[Diy_efi] pic based afm -> maf conversion

[Daniel R. Nicoson]

I may have missed something on this thread but I think the MAF vs MAP debate
is irrelevant in this case.  Ashley stated the change was from an airflow
meter (AFM) to a MAF.  Unless Ashley has control of the code calculating
resulting PW etc, MAP isn't an option.  Being a BMW I'd bet he doesn't have
control of the calculations inside the ECU.

The MAF will give the ECU very similar info to what a  AFM would.  Airflow
moving the vane is a form of mass airflow measurement, just not as precise
as a MAF (probably a whole other debate...)  So the signal out of his
MAF=>PIC=>ECU will be pretty useful.  He might be off by some temperature
input from the airflow meter but he could leave that temp sensor in place.
Honestly, you might not even need a PIC.

The reason I say MAP isn't a consideration, if we can't control the ECU
function, then the ECU won't make the proper correlation between RPM, MAP &
temp etc.  It is expecting a 0-5v signal "representing" mass air flow, which
the original sensor provided and now the MAF will provide.  As already
discussed the MAP does not provide the same info as any form of mass
airflow.

I think your mapping exercise could be done very simply (easy for me to
say).  Configure your AFM and MAF in the same inlet tract so the air going
through the MAF then goes through the AFM, log the output of both devices as
you drive (might have to do this on your work bench in which case you need a
big "sucker" to pull the air).  Now you have the "equivalent" transfer
functions.  If two crazy curves result, the PIC can just do table lookups
all day to get your ECU input correct.  If the two curves are fairly similar
in shape then you possibly could get by with an analog circuit, opamps &
resistors.

What is the goal of this exercise other than to do it?

If you are truly running a LOT more air through the engine (30% or more),
will the current ECU calibration accept more air?  In other words if you are
maxing out the AFM transfer function, it won't matter if the MAF signal
represents more air, the "transfer function" in the ECU will still be maxed
out.

Someone suggested the Mustang MAF's.  I tune a 1994 5.0.  I can tell you
that those meters will support up to about 300-320 Hp with a 0-5volt output.
These have some nice features in that the actual sensor is in a recessed air
passage in the MAF.  That is good because it makes the MAF less sensitive to
turbulence in the intake tract.  If your motor is a smaller 4 cylinder and
won't start to pull as much air as a 5.0, leave the stock "screen" on this
Ford MAF.  That screen forces the air to be smooth going through the MAF for
good accurate readings.  If the your engine can only pull enough air for say
200 hp, this screen won't hurt since you can flow almost 300 Hp of air
through the MAF w/screen.  In fact I just looked up, my stock 1994 Mustang
5.0 MAF give you 932 kg/hr of air at 5.0volts.

I will be watching how you fare with this project.  I have a 1998 BMW 540 &
a 1998 BMW 528.  The BMW world hasn't been hacked as fully as the Ford and
GM world.  I'm curious to see how far you can go modifying your BMW.  On my
Mustang I have the luxury of full control of the fuel & spark tables, MAF
function and a bunch of other stuff to change since I use the TwEECer.  So I
can let the Ford ECU do all the work for me as I change parts.  I don't
think you have that kind of control on your BMW, makes tuning much more
difficult.

A couple other things:

Get over the idea that a MAF has more restriction than MAP.  Not true,
actually irrelevant again.  IF the MAF is properly sized to flow enough air
in a given application, then it will flow plenty of air.  Remember, you
sized it for the application, so how could it restrict you?  Yes, a larger
MAF will flow more air than a smaller MAF if asked to but if both MAF's flow
enough for the given engine, the larger MAF will never be asked to exceed
the flow of the smaller.

The discussion below about reversion from wild cams and MAF sensitivities to
position in the intake tract are worth considering.  In this case it sounds
like the engine is still pretty stock so reversion shouldn't be a real big
bear (BMW's generally like high RPM  though so there could be some
significant cam overlap causing some reversion).  The farther the MAF is
from the intake valve, the less likely this will be a problem.  Lots of
Mustangs out there with pretty wild cams running pretty good, don't be
scared off by this one.

As far as sensitivity to upstream of the MAF, do it like the mid 1990's
Mustangs, use a "cone shaped paper filter" (from a Mustang of course) and
have your MAF pull the air in directly.  That way the air is pretty smooth
as it enters the MAF.  I use the "big" cone filter from Ford's V-10 pickup
engine on my 5.0.  My MAF sucks right out of the filter, works great.

Cool project, good luck!

Dan Nicoson

  -----Original Message-----
  From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org]On
Behalf Of Tom Visel
  Sent: Wednesday, January 18, 2006 2:23 PM
  To: diy_efi at diy-efi.org
  Subject: Re: [Diy_efi] pic based afm -> maf conversion


  Manifold pressure vs. RPM (speed-density) gives you a very handy way of
giving fine control over fueling in known areas where a MAF interpreter
would not.  MAF lets you play without paying as much, but if the playing's
over, the keenest edge will be found with a speed density system tuned using
a wide-band O2.  Either way, you would have to eliminate or make constant
the ECM's barometric pressure input, as this would be compensated for twice
otherwise - once in your black box and once when the computer tries to
figure it into what it thinks is the VAF (vane air flow or volume air flow)
sensor's output.

  BTW, the reason for the ubiquity of MAFs on modern vehicles is for tighter
emissions controls: to allow for the measurement of EGR flow, and to allow
for a MAP sensor as a redundant system and for rationality checks to see if
the MAF's calibration is drifting.  Almost every vehicle has both a MAF and
a MAP these days (at least in North America.)

  TomV


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