[Diy_efi] Calibrating Hot-wire MAF sensors

[Grant Beaty]

My car (97 Toyota Supra) comes from the factory with a hot-wire mass air
sensor. The sensor itself is fine, it just doesn't flow enough. At higher HP
levels there is a pretty big pressure drop across the sensor (~2 psi,
although some of that is probably the filter).

I've been thinking of a few ways to solve this problem without spending a
lot of $$ on an aftermarket solution. One way is to put two factory MAFs in
parallel, each metering air. Another is to gut the MAF, and place it inside
a larger 4" pipe.

Regardless of what I do, the voltage signal to the ECU will be corrupted.
>From what I understand, the MAF voltage follows a relatively simple formula
of:

voltage = (airflow ^ .25) + constant

I measured a some of the MAF voltages and airflow (reported from my OBD2
logger), and found that formula to be accurate (at least with the airflow I
could get reving the car in my driveway). Problem is, the constant was
actually a negative value (-.37v). I don't know if thats because my volt
meter isn't accurate, or if the formula is actually much more complicated.

If the formula is sound, it would be pretty easy to build an op-amp circuit
to change the MAF voltage for different situations (ie different flow rates
or larger injectors), ajustable via a pot. But if the formula isn't always
accurate, I would have to use a more complicated solution. Has anyone here
done anything like this before with a hot-wire sensor?

Thanks for any help,
Grant Beaty
'97 T66 6-speed Supra


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