Engine knock produces a distinctive sound that is hard to mistake. It can occur for many reasons, including bad or contaminated fuel, using a lower octane than the application requires, mechanical faults, or excessive cylinder pressure from too much boost or ignition timing that is too far advanced. Modern vehicles use what's known as knock sensors, to combat this. When the vehicles ECU “hears” knock through these sensors, it typically retards ignition timing immediately to prevent damage and bring the engine out of a knock scenario. Strategies vary by manufacturer, but the universal, standard concept is “Knock Retard”, this represents the instantaneous amount of ignition timing removed, or, "retarded" in response to a knock event. The size of this reduction generally tracks the severity of the knock. Some systems also apply a long-term correction that reduces advance over time so the engine does not return to the knock threshold.
In stock configuration, Type S vehicles expose a logging item labeled from the manufacturer as “Knock Retard,” and the OEM diagnostic software instructs technicians to use it to determine whether knock is occurring. Unfortunately, this label is deeply misleading. This item, "Knock Retard" as they call it, is actually an additive term, that advances ignition timing, not a subtractive term that retards it. We know this to be true because the “Knock Retard” logging item on Type S vehicles actually represents the system’s "Dynamic Advance". The ECU sends this value over the CAN-BUS as an unsigned 8-bit integer, proving this value cannot be negative. Because this item is used as an addition to the base timing and cannot go below zero, and because it cannot be negative, it cannot, at all, represent ignition timing being reduced, pulled, or "retarded". Despite it's name, it is not a reflection of true real-time "Knock Retard." Compounding the confusion, the real, genuinely critical knock-control items are not exposed in common scan tools or even the manufacturers diagnostic tool, they are withheld entirely from public consumption.
In reality, Type S vehicles use a "Dynamic Advance Correction" strategy. When the system determines there is no knock and conditions are favorable (good fuel, suitable weather), it advances the ignition timing to increase performance. This added timing is the known as “Dynamic Advance,” which is what actually populates the misnamed “Knock Retard”. Now, There is a true, immediate negative correction item that exists, and it reacts based upon the knock sensor noise index. This is the real "Knock Retard", it is a subtractive term, that the ECU applies the moment the sensors flag excessive noise. It behaves as you would expect, but once again, it is not provided as a logging item, it is hidden away from public consumption. To decide how much "Dynamic Advance" to allow, the ECU uses a “Dynamic Advance Multiplier” again, a value hidden from public consumption, that ranges from 0.00 to 1.00. This multiplier drops when the hidden, real "Knock Retard" has to intervene aggressively during significant knock events, and when it does not, the multiplier quickly moves towards 1.00, to advance the ignition timing. The equation is as follows:
Ignition Timing = Base Timing + Dynamic Advance (Manufacturer Calls This Knock Retard)
Dynamic Advance = Dynamic Advance (Hidden) * Dynamic Advance Multiplier (Hidden) + Actual Knock Retard (Hidden) + Miscellaneous Correction (Hidden) + Miscellaneous Correction (Hidden)
In stock configuration, this system isn't too helpful, the DAM (Dynamic Advance Multiplier) will push the Dynamic Advance to max within seconds of a detonation event, rapidly re-entering the knock threshold. With our 2.2 release, we give the end user the ability to monitor the vital indicator that is the true "Knock Retard". We look forward to collecting data within the community utilizing our 2.2 release to better understand this system as a whole, and when our mobile tuning platform launches, all of these items will be accessible and properly labeled.