Motronic for turbo: AFM pull thru vs blow thru

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  • funcrew
    E30 Fanatic
    • Jun 2007
    • 1223
    #1

    Motronic for turbo: AFM pull thru vs blow thru

    I'm trying to figure out why the Motronic gurus are now against blow-through use of the OEM or M30 AFM for M20 turbo builds. The barn door flapper senses the same mass air flow regardless of the pressure inside the meter. Granted I struggled through fluid dynamics class, but I remember that [high velocity X low pressure (pull thru)] vs [low velocity X high pressure (blow thru)] yields the same force against the flapper for a given scfm. It seems that the AFM would max out at the same scfm in either position, when the flap reaches the end of its mechanical travel or exceeds the range of the sensor, whichever happens first. Increasing the flapper spring rate would allow more air flow before you maxed out - obviously you would have to adjust your tune for the altered sensor output to air flow relationship.

    Using Ostrich 2.0 and suitable tuning software, can you see the AFM reading directly? if so, you should be able to see the flow reading flatline if you exceed the range of the instrument.

    Since this build has been done both ways, did changing from blow thru to pull thru make a difference on the same car? I'm always willing to ditch theory if empirical results tell a different story.

    Any experience on this question would be much appreciated.
    Last edited by funcrew; 03-17-2015, 05:01 PM. Reason: changed cfm (volumetric) to scfm (mass) for flow rate.
    "If the sky were to fall tomorrow, the tall would die first."

    -Dr. Paul Forrester



    Do I LOOK like I need a psychological evaluation???
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    • bmwman91
      No R3VLimiter
      • Oct 2004
      • 3128
      #2
      The "AFM" that we all refer to is actually properly called a VAM - Volume Air Meter. Its output is log-proportional to the VOLUME flow rate of air through it (cubic meters per second). So, no, it will not give a correct reading in blow-through mode since the density of the air will be changing a lot between no boost and full boost. That is also why the VAM has a thermistor in it to give the DME a reading of air temperature. Air density also changes with temperature.

      The DME's "load" variable is basically proportional to the mass flow rate of air coming in to the engine. It computes this by taking the volume air flow rate and air temperatures, and assuming a constant pressure, which can easily be equated to mass air flow rate through the ideal gas law. There is a barometric switch in the DME as far as I recall that will signal the DME when driving above 1000 meters of elevation, if I recall correctly. You are correct that the inertia of high pressure air is higher for a given velocity, meaning that it can exert more force against something when blowing against it. That is a factor in transient conditions (a sudden blast of air), but I don't believe that it makes a considerable difference in the VAM's door position under steady-state conditions or relatively slowly-varying ones (such as when revving up the engine).

      Hopefully it makes sense why you want to use a VAM in pull-through configuration. You need the incoming air to be at ambient air pressure.

      This is a big reason why MAF sensors are so popular. Their output is proportional to the mass flow rate of air (kg/s), which is essentially a direct estimation of engine load. The DME works to maintain a stoichiometric air/fuel ratio, which is the proportion of air MASS to fuel MASS expressed as a flow rate in time. A MAF takes all the BS out of things...its principle of operation inherently accounts for changes in air density due to pressure and temperature. On top of that, it responds much quickly than an AFM, which is actually a blessing and a curse in practice. Unfortunately, swapping a MAF into a car with a DME designed for a VAM is nothing trivial. I spent 5 years developing a MAF conversion for my E30 and it took me through the equivalent of a bachelor's in EE as well as some grad-level signal processing classes (I'm a MechE by degree, but much of what I do in and out of work is EE these days).
      Last edited by bmwman91; 03-30-2015, 10:34 PM.

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        • funcrew
          E30 Fanatic
          • Jun 2007
          • 1223
          #3
          My apologies for being stubborn. Using an exaggerated example, let's say you flowed water through the AFM at the same velocity as air. The water would push the vane out of the way much more forcefully than the air. More or heavier molecules bashing the vane at the same velocity equals more force, more vane angle, and a higher signal to the DME. Denser, boosted air would do the same thing, just to a lesser extent. I'm just talking physics, you guys know a lot more about tuning than I do.

          Any takers? Anybody do a before and after comparison in both AFM positions?
          "If the sky were to fall tomorrow, the tall would die first."

          -Dr. Paul Forrester



          Do I LOOK like I need a psychological evaluation???

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            • DesertBMW
              E30 Enthusiast
              • Aug 2011
              • 1023
              #4
              There are other turbo cars made from the factory that had barn door type AFM like Toyota MR2 and Porsche 944. Both of these are pull trough.

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