Air Flow Meter discussion (WAS: "I'm missing something...)

Scot Sealander Sealand at clarityconnect.com
Sun Nov 18 20:46:01 GMT 2001


Kevin _ wrote:
 
> Unfortunately, in the automotive world, all the aforementioned sensors
> besides the MAP are VERY slow. 

Here's a post from GMECM a few weeks ago:

========================================================================


Interesting the way the recent discussion on MAF/MAP can bring out
solidly held beliefs.  ;-)  Here is some of my experience with it.

I have a 1990 turbo Grand Prix, and that is a turbocharged 3.1L FWD
engine in a GM "W" body.  It uses the speed density (SD) code from GM in
a 1227727 ECM.  In order to help tune the the car, I put a B-body MAF
from the Impala SS LT1 in the intake air stream right after the air
filter.  The MAF data is only used for observation, and is not used to
run the engine.

To help get the MAF data out, some new variables were created.  One is
AirFlowRatio.  This is the ratio of MAF airflow to 2x the calculated SD
airflow.  The reason for this is to get it about 50%, so DIACOM can
display the ratio in a PWM field.  On transients, the MAF responds
faster than the SD, as the AirFlowRatio blips to 58-60% or so, then the
ratio goes back to 50%.  A question comes to mind is "How did people
come to the conclusion that the MAF is slow to respond?".  Measured
data, or something read? In case your curious, the SD airflow is
calculated from the Sync PW.  

I have some detailed data on airflow ratio, say at 2000 RPM from 25 kPa
to 90 kPa.  The ratio is not constant at 50%, but varies some.  Quite
close at the higher manifold pressures, rising some in ratio at 50-60
kPa, then falling to under 50% at minimum 25-30 kPa. Pretty fascinating
stuff.  The ratio does reflect all kinds of errors contained in both
strategies.

The MAF is also used to calculate VE.  I had some trouble with this
variable, and started to look more closely at the code used to calculate
it.  Was tossing some significant digits out by mistake.  Pretty
impressed with how well it can work.

As far as MAF variables, the hood of the TGP has almost no room under
it.  The air filter is tiny, and mounted between the engine and the
radiator, about midway between the fenders.  The MAF is attached to the
air filter by an adapter that goes from about 2.25 inches to about 4
inches for the MAF.  Right after the MAF is another adapter that goes
from 4 inches back to 2.25 inches.  Then an immediate tight 90 deg turn
into the compressor inlet.  No doubt a far cry from the Impala SS intake
ducting.  So how does the MAF read when connected to a very different
set up?  Not too bad actually.  If anyone is truly interested, I can dig
up some more data.

This went on too long to reach a conclusion that the MAF reads quite
well with large changes in original application, and also responds
faster than the SD algorithm. Your mileage may vary. ;-)

==========================================================================



> On the other hand, a general MAP sensor reacts in about 1ms. 

Maybe so, but how fast do you actually sample it?



> This is by far the superior way to go for making a drivable EFI setup.

Sounds like opinion to me, but I do believe you are entitled to it.  I
would like to know some of your background and how you formed your
opinion.


Scot Sealander

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