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Thread: Open loop afr

  1. #1
    Fuel Injected!
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    Open loop afr

    I am using 6395/7427, $OE, advanced $OE TP5 v250. There is a parameter "open loop afr enable" and "open loop afr vs temp vs vacuum"
    if you enable how does it deal with the table. Being as you are in open loop the nb supposedly not controlling but still indicating, would it take some calculation from within the bin and add? Hoping someone might know.
    6395, BHDF, 7.4 BBC lightly modded now 6395 BMHM back to BHDF

  2. #2
    Super Moderator dave w's Avatar
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    My explanation might not be 100% correct?

    The PCM "Assumes" the VE table is correct. The PCM calculates Open Loop AFR from the VE table, by adding / subtracting injector pulse width for an "Assumed AFR".

    I use Wide Band O2 sensor readings to adjust the "Open Loop AFR vs. Temp vs. Vacuum.

    dave w

  3. #3
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    Thanks Dave, your confirmation makes it worth while to burn some chips experimenting.
    6395, BHDF, 7.4 BBC lightly modded now 6395 BMHM back to BHDF

  4. #4
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    Well I did some testing and it does work as advertised. Whether there is any real use for it I don't know. If you have a correct ve table that is what you want you don't really need any corrections. I do think that it can be used in closed loop if it set right and that might be of some value to some. Some time when I'm bored I will try it. In the trucks gm doesn't use it and whether that is an emission reason or something else I don't know.
    6395, BHDF, 7.4 BBC lightly modded now 6395 BMHM back to BHDF

  5. #5
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    So do changes made in the Open loop AFR table alter the values in the VE table or do changes in the VE table alter the values in the Open Loop AFR table?
    I currently have Closed loop disabled and I'm running directly off the VE table. If I was to check the "Set" box for Open loop AFR enable what happens?
    By the looks of the Open loop AFR table the only areas of any use are the ones highlighted in the lower right corner?

  6. #6
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    Changes in either will not alter the other.

    If it is using that table then it will calculate the fuel to get the AFR entered in the cell where the engine is operating. If the VE table is accurate then the AFR will be accurate.

    I'm not sure why you think the highlighted cells are the only ones being used. Low to medium throttle causing acceleration will move you to the middle of that table. Heavy throttle might get you close to the left before you trigger AE.

  7. #7
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    As a side note on this post the chart is interesting. One of the few places gm uses vacuum instead of map. The right side is the richest so I would assume the 0 column is 0 vacuum equal to roughly 100 map. The left side is 80 and being as it is the leanest there is no way it would be map and it should still be vac. We know vac doesn't go to 80 so the next guess is 80% so then comes the question 80% of what. 29.92? Without a gm tech manual some of this is guessing.
    6395, BHDF, 7.4 BBC lightly modded now 6395 BMHM back to BHDF

  8. #8
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    So assuming I'm running off the Open Loop AFR table and for example the engine is running in a cell that commands 14.0:1 AFR what happens if that's not the AFR I would would have if I was running as I am now off the VE table? If the VE table isn't accurate what happens?

    I was looking at the Open Loop AFR table in respect to inches of vacuum and that's why I highlighted the area between 20" and 0" but it does make sense that it's more a percentage scale of some sort.

  9. #9
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    I am just off the top remembering when I did the test in open loop. I have my open loop ve quite lean in the low load/rpm area like high 15,s. I then set for open loop afr and set those to high 15's. It started cold and drove fairly well until it warmed up and then went lean and started to miss. I stopped and put my good chip in and came to the feeling that if you're open ve is good that is all you probably need. I am only guessing but think when you set the open loop afr the pcm assumes that your ve is 14.7 and does calc's from that.
    6395, BHDF, 7.4 BBC lightly modded now 6395 BMHM back to BHDF

  10. #10
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    Interesting, my VE table is pretty good at the moment so I might just work from that at this stage. Thanks.

  11. #11
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    It's likely just 100 - MAP and was done in the XDF setup.

    The VE table is not the AFR. It is a table telling the PCM the volume efficiency of the engine at each operating point. The VE table and the engine or cylinder displacement are used in a calculation by the PCM to determine how much air is entering the engine. The VE is used to calculate air, not fuel.

    Then, knowing the air entering the engine, the PCM can use an AFR value from somewhere else and calculate the amount of fuel to add to the air.

    If the VE table isn't accurate then the air calculation isn't accurate and when the air isn't accurate the fuel calculation based off the air isn't accurate.
    Last edited by lionelhutz; 07-13-2016 at 07:07 PM.

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