I'm missing something...

Diehl, Jeffrey jdiehl at sandia.gov
Mon Nov 12 19:56:10 GMT 2001


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. 

-----Original Message-----
From: NickG
To: diy_efi at diy-efi.org
Sent: 11/11/2001 6:44 AM
Subject: Re: I'm missing something...

A flapper door AFM can not compensate for changes in air density. The
AFMs
are 'airflow meters', but not 'mass airflow meters' like the current
mass
air meters. Thus, changes in elevation are not detected. That is why the
car
in question started running rich at higher elevations.

But, I would have expected the ECU to tempensate and adjust the mixture
based on the O2 sensor's feedback. Unless, of course, the system is
programmed to run in open loop or has adaption turned off.

Nick
'95 M3 turbo

> I was riding in a guy's supercharged car and we were driving up the
> mountain.  When we got to the top, we had gained ~5000ft in elevation.
Half
> way up, his car started to run very rich, according to the "tip of the
nose
> AFR sensor."  I would expect this, of course, except:
>
> This car is equiped with a flapper door AFM which I've been told are
self
> compensating for air temp and elevation.  This made sense at the time
since
> I believed that the air's MASS was opening the door to the AFM.  As
the
air
> got hotter or thinner, the door wouldn't open as much at a given RPM
and
the
> ECU would be able to compensate.


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