Many of you have noticed while logging your cars, that you see instances of high knock-correction values and low-rpm and low load situations. There has been much speculation about the cause and effects of these occurrences.

To put your apprehension to rest, Tim Bailey, one of our most experienced and knowledgeable tuners has offered the following explanation of this phenomenon:

Modern engine control computers (ECU) monitor and constantly adjust engine parameters including fuel delivery, variable camshaft timing, and ignition timing. Under light load (low boost or vacuum conditions) the objective of this active adjustment is optimized fuel economy and power.

An important component of this active adjustment is changes to ignition timing. Utilizing the highest possible ignition timing is a simple way to enhance the mechanical energy for any given combustion cycle. This enhancement extracts the most torque for a given quantity of spent fuel.

The processes that manage optimal ignition timing at light load are complex and vary by vehicle type. Some ignition timing adjustments are active and based upon sensor feedback. One such sensor is the "knock sensor." This sensor is a simple microphone that listens for engine noise. Complex signal processing inside the ECU helps discriminate detonation from other types of engine noise. The thresholds for noise vs knock vary not only by engine speed but by engine load. The ECU will react to engine noise by actively removing ignition timing. This low-load "knock correction" is a normal and desirable process that promotes optimal ignition timing.

Other timing adjustments are programmed compensations that are predictive in nature. For example, on some vehicles a sharp increase in throttle position predicts a fast change in engine load. In preparation for a predicted change in engine load the ECU will preemptively remove some ignition timing. This compensation is not really knock correction at all but is instead a predictive ignition timing adjustment. Similar compensations exist for a variety of conditions including but not limited to changes in load, air temperature, boost temperature, and acceleration. Tuning strategies for predictive corrections vary by vehicle but are present in some form in every modern ECU.

Unfortunately, the monitors that allow us to monitor the ECU’s activities do not discriminate between predictive timing adjustments and those responding to sensor input. However, regardless of the type of compensatory timing adjustment, at LIGHT load, these corrections are a normal means with which to optimize power and fuel economy. Under light load there is simply not enough cylinder pressure to allow engine damage. As a result, light load "knock correction" is not something to worry about. This is a normal process that is even more apparent when utilizing stock calibrations designed to utilize a range of fuel quality.

Travis Geny
COBB Tuning