Sensor Questions?

[Edward Hernandez R]

"Hitachi later developed a MAF that measures a portion of the entire
air flow ("bypass type").  The advantages include less dirt build up
and damage immunity from intake backfiring.  A disadvantage is that
the reverse air flow is not measured, so if there is a lot of
pulsation at the meter, it will cause errors (?)."

Not necessarily. The hot-wire principle does not account for which
direction air is flowing, so if if air goes in, then some flows back
out, it will measure both flows. This is a far larger error than
measuring only what went in.

"Quick question: what exactly is going to cause this sudden increase
in the amount of required air that is going to draw a vacuum?"

Let me explain the crux of the problem: if the MAF sensor is located
upstream of the throttle body, there will always be a delay during
throttle transients. Air is compliant, like a spring. As an extreme
but common example, when you go to WOT it takes time for the manifold
to fill with air at atmospheric pressure because it was at some level
of vacuum just prior to throttle opening. On top of that, the engine
is always "draining" the manifold. By the time the MAF sensor "sees"
this demand for air fill the manifold(and asked the ECU to provide
more fuel), the engine has consumed enough air(and sped up) to cause a
lean stumble. The closer the MAF sensor is to the throttle plate(s),
the faster its response. However, some of you have correctly pointed
out the risk of the MAF sensor picking up intake pulsations as you
place it close to the inlet valves.

"Just remember that a MAF system also has to be calibrated to the
engine it is on.  The MAF give out a voltage (or frequency) depending
on how much air is going through it.  It does not tell you "currently,
there is x Kg/sec flowing through me".

MAF sensors need to be calibrated for completely different reasons
than do SD systems. MAF sensors need to be calibrated primarily to the
airbox they are fitted to in order to learn their transfer function.
That transfer function is what converts voltage(or frequency) to kg/hr
of air. This has nothing to do with the engine they are fitted to. The
airbox determines the flow field in the MAF sensor, which is
particulary critical to the transfer function of bypass MAF sensors.
"Full field" sensors(like Bosch) are less sensitive, but they do have
flow straighteners to help them be a bit more robust at the cost of
some flow capacity. It is the acceleration enrichment which must be
calibrated to the engine due to throttle transients, and those
transients depends greatly on the intake manifold itself, the engine
displacement, cam events, and MAF sensor location. This is one
parameter that is easily(amd safely) tuned by seat of the pants, and
is also why the Mustang magazine car had problems.

Someone asked if MAF sensor placement on turbo engine affects thei
measurement because the pressure and temperature is different after
the compressor. All air must pass through the MAF whether or not its
density changes downstream , so placement is a matter of response
time, not accuracy.