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Thread: The final truth: LTFT's and WOT

  1. #1
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    The final truth: LTFT's and WOT

    Please comment if there's something wrong in the following logic:

    1. PCM needs to be in Closed Loop in order to generate LTFT's (via STFT's)
    2. There are several preconditions that needs to be fulfilled in order to PCM to be or to stay with Closed Loop. Following parameters define most relevant ones:
    __2.1 Engine temperature + bunch of other parameters, such as OLFA
    __2.2 MAP boundaries (?)
    __2.3 PE enable MAP value
    __2.4 PE enable TPS value (hot/cold)
    3. Whenever engine enters into PE mode*, no LTFT's are generated anymore
    __3.1 PE mode* equals Open Loop in practice, right?
    __3.2 For example in my car, TPS % for going PE is 55% at 3600 rpm

    Now, to my understanding this means that in the VE and in the areas of low vacuum level (high numerical value, i.e. somewhere between 85...105 KPa) no LTFT's ever occur. This is because on those areas in the VE, the engine is always in Open Loop and thus it does not generate LTFT's.

    If, and only if what I say above is true, then I do not understand what is the logic for positive LTFT's affecting WOT. Does the PCM remember the last positive LTFT just before PE mode is triggered by, say, throttle position change?

  2. #2
    There are 3 mods :
    1 .close loop O2 sensor targetting 14.7 AFR or your stoich useing STFT and LTFT .
    2 .Open loop when the engine is cold ,car whithout O2 sensors and if you whant to disable the O2 sensor and keep the car in open loop mod .( so your PCM use EQ Open loop table ) .
    3 .PE mod use power enrechment EQ ratoi vs RPM table only if you non boost ( NO use for LTFT in PE mod ) .

    + % LTFT and STFT mean yor PCM adding fuel cuz ( lean ).
    - % LTFT and STFT mean your PCM takkingout fuel cuz ( rich ).
    98 Z28
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  3. #3
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    Quote Originally Posted by gsegse View Post
    There are 3 mods :
    1 .close loop O2 sensor targetting 14.7 AFR or your stoich useing STFT and LTFT .
    2 .Open loop when the engine is cold ,car whithout O2 sensors and if you whant to disable the O2 sensor and keep the car in open loop mod .( so your PCM use EQ Open loop table ) .
    3 .PE mod use power enrechment EQ ratoi vs RPM table only if you non boost ( NO use for LTFT in PE mod ) .

    + % LTFT and STFT mean yor PCM adding fuel cuz ( lean ).
    - % LTFT and STFT mean your PCM takkingout fuel cuz ( rich ).
    Sure, I do understand that.

    But I do not understand how there can be any corrections based on LTFT's & STFT's in the VE areas where the engine is NEVER at Closed Loop. How can you have any corrections based on LTFT's, if no STFT's and LTFT's were ever created?

    Here's an example of VE from 2003 Z06. In the circled area, the engine is very probably always in the Open Loop. So where and how LTFT corrections are made in these areas?:


  4. #4
    If your in open loop they are no corrections made.

    Open Loop = not using O2 sensors, relying on MAF or VE or both for fueling.
    Closed Loop = using O2 sensors for correction

  5. #5
    If LTFTs are positive they can carry over into PE.
    STFTs do not influence PE, they will be 0.0% when commanded EQ>1.00

  6. #6
    Quote Originally Posted by Pekka_Perkeles View Post
    Sure, I do understand that.

    But I do not understand how there can be any corrections based on LTFT's & STFT's in the VE areas where the engine is NEVER at Closed Loop. How can you have any corrections based on LTFT's, if no STFT's and LTFT's were ever created?
    I understand your question. To answer it comprehensively you would need to look further into the Fuel Trim Cells (FTC) and how they are setup to find out exacly which ones are applied positively to WOT for example.

    But yeah as you and others pointed out there is no feedback or reliance on O2s because you are in Open Loop but if the PCM has lean (+ve) LTFTs in a number of FTCs (depending on how you have set them up) 'preceding' (couldn't think of a better term to describe it) WOT then the PCM and its strategies has in the interests of self-preservation is programmed to ASSume that because it is lean preceding Open Loop (high load) then it is likely to be lean during this and adds the corresponding LTFTs that have been derived from a slightly different part of the airflow table.

    I hope that made sense.

    Personally, for a shitload of reasons I am not a fan of LTFTs not least of which is because of this, so personally I turn them off and rely on STFTs for closed loop control which is where LTFTs come from anyways and since we can accurately model the airflow I don't see the need for the 'clutch' that LTFTs were designed for - just my opinion and experiences.

    Cheers

  7. #7
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    I always turn LTFT's off once the tune is done. Once you do a quick check of them just to see if they are within what youwant turn the suckers off cause all they do is collect trash afterwards in my opinion.
    Last edited by Z06SUPERCHARGED; 03-14-2010 at 11:20 AM.
    16 psi on E85

  8. #8
    Senior Tuner LSxpwrdZ's Avatar
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    Not a fan of LTFT either!
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  9. #9
    Senior Tuner Frost's Avatar
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    Well while we are talking about dislikes... I dislike trim tuning! Use your wideband!
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  10. #10
    Senior Tuner S2H's Avatar
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    Quote Originally Posted by Z06SUPERCHARGED View Post
    I always turn LTFT's off once the tune is done. Once you do a quick check of them just to see if they are within what youwant turn the suckers off cause all they do is collect trash afterwards in my opinion.
    your narrow band o2's are doing their job correctly most of the time...
    only time they dont is when they are worn out or when they are experiencing an over-temp situation
    and only then do they drift away from being accurate, and usually only by a couple of 10ths... not really enough to worry about in clsoed lop situations

    I almost always turn the LTFT back on when the tune is complete so that the car can correct for any fueling issues that might occur(temperature/weather related or mechanical related)


    tune in open loop with your wideband... if your wideband and your fuel trims dont agree at part throttle(closed loop) then question if your wideband is accurate... it could be out of calibration, have a voltage offset, just be a bad sensor, or be in a bad location...some wideband sensors dont like high heat and need to be further downstream than the stock o2 sensor locations or need a heatsink added to them to keep them from overheating.

    once your tune is complete you can re-enable your fuel trims to correct for minor fueling differences day to day.
    it will only be a few points either way(+/- 4% ...small amounts...less than .5 afr typically... if its doing more...then you really need to double check your wideband or spend a few bucks and buy a better one that will be more accurate.
    -Scott -

  11. #11
    Tuning Addict WS6FirebirdTA00's Avatar
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    Quote Originally Posted by Z06SUPERCHARGED View Post
    I always turn LTFT's off once the tune is done. Once you do a quick check of them just to see if they are within what youwant turn the suckers off cause all they do is collect trash afterwards in my opinion.
    I agree. They can easily add fuel to WOT when it is not needed (when it locks + for that FTC) because some goofy numbers.

    Once tuned properly, the STFT swing is so minimal that the LTFTs are almost useless. I have rechecked cars in weather extremes and there is still only an average of about a 1-3% error.
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  12. #12
    Senior Tuner Frost's Avatar
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    Quote Originally Posted by WS6FirebirdTA00 View Post
    I agree. They can easily add fuel to WOT when it is not needed (when it locks + for that FTC) because some goofy numbers.

    Once tuned properly, the STFT swing is so minimal that the LTFTs are almost useless. I have rechecked cars in weather extremes and there is still only an average of about a 1-3% error.

    yessir
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  13. #13
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    Quote Originally Posted by gsegse View Post

    + % LTFT and STFT mean yor PCM adding fuel cuz ( lean ).
    - % LTFT and STFT mean your PCM takkingout fuel cuz ( rich ).
    Sorry for the thread jack, but I didn't think this was worth starting a new thread.
    Been on here over six monthes, reading, searching, trying not to ask stupid questions. But this has been buging me for a while
    If your commanding 14.7 AFR and your LTFT show 10. What will your wideband read?
    This is assuming everything is working correctly and STFT are 0.
    Thanks

    Tony
    05 CTS-V l LS6 l Maggie l Magnaflow Cat-back l Meth l 439rwhp/414rwtq

  14. #14
    Senior Tuner Mep_q8's Avatar
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    it will read 14.7 or there about

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  15. #15
    Advanced Tuner axekick's Avatar
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    Quote Originally Posted by Trailblazin' View Post
    it will read 14.7 or there about
    This is true. The reason for the trim is to keep the engine running at 14.7.
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  16. #16
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    It seems that there are opinions having LTFT's enabled or not. That's perfectly fine. I guess it's the same with dialing VE table with either stock narrowband o2's vs. wideband. I've tried both ways and they seem to work.

    However, may I suggest going back to the original question. What I'd still like to understand is this:

    Quote Originally Posted by SSUte01 View Post
    I understand your question. To answer it comprehensively you would need to look further into the Fuel Trim Cells (FTC) and how they are setup to find out exacly which ones are applied positively to WOT for example.
    Now, could somebody even roughly explain what's the dependency between certain FTC and the applicability of correction factor when entering into Open Loop?

    I understand that this information may be something that's not very eagerly shared in an open forum without any financial benefit, and I can live with that. I guess a topic like this could be covered with Greg's advanced class or in The Tuning School's advance courses, but it's pretty much out of question for me since I'm located overseas.

  17. #17
    Tuning Addict WS6FirebirdTA00's Avatar
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    I think the OL FTC is locked on based on the last value prior to OL, or at least that is how it is updated. I could be wrong though.

    How do you tune a whole VE table with stock narrow bands when they don't work in OL cells?

    Leaving LTFTs disabled prevents false trimming in OL.
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  18. #18
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    Quote Originally Posted by WS6FirebirdTA00 View Post
    How do you tune a whole VE table with stock narrow bands when they don't work in OL cells?
    I was expecting something like this when I wrote my comment! So obviously I can't tune the whole VE table with just narrowband.

    Meanwhile, let's keep searching the final truth about the WOT & LTFT behavior!

  19. #19
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    Quote Originally Posted by WS6FirebirdTA00 View Post
    Leaving LTFTs disabled prevents false trimming in OL.
    Good point.

  20. #20
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    Quote Originally Posted by Pekka_Perkeles View Post
    Meanwhile, let's keep searching the final truth about the WOT & LTFT behavior!
    If you just disable them then you dont have to worry about them.,,,pretty simple procedure.
    16 psi on E85