Base Spark Threshold cells?

[ArKay99]

B5916 Base Spark Closed Throttle Threshold
B5917 Base Spark Vehicle Speed

I've been looking at timing during very low load in regards to surging/bucking issues lately and have been seeing some very interseting results. These results have brought me to look at these 2 cells.

Mine are set to:
B5916 = 1.191%
B5917 = 256mph

The descriptions are:
B5916: If the throttle is below this value and vehicle speed is below threshold, then it is considered closed and the base spark tables will be used
B5917: If the vehicle speed is below this value and the throttle is below threshold, then the base spark tables will be used.

I am using a drive by wire setup and my throttle position never goes below ~8%. However I do know that my pcm transitions from the high octane to base spark table. However, it seems that it oscillates between the 2 at the very low load. My throttle opening is somewhere around 10% when this occurs.

My question for B5916 is: It seems that in the case of drive by wire that this value is % of change or something else is controlling the transition. Is this the case?
My question for B5619 is: If this is set at maximum it would seem that the base spark table should be entered as soon as the pedal reaches the idle position. Why would I want to be using idle spark values with rolling air and fuel values?

[405HP_Z06]

Great post, I've been wondering the same and have not looked at these two parameters. :sly:

[Delco]

In effect it is closed throttle spark not just idle spark , used to help bring the engine down to idle and when coasting , later calibrations split this up further with coasting spark.

With ETC the PCM works out where idle throttle is then calculates its zero thottle position and stores that in ram.

[405HP_Z06]

In effect it is closed throttle spark not just idle spark , used to help bring the engine down to idle and when coasting , later calibrations split this up further with coasting spark.

With ETC the PCM works out where idle throttle is then calculates its zero thottle position and stores that in ram.

Thanks Delco, so for ETC vehicles that idle at 7-10% throttle position the PCM determines this is approximately 0% and uses the tables to reference from there?

[ArKay99]

Thanks Delco, so for ETC vehicles that idle at 7-10% throttle position the PCM determines this is approximately 0% and uses the tables to reference from there?

That's what I infer from his answer. That makes sense. However, as my RAF changes, along with my ETC, and IAT, so does my idle throttle position. I wonder if this value (B5916) needs to be larger for h/c cars that have larger values in the RAF in order to keep pace with the larger than stock throttle position?