Programming 101

[Bruce Plecan]

Again, this is how I understnd things to be, and/or how they seem to work.
If you have a better explaination, feel free to splain it to us.  This is
for the
new guy trying to get a handle one engine management.  This is from my
experiences, and comparing notes with others, as specific to the 747/749/730
series of gm ecms, but I'd imagine that it carries over to most other late
C-3 P-4 ecms.

MAP filtering is exactly as it sounds, a filter.  What happens is that a MAP
sensor (or a MAF also) is a very sensitive analog device.  It literally can
pick up cylinder pulses in the manifold.  Because of this and the relative
instability of the raw data, the signal is filtered into the A/D for a
smooth
signal.  How it is usually done is with a rolling average routine, using a
time constant.  It is worse on a 4 cyl than an 8.  So you would need to
filter it more.  To "smooth" a signal they will usually sample a signal
during a calibratable period of time, say once every 1 or 3 or ?
msec.  It then takes the average of these samples and feeds it to the
program.  A MAP sensor is only a device that "infers" the amount of
 air that an engine is using based upon the speed of the engine.
Now also bear in mind that too much filtering can make the idle lame
in response, to various conditions, ie idling forward in gear, A/C
cycling on+off, Neutral to Drive, sometimes even off idle transistions.
Also, too far off and the engine can stall before the anti-stall programming
can come into play..   If you feel that you have to get into this keep
EXCELLENT notes.  You can really get turned around..
   Because of this the pulse width is determined based upon that "air"
calculation to give you your desired air fuel.  If you have a signal that
is jagged, you would be chasing the pulse width all over the place and
have one hell of a time maintaining stability.
What happens also is that programmed in is the density and weight of air.
It then knows that for every "intake" the theoretical amount of air that it
can take in is 100% of the volume of that cylinder.  So VE is the % of this
maximum amount.
8 cyl.  350 cu. in. engine, 2000 rpm.
weight of air .0739 lb. cu/ft
1728 cu. in. per cu. ft.
175 potential cu. in. of air for every revolution.
2000 revolutions = 350,000 cu. in. potential of air.
350,000/1728 cu in. = 202.54 cfm.
14.968 pounds of air.
50 % VE = 7.48 lbs. of air/min.
divide by the weight of fuel and viola air/fuel ratio at the desired
amount.  That is why you have to put in the stoich value, and the
other A/F values for idle and WOT, and start up to determine your
pulse width.  Boro is a big factor, because your density goes
down and you increase in altitude.  That is why that is there.
Also remember that EGR is putting an inert gas into the
combustion chamber.  If you just take the MAP reading, (which
is lower than  the same charge without EGR) you would be
asking for to little fuel.  That is why you have the EGR correction
factor.  It knows the amount of EGR based upon the duty cycle,
and then multiplies the value by an amount to tell the computer
that I really have more air in their than what the MAP sensor is
telling me that I have.
This GM system is pretty crude, but realise the year.  The current
calculations are very sophisticated and don't just directly multiply a base
value of pulse width.  Well, it does, but it goes through 10,000 other
calculations before it gets there.
Highway spark adder is a fuel economy thing.  What some
companies were doing is leaning out the fuel in open loop (i.e.; 16:1)
and adding spark to get a little fuel economy advantage on the
highway.  You needed to be over a certain speed, and have been
cruising for a calibratable period of time and then it would go into
this mode.  Some did not lean out the fuel, just add spark and
it would give you .1 - .5 fuel economy benefit.  Not much but multiplied by
1,000,000 vehicles and CAFE goes up to meet govt. regs.  This has now been
deemed a defeat device by the government and you can no longer do it.  It
also sends you NOX emissions out the roof.  Therefore we now have highway
NOX standards so that they can catch anyone who may still want to do this on
production vehicles.
Transient MAP filter again would be the same as I described before but in a
transient condition (throttle increase).  It may be necessary to speed up
the filtering in this mode.
Bruce and the staff at CSH, HQ