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

-- SteveRavet - 12 Mar 2007