I'm missing something...

[James Montebello]

FWIW, the Bosch L-Jet setup on my Alfa has a separate barometric sensor,
which reinforces the point that the flapper-type meter won't handle baro
changes on its own.

Not sure if all L-Jet systems had this sensor.  The only other flapper
system I dealt with was a turbo, so the 2-bar MAP sensor it had was there
for measuring boost, but could also fudge an adjustment for baro changes.  
It didn't have a separate baro sensor, but it also wasn't Bosch, but an
Nippon Denso copy.

james montebello

On Tue, 13 Nov 2001, Chris Conlon wrote:

> At 11:30 PM 11/9/01 -0700, Diehl, Jeffrey wrote:
> 
> >This car is equiped with a flapper door AFM which I've been told are self
> >compensating for air temp and elevation.
> 
> The flap-type AFM does not properly compensate for either of those
> factors, by itself. The AFM responds to both air flow velocity and
> air density, but the terms are different orders. So, you have a
> single equation with 2 unknowns (density and velocity), and you
> can't get a single-point solution to that. (If the density and
> velocity terms had the same order it would not matter, since mass
> flow is linear in both those terms, and their product would be
> all that mattered.)
> 
> The AFM pretty much assumes some known density to start with (probably
> sea level). The ECU can (presumably does) use the temperature sensor
> to make a guess at the air density, based on that "known" starting
> point. With a guess at the density you can solve for velocity. When
> the actual baseline density gets too far away from the assumed baseline,
> the meter reads incorrectly.
> 
>    Chris C.
> 
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