bjankuski

My bin file which is from a 1987 vette has a ECM constant that states "max spark advance = 41.8 degrees", but my main spark table has advance numbers of 48 degrees and then the highway mode adds another 3 or 4 degrees for 50 plus degrees of advance at light cruise. My question is what is the ECM constant controlling? I made an assumption that the spark advance would not exceed 41.8 degrees is this assumption true? Also is the ECM constant number taking into account the initial spark advance of 6 degrees or is it adding the 41.8 to the 6 degrees for a total advance of 48 degrees? Any help would be appreciated.

Grumpy

Yes.
The initial timing values are included in the info the ecm is dealing with. If you set the max timing to 42, the ecm will limit the max timing to 42, based on the info you feed it.

RBob

iTrader: (1)

This parameter is based on the physical limitations of the distributor.

RBob.

bjankuski

Thanks for the answers!

Why did GM put numbers as high as 48 degrees in the main spark table when they had the ECM constant at 41.8 degrees? That just doesn't make any sense to me.

RBob

iTrader: (1)

41.8 and 6 initial equals 47.8 at the crank.

RBob.

JPrevost

So initial SA isn't taken into consideration with "max SA"? I thought it was. I'm pretty sure it is. I have initial set to 8 actual and in the calibration, then I have 42 as max SA. Main SA table has values over 42 in cruise and decel but my final SA is always trunkated to 42, at least that's what it displays. If that isn't the case then I'm actually running 50 . That might explain some of the detonation I was experiencing on the highway with winter gas . Although the spark plugs all look the same to me . I'm at the point where the only thing I can tell with spark plugs is oil, detonation, too rich, too hot, too cold, and WAY lean. Anything around stoich looks the same.
Bob, I'm going to give you a call tonight if you're in.

Grumpy

Because there are several adders that come into play that would push the timing over 45d (distributor), and that would be to point where the rotor would be pointing at the next terminal and trying to fire the wrong cylinder. One reason for DIS, and CNP is the ability to run larger amounts of advance since they don't have the physical limitations of the cap and rotor, to worry about.

RBob

iTrader: (1)

'8746 and outputting $0061 will be the timing value as programmed into the distributor. Will need to multiply that value by .352 and add the initial to it for the true at the crank timing relative to TDC.

Now where it gets ugly is with code such as $8D. The $8D ALDL stream outputs two timing values. The first includes the initial timing but excludes any knock retard. The other value excludes the initial timing but includes the any knock retard.

So the question becomes. . . Which timing value is the scan tool displaying?

RBob.

TRAXION

Just to clarify ... for the $8D this means that the ECM will add up to 42 degrees in addition to the amount of advance already dialed in at the dizzy. As an example ... I have the stock max setting of ~42d but run 10d initial timing. Some parts of my timing have 50d of advance and I DO get 50d of total advance.

Tim

JPrevost

Interesting. Now I know why the car is being stupid and liking less timing than I thought it would want. I had thought that final SA took into account all tables, bias values, AND initial. But not.... very interesting .

dimented24x7

iTrader: (2)

just an observation, Several days ago i finally changed the initial SA to match what i had at the distributer and i noticed that the timing over the entire engines operation apeared to have changed. I had to go back and redo some (the apparent loss of timing was a good thing in some spots) of the spark curve to get it back to where its supposed to be. Maybe tommorow if i have some free time ill go out and tinker with the initial SA while observing the timing and see what happens. This is on my 8063, btw so it may be diffent then the 8746. If so why still have the initial SA included if it really has little or no effect?

Grumpy

;-------------------------------
; SUM OF THE ADVANCES:
;
; SAP = SAMAIN + SAALDL + ATSSPK + SATC + SATI
; - SABST - VSSSA - KCTBIAS - KATSBIAS - SRHR
;
;-------------------------------

SUMADV LDAA #0 ;
ADCA #0 ;
ADDB TEMP+7 ; Main SA
ADCA #0 ;
ADDB TEMP+6 ; Coolant SA
ADCA #0 ;
ADDB SATI ; Choke SA
ADCA #0 ;
;
SUBB SABST ; Boost modifier
SBCA #0 ;
SUBB KCTBIAS ; Coolant temperature table bias
SBCA #0 ;
SUBB KATSBIAS ; Intake air temperature table bias
SBCA #0 ;
SUBB TEMP+5 ; VSS modifier
SBCA #0 ;
SUBB TEMP+4 ; Hot restart retard
SBCA #0 ;
;
;
STDADV STD SAP ; SAP SCALED 8


**************************************
* LIMIT SPARK ON THE ADVANCE END *
**************************************

LDD KMAXADV2 ; EXTEND KMAXADV TO 16 BIT 2'S COMPLEMENT
SUBD SAREF ; IF SAREF > KMAXADV, SET SAREF = KMAXADV
BGT LIM020 ;
ADDD SAREF ;
STD SAREF ;


Maybe the 8D ignores the ref angle, but here's a snippet where it's included in the spark total advance comp..

Can you post the 8D total spark calc., code comments?
It'd be interesting to see that if in some code it's one way, and in another, not.