[Diy_efi] pic based afm -> maf conversion

[Daniel R. Nicoson]

Comments in line below:

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 6:46 PM
  To: diy_efi at diy-efi.org
  Subject: Re: [Diy_efi] pic based afm -> maf conversion


  "very similar info to what a  AFM would" ... but not the same.  If "close
enough" tuning were OK, the AFM would still be there, right?

  The output curves in the test rig described will have a certain
relationship for one density of air - barometric pressure, temp, humidity -
and another relationship altogether when the density changes.  The same
number of CFM that max out the AFM's output could be so dense that the MAF
is maxed out, too, or (at altitude, on a summer day) they could make for a
much smaller (raw) reading from that same MAF.  That's the primary weakness
in vane air flow meters - they measure volume, not mass.  Most L-Jet based
systems have some way to determine barometric pressure and intake air temp -
this last usually in the AFM, by the way - so they can derive the mass of
the air.  Since the sensor will be giving a signal that relates to the air
mass, a way must be found to remove the barometric pressure measurement (the
only remaining measurement external to the now-absent AFM) from the
computer's calculations.

  I haven't researched the vane meters, never needed to.  I think my
inclination was that the dynamic flow of air moving that meter would include
the effects of density (since dynamic pressure=1/2*roe*v^2).  I guess it
doesn't really show up in real life, Tom, you sound like you know this one
cold.  Point conceeded.


  A couple of thoughts:

  If you remove the baro sensor (by making its value constant,) the computer
will not - if it were going to - advance or retard the timing in response to
weather conditions and/or altitude.  This could be no big deal - the
computer might not do any futzing with the timing anyway based on baro
readings.  It could be a problem manifested as lack of torque (high
altitude) or pinging (low altitude.)  Probably nothing, but the only way out
of it would be to remove the baro component from your MAF signal before
supplying it to the ECM.  Any takers on this last one?

  I would just leave the stock baro and temp sensors in a similar position.

  If the ECM is counting on air door overswing to provide acceleration
enrichment, the engine may end up with a stumble on sudden throttle opening,
perhaps necessitating the watching of the MAF or TPS for rapid increases.

  In most of the systems I've dealt with, air temp has more influence on
spark control than fuel.  Does your old AFM have the ACT/IAT sensor built
into it?  IDK about your Bosch sensor, but there are MAFs out there that
have ACT sensors as part of the package.  If it keeps your ECM from getting
stupid and going into damage control mode, it would be worth the effort to
include an air charge temp sensor.

  TomV


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