[Gmecm] converting dist and DIS

[Steve Ravet]

I got a note from a former list member (Shannen Durphey) just today
regarding the difference in code between DIS and distributor.  It
includes a bin for a 2.2L motor, which I've placed in the uploads
directory called 2pt2_dis.bin.  Enjoy,

--steve

I've attached a calibration (2pt2_dis.bin) which I wonder if you'd be
kind enough to forward to the list ftp site, along with a general post
to the list.  This calibration may be of interest to some.  It's a $58
calibration for a 1994 2.2 PFI turbo engine.  The engine and most of the
fuel system are stock.  Pertinent details are DIS, T25 turbo (non
intercooled), 8psi, and (4) 55#/hr TBI injectors.  Base fuel pressure is
about 14 psi, with the injectors firing in quasi-asynchronous mode at
idle and under most light throttle conditions.  The ecm has been
modified with (2) upgraded injector drivers to handle the additional
current requirements of the TBI injectors.  The vehicle is driven daily
and has been for several years, making a few long trips including one
1800 mile jaunt to the middle of the country and back. 
It is, in short, reliable and consistent.

I'm surprised that the question of DIS / distributor swaps hasn't been
answered by the gmecm list.  I see from time to time people making
inquiries of varying depth as to the requirements to make it happen. 
Although it took me years of work, I'd expect that someone else would
have determined the answer by now.  Guess not.

Anyone who has suspected the answer lies with the "reference angle" is
partly correct.  This angle is usually from 0 to 10 degrees in a 
distributor based calibration.   DIS angles are 33 degrees for the 
Buick v6 system, and 60 degrees for the more common 7 notch system found
on many 4 and 6 cylinder engines.

In order to deliver correct spark advance, the ecm must know at what
point, in crankshaft degrees, the reference pulse occurs.  The total
timing is a combination of reference angle and additional delivered
angle.  The additional angle is determined by subtraction:  Desired
timing minus reference angle = delivered timing.  Since the ecm cannot
measure crank position directly, it uses the # of cylinders constant,
the time between reference pulses, and the base angle to convert
additional degrees calculated above into a "wait time."  In the end, the
correct spark timing is delivered by counting off "wait time" 
after the reference pulse.

After the delivered timing is calculated, but before it's delivered to
the ignition hardware, it is tested to ensure it conforms to the
physical limitations of the ignition system it is intended for.  Both
maximum and minimum advance values must conform to prevent accidental
ignition in the wrong cylinder.  These limits are called out as "maximum
spark advance relative to reference" and "maximum spark retard relative
to reference" in most disassemblies found on the 'net. 
  A check of a stock $58 calibration shows fairly typical 4 cylinder
distributor based limits: 60 degrees maximum advance and 3.52 degrees
maximum retard.

 From here we should sidestep to a discussion of just how the terms
"advance" and "retard" are understood by most readers.  It seems fairly
apparent that the timing values above allow a range of 63.52 degrees.
This range, of course, is from maximum retard (less than zero, ATDC) to
maximum advance (more than zero, BTDC).  But is this really correct?
Maybe the range is only 56.48 degrees, from +3.52 degrees to +60
degrees.  But if "maximum retarded spark" is positive, what ensures that
"advanced spark" is also positive?  Or maybe those limits are
interpreted completely wrong... maybe they mean "maximum retard at any
given time", in other words, maybe they represent the maximum change in
timing allowed at any given calculation???  After all, if timing was at
60 degrees BTDC, and it's now at 56.48 degrees, isn't the new value
"retarded" from the previous?  The final answer is that the creator of,
and definitely the reader of the disassembly might be better off to call
these values "maximum timing value" and "minimum timing value" with no
reference to the condition of advance 
or retard.   With maximum timing of 60 deg and minimum timing of 3.52 
deg, the total range is 56.48 deg.

Now back to the DIS conversion.  Past attempts, at least those posted,
focused on changing the reference angle in th ecalibration to match the
ignition system in use.  This typically resulted in a "locked" 
timing value which was generally somewhere around 0 - 5 deg BTDC. 
Let's look at why this happened.  We'll pick an arbitrary desired timing
value of 20 deg BTDC.  As in a real example posted to this list, we'll
replace the distributor based reference angle of 6 degrees with a DIS
friendly 60 deg value.  Here's the gist of what happens:

Ecm performs magic and arrives at desired timing 20 deg BTDC.

Desired timing - reference angle = delivered timing
  20deg         -        60deg    =     -40deg

Check ignition system limits:
Is -40 delivered timing greater than maximum allowed value of 60?
No:  leave delivered timing alone

Is -40 delivered less than mimium allowed value of 3.52?
yes:  Set delivered timing to minimum allowed value of 3.52

Hmmm... So for any value of desired timing up to a ghastly 63.52 deg
BTDC, this calibration applies sanity checks and corrects the value to a
positive 3.52 degrees.  Timing is essentially "locked" and no amount of
MAP, RPM, temp, or TPS changes can dislodge it.  Obviously we need to
use some limits more appropriate to the DIS ignition system.  A simple
check of Rob Rauscher's extremely helpful $A1 disassembly shows a
maximum value of -9.9 deg and a minimum value of -70 deg.  Be 
warned:    These are 16 bit 2's complement numbers.  They appear in a 
binary as $FFE4 and $FF39, respectively.

There have been no additional changes necessary in the DIS swaps I'm
aware of.  Some people have chosen different limits for their
calibrations.  I'm using limits closer to the original distributor based
values.  Dwell calculations are left untouched as the DIS module handles
dwell independently of the ECM.  Timing tables do not have to be
reworked because of a DIS swap, although they will need to match the
needs of the engine they are applied to.

And a final note:  It is not desirable to offset a crank position sensor
-60 deg to to allow an unmodified distributor calibration to be used
with the 7 notch DIS system.  During cranking and at any time the 5V
bypass line is held low, ignition pulses are delivered to the coils 60
deg. retarded from the reference pulse.  These pulses will be delivered
60 deg ATDC with an offset crank sensor.  Starting will be extremely
difficult, requiring large amounts of fuel and quite possibly backfiring
severely through the exhaust.  There is no need to alter the original
relationship between the crank sensor and notch position to convert
between DIS and distributor calibrations.

Have fun.

Shannen

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