AFM vs MAF Here is the deal

[mark kibort <mkibort@xxxxxxxxxxxxxx>]

Ed, =

Thanks for your response. Make no mistake , a AFM is a mass flow device.
very accurate as =

you can plot or match fuel for mass flow thru it.  Volume flow would only=

be a speed indicator.  This is =

clearly not the case as aerodynamically , the flapper valve moves with it=
s
stablizer counter flap , in accordance =

to MASS flow.   at altitude where the air may be 3% less dense, for the
same air speed  in to the engine, you would have less dense air thus
deflecting the flapper valve less and creating less voltage for the ECU
signaling less fuel.   One of the only limitations is its max point.   at=

which time, the system goes uses only RPM to increase fuel injector
duration.
The MAF sensor has a "hot wire" that is very sensitve to temp changes.
Greater temp change, more  or less mass has to pass thru it. =

 Classic example of old vs new technology.  one just doing the exact same=

job , but more efficiently.

You mentioned open loop. This  occures during  a part  to full throttle
condition , but it refers to the ECU ignoring the O2 sensor for keeping
your mixture fuel efficient  (lean) and goes to the fuel air mapping for =
 a
much richer part to full throttle conditon.
Bosch's AFM design was very good.   It does not have "divergant " behavio=
r
at  high flow rates.  it is very accuate in measuring High flow rates do =
to
the exponential force needed to move the flapper valve at high rates.  =

this is just calabrated  and fuel is matched.  Open loop, does not mean t=
he
AFM is ignored.  Quite the contrary, it is only used as the 02 sensor is
ignored at full throttle.  =


For a good example of this , my 928S track car, has a AFM like the E30
(same relative size)   I put on 75 hp of mods bringing me up to 300 hp . =
 =

On the dyno with the Horibra fuel meter, you could see the fuel system
keeping the ratios in tact.  The bosche book says the AFM has no pressure=

drop associated with it.  I tend to not believe it and  do believe your 1=
0
hp gain for getting rid of that restrictive element.   10HP means you
probably got rid of close to a .75psi pressure drop.  (just like we saw
with the addition of the eRAM electric supercharger) =

 Great stuff!!!

Cheers,

Mark Kibort


Ed says:
>
I thought an AFM was a device for measuring volume flow, not mass flow.
Although it is very accurate at lower flow rates, its divergant behavior
does not lend itself well to accurate high flow measurement.  Is this why=

the Bosch ECU runs in open loop at full throttle?

Disclaimer:  I took one semester of automatic controls.  All statements
made
by me are either uninformed opinions or wild guesses pulled out of my
nether
regions.  I'm guessing here.  Some of the more knowledgable engine expert=
s
may have a better idea of what's going on here.  I'm open to opinions.

Ed<

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