I'm missing something...

[James Montebello]

>Isn't the reason for the MAP is to calculate the % Oxygen in
>the air at different elevations?

>MAF would still measure just the amount of Air entering the engine
> , even if it's Oxygen content is much lower at higher elevations...

>or do I have it all wrong?


You have it all wrong. :-)

The MAP sensor is only measuring pressure in the manifold, which is
predominantly affected by throttle position and load.  Indirectly, it's
also measuring barometric pressure (elevation), which is why a system
based on a MAP sensor can usually get by without a baro sensor.  % of O2
in the manifold can be inferred from the pressure, but that's not the
primary reason to have a MAP sensor in the system.

The amount of O2 in air relative to the other things in air (primarily
nitrogen) is the same at higher elevations, btw.  A liter of air at sea
level and a liter of air at 2000m each contain the same percentage of O2.  
However, the higher elevation liter contains fewer molecules of
everything.  So, if you measure a liter of air past a point, you have no
idea how much O2 is in it, so measuring by volume is useless.  Now, a MASS
airflow meter is supposed to measure air by mass, not by volume.  A high
elevation liter of air will weigh less than a low elevation liter of air.  
That should allow you to infer the amount of O2 going past a point.

Now, the open question is still whether a flapper-door type meter is
actually measuring mass, or just volume.  Personally, I think mass will at
least have a strong influence on the reading from such a meter.  I'm
willing to be proved wrong on that point.

james montebello  

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