[Gmecm] Modify $8D to work on MPFI 3.1L DIS engine

[Gary Evans]

On Aug 31, 2006, at 6:46 PM, Brendan Patten wrote:

> Okay, I want to run the ever popular $8D code on my 1990 Cavalier 3.1L

<rant> Ah, see this is where a forum format (or at least a search 
function!) would be immensely helpful. This subject was just covered in 
detail a while back and I spent the last 20 minutes looking all over 
the place for the relevant post. </rant>.

Anyway, here it is in all its glory. I found it very enlightening and 
it answers a lot of your questions I think. If you get it working, let 
us know. I would like to do the same on my car but have not gotten 
around to trying it yet...

Gary Evans
1977 Jaguar XJ12

> =============
> 	From: 	  Steve.Ravet at arm.com
> 	Subject: 	[Gmecm] converting dist and DIS
> 	Date: 	February 23, 2006 6:05:31 PM PST
> 	To: 	  gmecm at diy-efi.org
> 	Reply-To: 	  gmecm at diy-efi.org
>
> 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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