga vet 180's Avatar
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    Quote Originally Posted by GSRman View Post
    Or a bigger turbo can spool faster because of increased exhaust gas.
    Yeah the difference is quite big. Being able to run a much more aggressive tune to get the turbo to full boost does wonders for power under the curve
    Quote Originally Posted by Babalouie View Post
    Geez we're a bunch of softcocks...we have a 911 and we're obsessing over non-functional ducts and indicator colours

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