Chasing a spark blip....

[Redline Motorsports]

Been doing a ton of cammed LS7 cars and have found a few situations where at low speed-low loads we are getting an occasional skip.

I have at least captured it in the screen shot shown. When it starts to "buck" the timing jumps from 45 degrees ( the commanded value off the high oct table) to values between 16-20 degrees. I can't seem to identify what other table is commanding a timing change at such a steady state of operation. MAF and dynclair are as stable as can be during this event as well as a solid AFR.

[The Alchemist]

ah haaaaa !!!!

I had this too in my first cammed car I did and it bugged me for ages until I sussed it out. At low speed 2 to 3 km/h to car would sometimes log timing going up and down from 9deg up to 33 deg then back again, up & down Up & down, speed up to 5km/h and it would log at a constant timing according to the timing on the map.
I finally put it down to the slight bucking, as you describe, causing the rotational speed and therfore trigger times to accel and decel between triggers which would make the displayed timing jump up and down in time to the bucking.
Thats all I could put it down to in the end and I logged everything to try and find the cause!!!!!!!!!!!!
In the end giving it a bit more timing low down "fixed" the problem or greatly improved it.

Cheers,

Mike

[Redline Motorsports]

Sure.....rub it in! LOL!

I agree that timing, in the past has helped, but the high value in that log is about 44 degrees at cruise. Seems like a lot already but I guess a few more degrees is worth the try.

As complicated as the PCM is it still is looking at inputs to control outputs. What is the PCM seeing that has it is commanding a drastic timing reduction not done through KR controls.. I see no timing tables, besides knock tables, that has permission to alter the main spark table. I was hoping to see the engine load jump across the spark table which could cause that kind of swing but its rock solid...

Got a few ideas I am going to try out tomorrow......just doing a little pre-research!

[Redline Motorsports]

I am starting to wonder if at that light cruise load that we might be pulling off the coast down spark table as it seems to be right on with the lower values.....

It has to be timing from another table.

Howard

[GMPX]

There is DMA spark PID's for the E38, try logging some of those to find the culprit.
I saw on a log today on a 3.6L (E67) for no logical reason the ECM would pull 25deg of timing for torque reduction, even though the throttle was steady at 38%, 3000RPM, steady airflow. It was just weird.

Cheers,
Ross

[TAQuickness]

I think you're on the right track Howard. I've seen similar on LS1 pcm's when the PCM transitioned from the high/low octane blend to the base timing tables.

[Redline Motorsports]

Regarding the high and low tables; I always make them the same until I'm done with the entire tuning process. The values that are the low blip values are much lower then the stock low octane so I started looking elsewhere anyway.

I really feel that there is a lot of interpolation going on that we never really focus enough on. We need to better understand when we have reached the switch over point from different conditions. I guess we are dealing with transient spark!

I haven't looked but can we monitor a pid to show the "state" for coast down, DFCO, CFCO, etc?? Kind of like closed loop status?

Howard

[cmitchell17]

I think this might be happening to me. Right before the shift to second at about 20-30%throttle it feels like it drops the timing right before the shift.

It also feels like my torque reduction is not timed right. Like it gives a hard shift with stock pressures and then it pulls the timing at the wrong time.

[Redline Motorsports]

I think this might be happening to me. Right before the shift to second at about 20-30%throttle it feels like it drops the timing right before the shift.

It also feels like my torque reduction is not timed right. Like it gives a hard shift with stock pressures and then it pulls the timing at the wrong time.

This is not just something I feel but something I am able to catch in the log. Are you working on a ZO6?

[Highlander]

What did you ever do to sort this out????