Air Flow Meter Modification

[Terry McLane 312.630.0533]

    The L-Jetronic transmits a "wave" pattern, as the air opens.  
    Interestingly enough, if you look at the specs for a particular car, 
    (All-Trac Turbo for example), it says when the air meter is closed, 
    resistance should be 400-600 ohms.  When wide open it should be 20-2000 
    ohms!  There are (as I remember) about 8 different gradations in the 
    meter, with a linear resistance in between.  Supposedly, each gradation 
    is a fixed multiplier of air (logarithmic) like step 2 is 3 times more 
    air than step 1, and step 3 is 3 times more than step 2, etc.
    
    I have also looked at replacing the velocity meter on my car, but it's 
    rather difficult.  I can get a VPC from HKS, which switches the car 
    over to speed density, but it costs about $1000 US. 
    
    If you were to wire the meter to the throttle body, (assuming you keep 
    the thermistor in the intake air stream), you would be forcing a linear 
    relationship between air flow and throttle body opening.  This would 
    probably cause some bogs and surges, and wouldn't work at all with a 
    turbocharged vehicle.  
    
    A somewhat better solution might be to hook up a vacuum pot to the 
    intake manifold, and use the engine vacuum to move the wiper.  By using 
    different spring rates in the can (look at an old vacuum secondary 
    carburetor or distributor advance unit) you could move the flapper 
    door.  This would be a sort of mechanical speed density system. It 
    would also build in a little bit of lag, which the computer would 
    expect.
    
    If you're more ambitious, you could always build a circuit using one of 
    the non-restrictive Hitachi MAF sensors, use a Pic16 or 68HC11 and a 
    D/A converter to map the MAF output to the meter output, build a table 
    and then send it to the computer.  Since the MAF already adjusts for 
    temp, you could then use a potentiometer on the thermistor leads, 
    giving the ability to tune the entire fuel curve by making the ECU 
    think the air is colder (richer) or warmer (leaner).
    
    If anyone has any other solutions, I'd like to hear them as well.
    Terry