I'm missing something...

rob files rncfiles at hotmail.com
Mon Nov 12 22:01:29 GMT 2001


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

>From: "Diehl, Jeffrey" <jdiehl at sandia.gov>
>Reply-To: diy_efi at diy-efi.org
>To: "'NickG '" <nikog at mediaone.net>,        "'diy_efi at diy-efi.org '" 
><diy_efi at diy-efi.org>
>Subject: RE: I'm missing something...
>Date: Mon, 12 Nov 2001 01:41:52 -0700
>
>I'm not trying to be arguementitive, but I really want to understand this.
>
>My understanding was that the mass of the incoming air was what forced the
>flapper door open.  If this is so, then it should compensate for density,
>right?
>
>If that isn't how it works, then is it vacuum which opens the door?
>
>Any comments are more than welcome.
>
>Thanx,
>Mike Diehl.
>


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