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[Andrew R. Ghali]

Guys, for what it's worth, I believe flapper AFM's actually are reading
air MOMENTUM - in other words, higher velocity air at lower pressure can
cause the same reading as denser, lower velocity air.  Now, whether that
is linear with volume, I do not know.

Andrew

On Mon, 12 Nov 2001 13:06:57 -0800, "rob files" <rncfiles at hotmail.com> wrote:
>
>Hi Mike,
>
>It is called an air flow meter for a reason...it can only measure air 
>*flow*.  The units of flow are volume per time....so the AFM can tell the 
>ECU, "I am measuring 20 cubic meters of air/hour."  Now you can see why you 
>would need a density correction for this, 20 m^3 is a volume and depending 
>on temp and pressure, you could have a varying amount of oxygen molecules 
>(which is all the ECU wants to know).  It's very likely that the vehicle you 
>were riding in does not do continous atmospheric correction and that is why 
>the a/f ratio went out of wack with the increase in elevation.
>
>BTW, Mass Air Flow meters (the hotwire type) measures the mass of air 
>directly without the need for density correction.  MAP sensors can only 
>provide the manifold pressure which the ECU uses along with density 
>correction, RPM and some constant VE value to calculate the mass of oxygen 
>entering the engine.
>
>That is how I understand tthings anyway
>- -Rob

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