[Diy_efi] Newbie with a simplified Motronic project

[Eric D Byrd]

Kyle,

I think all you need for basic fuel and ignition control is MAP, IAT and
RPM.  The reason MAF is good is that it accounts for both pressure and
temperature.  An accurate MAF signal, interpreted by an accurately tuned
ECU table, is just as good as MAP + IAT interpreted by an accurately
tuned ECU table.  Consider the equation of state, that it has three
variables, pressure, temperature and density.  Fix pressure and temp, and
you have density.  What an ECU really wants is an accurate measure of
density, because that's how you set fuel mixture.  And to set timing, you
need to know density and water temp, because the fireball burns at a rate
dependent on density and temp in the chamber at TDC.  Essentially
dependent on what is known as the "second speed of sound", a term
applicable usually only to heat transfer issues in engineering.  With
those in mind, I still wonder why they bothered with making an MAF
sensor.  It isn't more accurate, simply because all your accuracy rests
on the accuracy of your tune in your ECU.  I guess it's because you don't
"need" the IAT, but in fact you still do, because IAT does affect the
temp of the charge at ignition time, regardless of water temp.  IAT can
fluctuate wildly without greatly affecting water temp.  So even if you
have MAF, you still need IAT to set both mixture and timing.  So I say
you can do this with just MAP.  I have a 2G Honda CRX, OBD0, with a PM6
ECU that does a very good job controlling mixture with the stock O2
sensor during closed loop, and my CRX has a MAP/IAT combo and no MAF at
all.  Unless there's something I don't know, I don't begrudge the absence
of the MAF.

And no, I don't think you need a cam position sensor for your basic
project, simply because the timing of spark or induction never changes. 
For every TDC spark event, it is immediately followed by a TDC intake
opening, with no spark.  If your new ECU doesn't run the engine *at all*,
you can guess it might be because you got them reversed.  And as far as I
know, valve openings or closings do not affect spark timing, simply
because ignition always occurs with all valves fully closed, at the end
of compression or thereabouts.  So, even if you have a variable cam gear
installed, that would not be relevant to ignition.

--Eric Byrd

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