1227749 Info

This is link to the '749 info on the GMECM web site.

http://www.diy-efi.org/gmecm/ecm_info/1227749/

Ludis's '749 schematics.

http://www.cruzers.com/~ludis/1227749schematic.html

Driver modifications for 8 P&H injectors.

http://p-928.home.comcast.net/749mods.html

Porsche 928s '749/Vortech

http://p-928.home.comcast.net/749.html

The Turbo Sunbird used 4 PFI P&H injectors, 2 on each driver. I'm using 8 PFI P&H injectors driven off the 2 high current injector drivers for my 928, and I've commandeered an unused pin to help carry the C10 current to 4 of my drivers. My setup is configured like the Sunbird.

-- BillShaw - 22 Sep 2006

• Conversion_of_1227727ECM_to_1227749_Underhood_type_ECM.pdf: Document detailing the factory mixing of 730 and 749 components in a Delco remanufactured ECM

This documents a discovery that makes the swapping of ICs between the 1227727/30 family of ECMs and the 1227749 family have a much higher potential of success, as GM's Delco Electronics was doing it themselves. One potential application is a waterproof enhanced 1227749 made from a 1227727 ECM...

749 DIS Info -

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