[Diy_efi] Newbie with a simplified Motronic project

[Brian Dessent]

Eric D Byrd wrote:
> 
> One thing I don't get about MAF vs. MAP.  Everyone's making out like MAF
> is self-correcting, and never any longer any need for fuel trim tables.
> How?  Both are sensors, that respond to a physical phenomenon by
> generating a voltage, and all such devices experience variations in
> response due to age.  MAF is no different.  The fuel map in the ECu is
> fixed, and eventually the MAF will signal a slightly different value for
> the same air flow, and the ECU will send a different fuel pulse.  Why
> don't MAFs require fuel trim tables?  From the post below, it appears
> they do, and that they get them from feedback from the O2.  And that is
> no different from a MAP.

The fundamental difference is this: at the end of the day you want to
know how many O2 molecules are in the cylinder (and a desired A:F ratio)
to calculate a pulsewidth.  A MAF sensor measures this directly, as its
output corresponds to grams/s or some equivalent unit.  Its output is
usable without much further processing, assuming that you know the
number of cylinders and the RPM.  With a speed density system you are
indirectly measuring the pressure of the air in the manifold (and more
precisely, at a single point in space of the manifold) and the
temperature of the air.  From this you can infer the mass, but it's not
a primary measurement.  So yes, sensors do have error, but in the case
of a MAF, there's a single source of error.  In the case of speed
density, you've got the MAP sensor, the MAT sensor, and the VE table,
all of which could have some drift/error/inaccuracy.

Brian

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