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Thread: Stoich Table Accuracy?

  1. #1
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    Stoich Table Accuracy?

    When I dialed my MAF in at 80% E, EQ error was +/- .5%. Now I'm back down to regular 93, showing 11% E, and the EQ error is around -4%. Pulled the stoich table values from a factory flex truck tune, 14.678 at 0% interpolated to 8.976. Is that table just that far off, or is there something I'm missing?

  2. #2
    Tuning Addict 5FDP's Avatar
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    I think it's pretty accurate, never really had reason to correct it before. Are we talking the ~4% with just normal driving in or at WOT?
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.

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    WOT specifically, although at a glance it looks like LTFT's are around -4% more throughout the log as well compared to my last E85 log

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    Advanced Tuner jsllc's Avatar
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    You are not taking into account the engine's ability to combust e80. All of these engines were built to run gas. Not near enough cylinder head pressure for efficient combustion of ethanol/alcohol. So you end up rich when burning gas.
    2012 ZL1 - Maggie Heartbeat, Port & Polish Heads, Custom Cam, Custom rotating assembly, steel sleeved LS9, No NOS and No water meth. 16psi
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    Quote Originally Posted by jsllc View Post
    You are not taking into account the engine's ability to combust e80. All of these engines were built to run gas. Not near enough cylinder head pressure for efficient combustion of ethanol/alcohol. So you end up rich when burning gas.
    Wouldn't it actually be that all the excess ethanol that isn't combusting make the resulting mixer read rich in the wideband, while a complete burn of gasoline would make it read leaner...?

  6. #6
    Advanced Tuner jsllc's Avatar
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    The math for ethanol is 100% There is no miscalc on that unless it is changed from defaults.

    Using your theory lambda 1 would be richer.

    An incomplete burn will show more free O2 for the WB to pickup so measure a higher lamba. You will and fuel to drive that down. A more complete burn will show less free O2 therefore lower lambda and you will reduce fuel to bring the number up.

    WB is an O2 sensor not a fuel burn sensor.
    2012 ZL1 - Maggie Heartbeat, Port & Polish Heads, Custom Cam, Custom rotating assembly, steel sleeved LS9, No NOS and No water meth. 16psi
    810rwhp and 820rwtq 91 Octane 6400 rpm
    948rwhp and 951rwtq 105 Octane 6400 rpm
    999rwhp and 997rwtq on 60% Ethanol 6400 rpm

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    Just meant a more complete burn of the fuel would result in a leaner reading due to less unburnt fuel remaining after combustion. But okay. So you're saying to just play with PE tables to get to my desired lambda even if the MAF table will be off? i.e. tell it to target ~.91 to get .87 actual?

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    Try setting your Injector Flow Rate Alcohol Multiplier to 1.0 for all and tweaking your MAF again to be inline. Then go back to E85 and see if there is less variance in LTFT's between fuels.
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  9. #9
    Advanced Tuner jsllc's Avatar
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    More complete burn means lower lambda. Less free O2.

    No you do not change PE. You change Ethanol modifiers.MAF is either right or not. Usually you tune with no Ethanol, then tune with it getting the modifiers right with no change to MAF.
    Last edited by jsllc; 05-14-2020 at 08:15 PM.
    2012 ZL1 - Maggie Heartbeat, Port & Polish Heads, Custom Cam, Custom rotating assembly, steel sleeved LS9, No NOS and No water meth. 16psi
    810rwhp and 820rwtq 91 Octane 6400 rpm
    948rwhp and 951rwtq 105 Octane 6400 rpm
    999rwhp and 997rwtq on 60% Ethanol 6400 rpm

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    Quote Originally Posted by TriPinTaZ View Post
    Try setting your Injector Flow Rate Alcohol Multiplier to 1.0 for all and tweaking your MAF again to be inline. Then go back to E85 and see if there is less variance in LTFT's between fuels.
    I'll give that a shot. The fact that the 75% cell was 3% and the 87.5% was around 3.5% is promising

  11. #11
    any update on changing the multiplier to 1?

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    Quote Originally Posted by str8dum1 View Post
    any update on changing the multiplier to 1?
    No sorry, damn tank of 93 just won't finish lol. I'll be filling up in a day or two with E again, got my MAF to around -.5% +/- .5% right now, I do a few pulls every day on the way home and multiply by half % to make the change slower and more precise. Hoping 6000-8500hz is all within .25% by the time I switch back over. Haven't bothered to do a full MAF tune since I have a set of cat deletes and a mamo tb to throw on when I get the chance, next step will be a cam after that so ill actually do a full airflow tune since that isn't happening for a year or so

  13. #13
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    So, filled up with E today. Tank is sitting at 75%. EQ error is still within .5% for the most part, so I'd say that changing that multiplier did the trick, thanks TriPinTaZ

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    Why is that multiplier setup for not 1.0 to begin with? What are they trying to compensate for?

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    Quote Originally Posted by BDPhoto519 View Post
    Why is that multiplier setup for not 1.0 to begin with? What are they trying to compensate for?

    It is adding fuel volume. It could just be that GM didn't finish the tables for these cars since they are not Flex Fuel from the factory. Or GM could have some other reason. We will never know.
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    I find myself making tweaks to the stoich table.. Only way to correct for a lean condition as alcohol content changes.

    You would think a by the book chemistry defined stoich table would be the way to go but it can be 7-10% consistently off part throttle and wide open.
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    Quote Originally Posted by Alvin View Post
    I find myself making tweaks to the stoich table.. Only way to correct for a lean condition as alcohol content changes.

    You would think a by the book chemistry defined stoich table would be the way to go but it can be 7-10% consistently off part throttle and wide open.
    Do you mean as content changes as in the first 5 min of driving after switching from pump gas to ethanol? Cause im sure thats gotta be normal as the fuel content is rapidly changing

  18. #18
    Advanced Tuner jsllc's Avatar
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    Quote Originally Posted by Alvin View Post
    I find myself making tweaks to the stoich table.. Only way to correct for a lean condition as alcohol content changes.

    You would think a by the book chemistry defined stoich table would be the way to go but it can be 7-10% consistently off part throttle and wide open.
    When flex-fuel is setup correctly there are none of these issues and stoich matches the chemistry formulas just like it does in a calibration from the factory. Only when the tune is wrong do these things happen.
    2012 ZL1 - Maggie Heartbeat, Port & Polish Heads, Custom Cam, Custom rotating assembly, steel sleeved LS9, No NOS and No water meth. 16psi
    810rwhp and 820rwtq 91 Octane 6400 rpm
    948rwhp and 951rwtq 105 Octane 6400 rpm
    999rwhp and 997rwtq on 60% Ethanol 6400 rpm

  19. #19
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    So it looks like the consensus here is to change the Alcohol Injector Flow Multiplier back to 1.0 stock? I am currently copied the values from a GENV 5.3 flex fuel truck, I would think this would be best because we know for a fact GM programmed this table to work with alcohol because yes some of the other LT1 tables for alcohol look like they were just forgotten or just random stuff was put in there, but you would think this would be a liability for GM though to do that kind of stuff?

  20. #20
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    Quote Originally Posted by jsllc View Post
    When flex-fuel is setup correctly there are none of these issues and stoich matches the chemistry formulas just like it does in a calibration from the factory. Only when the tune is wrong do these things happen.
    So why then does some factory calibrations not follow the chemistry? There's more to it than just balancing the equation. Like combustion efficiency for one!

    As someone who tunes a ton of cars I'm telling you it doesn't always work out.
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