GM > Engine > Spark Control > Spark Retard

Knock Learn
Knock Learn is a method the VCM uses to provide a longer term adjustment for fuel octane. If the VCM detects knock it will instantaneously reduce timing (Knock Retard) but will also modify the Knock Learn factor to affect the proportion the VCM uses to interpolate between the High Octane and Low Octane Main Spark tables. If no knock is detected over a reasonable period then the Knock Learn factor will slowly return to the High Octane table.

If a MAF DTC is set then the knock learn factor is set to the Low Octane tables. (HP Tuners speed density enhancements disable this behavior).

The various thresholds determine when the VCM will modify the knock learn factor. The knock learn factor can be logged using the scanner on many vehicles.

• Min MAP: Minimum MAP to enable knock learn.
• Max RPM: Maximum RPM to enable knock learn.
• Min RPM: Minimum RPM to enable knock learn.
• Min ECT: Minimum ECT to enable knock learn.
• Min IAT: Minimum IAT to enable knock learn.

  
  

• Total Knock Retard: Maximum knock retard to enable knock learn.

  
  

• Enable Multiplier vs. ECT: Proportionally enable or disable this feature in relation to ECT.

Knock Retard Recovery Rate (Decay)

• Knock Retard Decay vs RPM High CylAir: Knock Retard Decay Rate when CylAir exceeds threshold (below). Used on V6 engines.
• Knock Retard Decay vs RPM Low CylAir: Knock Retard Decay Rate when CylAir is below threshold (below). Used on V6 engines.
• Cyl Air Threshold: CylAir threshold to determine which decay rate to use. Used on V6 engines.

Knock Retard Fast Attack

• Knock Fast Attack Gain vs. Coolant Temp: This is the gain in relation to coolant temperature.
• Knock Fast Attack Gain vs. Baro: This is the gain in relation to barometric pressure.
• Knock Fast Attack Base Retard vs. RPM: This is the amount of timing that is subtracted in proportion to each knock detected. It is multiplied by the Temperature and Baro gain factors. The higher this number is the more aggressive Knock Retard will be.
• Knock Fast Attack Recovery Rate vs. RPM: This is the rate at which knock retard is removed in the absence of further knock. The higher this value is the faster knock retard will be removed.

Burst Knock Retard

• Burst Knock Enable Delta Cyl Air Threshold vs. RPM: If this value is exceeded the VCM will remove the amount of timing specified in the Burst KR vs RPM table (Note: for 97/98 LS1 this table is a single value).
• Burst Knock Retard vs. RPM: Amount of Knock Retard when the CylAir delta threshold is exceeded.
• Burst Knock Retard vs. ECT: Amount of Knock Retard when the CylAir delta threshold is exceeded (97/98 LS1 Only)
• Burst Knock Retard Multiplier vs. RPM: After the value from the Burst knock Retard vs. Run Time vs. Coolant Temp table is selected, this table provides the gain to that value based on engine RPM. Multiplier range is 0 through 8. The Burst knock Retard vs. Run Time vs. Coolant Temp and Burst Knock Retard Multiplier vs. RPM tables replace the Burst Knock Retard vs. RPM table in earlier calibrations.
• Burst knock Retard vs. Run Time vs. Coolant Temp: Base amount of Knock Retard/Advance when the CylAir delta threshold is exceeded. Table spark value can be from -25 to +55 degrees spark. During the first 120 seconds of engine run time, the values in the 0 through 120 seconds rows can be in use. After 120 seconds, the values in the 120 second row only are used. Depending on engine temperature, the spark change can be + or -. The Burst knock Retard vs. Run Time vs. Coolant Temp and Burst Knock Retard Multiplier vs. RPM tables replace the Burst Knock Retard vs. RPM table in earlier calibrations.

  
  

• Burst Knock Retard vs. CylAir Delta: Amount of Burst Knock Retard in relation to the detected change in cylinder airmass.

Maximum Knock Retard

• Maximum Knock Retard vs. MAP (Not in Power Enrich): This is the maximum amount of Knock Retard when not in PE Mode.
• Maximum Knock Retard vs. RPM (in Power Enrich): This is the maximum amount of Knock Retard when in PE Mode.
• Maximum Knock Retard vs. RPM: This is the maximum amount of Knock Retard allowed.
• Maximum Knock Retard vs. AFR: Used on V6 engines, this is the maximum amount of Knock Retard versus current commanded AFR.
• Load Dynamic Knock Retard Max: Maximum allowed Knock Retard based on load dynamics.

Minimum Spark Advance

• Minimum Spark Advance vs. RPM: This is the minimum limit for spark advance during any retard event. Spark will not be retarded below this value.

Static Spark Retard

• Static Spark Retard vs. RPM vs. Cylinder: Static spark retard per cylinder.
• Static Spark Retard Enable Cylair: Cylair must be above this to enable static spark retard per cylinder.