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Thread: Dynamic Airflow vs VE Airflow

  1. #1
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    Dynamic Airflow vs VE Airflow

    I swapped the turbos on my TT GTO (2006, E40 ECM) for a set of GT3071s, and in the process I switched back to an older MAF tuned map to get a starting point. This map has the dynamic airflow disabled, so the car is running on MAF only. (It is a SD OS Map however, with a 2.5 bar map sensor). MAF is the stock 06 GTO MAF, and above 5400RPM the MAF is pegged.

    I did a small 3rd gear pull to 5700rpm at wastegate boost (7psi).

    Pull Datalog below:



    In the center graph, yellow is the VE Airflow and cyan is the MAF Airflow. I was puzzeled by the jump in the yellow line early in the pull.

    You can see the data right before the jump:



    Then the next cell over after the jump:


    Before the jump, the VE Airflow is 11.81, and after the jump it is 25.36. The MAF Airflow is around 26.39 at this time. This switch occurs right at 3000 rpm, and I am trying to figure out what the cause is. Looking at the dynamic airflow page in HPT, there are the boundaries for the dynamic zones that corresponds with 3000 RPM. Based on the MAP before and after, the dynamic airflow zone would change from 13 to 17 at 3000 RPM. It is my understanding that these "Dynamic Zones" alter the calculation method for the VE/MAF blending. Does anyone know if the Dynamic Zones would/could correspond to such a significant change?

    Dynamic Zone Config (2006 GTO E40)


    I also noticed that at the point before the jump the "Dynamic Airflow" is 23.88, while the "Mass Air Flow" was 24.81. Right after the jump the "Dynamic Airflow" was exactly the same as the "VE Airflow" and follows each other exactly for the rest of the pull.

    (Data mentioned above)


    I suppose all of this does lead to the question about what is the difference between "VE Airflow" and "Dynamic Airflow".

    Is the "Dynamic Airflow" what fueling would be based on if we were in VE/SD mode, and is the "VE Airflow" what the algorithm would use for the dual combined Dynamic/MAF calculation?

    Jeff

  2. #2
    Senior Tuner DSteck's Avatar
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    Dynamic airflow is the number used no matter what mode you run (MAF only, SD, or hybrid). The way dynamic airflow is determined depends on the mode though.

    Post your VE table.

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  3. #3
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    Interesting.. my first thought was: Hmm.. are their weird values in the VE table? Of course on this map the Primary VE Table is stock, and the Boost is an extension of the Primary Last col + x%. Here they are:




    Take a look at a closer zoom up of the above log, this time including the Dynamic Airflow PID as well.



    The Dynamic Air (green) follows but is not the same as the MAF(cyan), and when the 'VE Airflow' (yellow) jumps up they are 100% overlayed (exactly the same).

    There are a couple of possible explanations:

    (1) The Dynamic Air is , as you suggested, the filtered and final value being used for fueling, derived from the MAF in this case. Since the VE Airflow PID follows the Dynamic Air exactly, it would lead one to believe the 'VE Airflow' PID is not a result of the VE calculation, but instead some other intermediate value (but MAF based in MAF mode)

    (2) Both the Dynamic Air and the VE are based of the VE table, and in this tune are not relevant at all. This could be quickly verified by simply cutting all my VE table vales in half.. since I am in MAF only this should have no effect on the car running... but will be visible in the log very quickly.

    In a thread some time ago Chris commented that the 'VE Airflow' PID was not a value used in the calculations, but rather some report out variable.

    Jeff

  4. #4
    Senior Tuner DSteck's Avatar
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    Dynamic air is not based off the VE in all cases. If you are in pure MAF mode, then dynamic air is just the MAF airflow, except filtered to be smooth.

    If I were you, I wouldn't worry about whatever that spike is. I would instead shut the MAF off and run it in SD, only log Dynamic Airflow, and tune your SD table.

    This probably sounds mean, but... don't waste your time worrying over things that don't matter. At the end of the day, dynamic airflow is what dictates fueling. If you want your VE to operate correctly, force the car into speed density and dial it in.

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  5. #5
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    Quote Originally Posted by DSteck View Post
    Dynamic air is not based off the VE in all cases. If you are in pure MAF mode, then dynamic air is just the MAF airflow, except filtered to be smooth.
    Yes, it does appear the Dynamic Airflow PID is a filtered version of the MAF PID, which would make sense as this configuration is setup for MAF only.

    Quote Originally Posted by DSteck View Post
    If I were you, I wouldn't worry about whatever that spike is. I would instead shut the MAF off and run it in SD, only log Dynamic Airflow, and tune your SD table.

    This probably sounds mean, but... don't waste your time worrying over things that don't matter. At the end of the day, dynamic airflow is what dictates fueling. If you want your VE to operate correctly, force the car into speed density and dial it in.
    Oh that does not sound mean at all. I had a working 2 bar SD tune previously, and I was just curious as to the meaning of the 'VE Airflow' value. It isn't critical to getting a new SD tune working, but more of a curiosity.

    Cheers,

    Jeff

  6. #6
    Senior Tuner LSxpwrdZ's Avatar
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    VE Airflow is just that the raw airflow calculated off the VE table. Mass Airflow is directly off the MAF sensor and looked up on the MAF table. Dynamic Airflow is the smoothed calculated airflow based off of multiple factors and modes that are currently enabled in the tune. Like DSTeck said if you are SD only it's a smoothed VE airflow, if it's MAF then dynamic is a smoothed version of MAF. If your are running mixed SD/MAF like stock dynamic is the meshed calculated smoothed version of them both. Throttle transients are usually biased towards SD and steady state and WOT is usually biased towards MAF on a stock style SD/MAF setup.
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    That's a helpful analysis, thanks. To the OP, could it be that even though you are in MAF only mode, that you may have the Dynamic Airflow High RPM disable still set to 3000?

    My other idea was if the VE table had been modified and wasn't correct, but you said it was a stock VE table, right?

  8. #8
    Advanced Tuner ajamesbond007's Avatar
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    This thread answered a lot of questions... thanks guys!
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    Quote Originally Posted by LSxpwrdZ View Post
    VE Airflow is just that the raw airflow calculated off the VE table.
    This is one of the reasons I started a thread. That statement above does not appear to be true for this ECM/Configuraiton. On MY E40 ECM in MAF mode (dynamic disabled), the PID for 'VE Airflow' is not based on the VE table after 3000 RPM. It is instead a copy of the 'Dynamic Airflow' PID. Below 3000 RPM it does appear to be based on the VE Table (I need to do a few steps to verify).

    The other statements are correct, and it does appear that the Dynamic Airflow is a filtered version of the MAF when in MAF mode.

    As mentioned by the other poster, it is not particularly relevant that the "VE Airflow" is not as I expect. He is correct in suggesting that if you want to see the VE airflow, put the car in SD mode and log the dynamic airflow.

    However it is worth noting that you cannot run the car in MAF mode and log the VE Airflow value with the expectation of seeing how close the VE tune is to the MAF tune. From what I can tell, if you are not in SD mode you cannot see what the SD calcs would have been for a particular condition.

    Quote Originally Posted by pontisteve View Post
    That's a helpful analysis, thanks. To the OP, could it be that even though you are in MAF only mode, that you may have the Dynamic Airflow High RPM disable still set to 3000?

    My other idea was if the VE table had been modified and wasn't correct, but you said it was a stock VE table, right?
    Stock VE Table, and DA High RPM is 500. However I am going to test a few other things out just to better understand how the VE Airflow PID is generated.


    Jeff

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    Running the car in MAF mode while whacking the VE table in half ought to bring things to light, no?

    And perhaps a test moving the DA High RPM to say 2200 to see if the correlation follows?

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    So setting High RPM Disable to like 200 RPM will force MAF only?

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    Quote Originally Posted by SargeZ06 View Post
    So setting High RPM Disable to like 200 RPM will force MAF only?
    Correct. But I would set the High RPM disable to a higher number, like 400 to 600 RPM. At 200, you're into starter motor territory, and maybe that would affect engine cranking. Then set the High RPM re-enable to about 100 RPM less than the disable rpm was set to.

    It's easy to get tripped up on the terminology. I find it helpful to clearly define the two modes of airflow calculation: Dynamic and Steady State.

    Dynamic by definition means moving, changing. Steady state means steady, consistent. Mass Airflow meters are very good at measuring airflow that's nice and steady, like a constant 2000 RPM for example. Sudden changes in airflow are not so well detected, because that airflow change in the motor has to travel thru the meter first, so there is a bit of lag in the response time from the actual MAF measurement to the actual engine.

    MAP sensors are very sensitive to changes in manifold vacuum. The instant you open the throttle up, the manifold vacuum changes, and the MAP sensor detects this right away. So the PCM quickly knows changes in MAP, but not so quickly for changes in MAF.

    For this reason, GM runs a blended model. At lower RPM, GM pays close attention to the MAP sensor. At higher RPM, the engine is breathing in air so fast that a change in airflow is much easier to detect, and faster to report back to the PCM. So at higher RPM, GM pays closer attention to the MAF.

    With MAP/speed density, airflow is estimated based on VE, IAT, and displacement. With MAF, airflow is actually directly measured, which is even better than estimating. So for sudden throttle changes, or low RPM, we should be looking at the MAP (dynamic airflow). For higher RPM, or a nice steady airflow amount that doesn't vary, we should be looking at MAF (steady state). And this is exactly what GM does with their blended model.

    The High RPM disable scalar simply tells the PCM at what point to permanently quit looking towards MAP. It's that balance point between high rpm consistency and low rpm fluctuation in Mass Airflow.

    But bear in mind, it's not an on/off switch. It's a blended model. So even at low RPM, the airflow calculation still is weighted more towards MAF than MAP. But as you suddenly change manifold pressure, it quickly looks more towards the MAP/VE calculations so it can estimate airflow.

    In short, this is why GM has such killer throttle response.

  13. #13
    That was a great explanation pontisteve

  14. #14
    Senior Tuner mowton's Avatar
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    Log both MAF (for tuning purposes) and Dynamic Air for PCM's "final answer" per above as well as SD tuning. I know I am late to the party on this thread but it is also a great idea to generate a g/cyl parameter in the Math User Defined area based on Dynamic Airflow, I call it Dynamic Cylinder Airflow or DCA. This way you have a true representation of the g/cyl which developed form the MAF airflow (like the pre-defineds) in itself could lead you to a wrong place in the timing table during transient scenario's.....I have attached the Parameter for you guys. It is for 8 cylinders but easily converted for other cylinder configurations.

    Log both MAF and Dynamic Airflow's and see how they deviate during transient conditions such as hard acceleration or deep decel.....

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