Do I need an Air Flow Meter?

[John Dammeyer]

My two cents here...

Regarding the need for a barometric sensor:  If and only if your intake
manifold was under static conditions when the intake valve opened would a
Barometer not be needed.  What I mean by this is if the intake valve was
open long enough so the pressure inside the cylinder was identical to the
pressure inside the intake manifold when the intake valve closed could you
use only a MAP sensor.

However,  life is dynamic and so is the airflow which is why volumetric
efficiency is part of the equation.  Volumetric efficiency is a measure of
how much air makes it into the cylinder given that the air doesn't start
moving quickly until the piston is at about 75 degrees ATDC.  Then at the
bottom of the stroke the piston stars moving upwards but air/fuel is still
moving into the cylinder; assuming the intake valve is still open.

  As the piston moves down,  it creates a low pressure area and air from the
high pressure area moves to equalize the pressure.  How fast the air moves
and how much air ends up inside the cyclinder depends on the _difference_ in
pressure.

So without knowing the ambient pressure and temperature you don't know the
mass of the air that has ended up in the cylinder because you don't know how
much pressure there was to push the air into the intake manifold past the
throttle plate.

Now there are systems that don't have the barometer and they do function but
they just aren't as accurate.   Even a Haltech that I opened up a few years
ago had a separate barometer on board.

I also read a while ago that the caraburator's venturi measures flow not
mass and that is why the carburator delivers a richer mixture at altitude;
velocity of the air is the same, drawing the same amount of fuel but less O2
molecules available.  Same thing with the flapper type injector controller.
Years ago my 76 SAAB would idle at a much higher RPM with a change of about
2000' wheras my Turbo 900 runs exactly the same regardless of the altitude.

Brings up an interesting question.  For a Turbo'd engine using a MAP sensor
obviously the need for the Barometer goes away if the engine is always under
a slight amount of boost.  Perhaps the barometer sensor needs to be
connected to the front of the throttle plate.  Perhaps that's why SAAB went
to a hot wire mass air flow sensor instead.

Regards,

John Dammeyer

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