Subaru Knock Monitoring - Everything You Need To Know – DRAFT

Applicable Vehicle:

All supported Subarus

Brief Background

Thanks to the power of the AccessPORT and AccessTUNER software, it’s now easier than ever to gain unparalleled insight into the current status of your engine’s tune and running condition at any given time. Monitoring for detonation and knock is likely the most frequent topic we receive inquiries on. Subaru’s factory knock control strategy is somewhat complex, though with some basic knowledge the picture is much clearer. First, a few items to remember:

• Detonation events are inevitable and will occur from time-to-time on any modern vehicle running on pump gas. Properly advanced ignition timing is important for maximizing fuel economy and power output while minimizing tailpipe emissions; in many instances this timing value is at the edge of the knock threshold for pump fuels and occasional detonation is the result as external conditions vary slightly. This is why all factory EFI implementations feature knock detection and control systems.

• No detonation/knock detection system is perfectly accurate. We choose to err to the side of safety when it comes to listening for and responding to noise detected from non-detonation causes (“false knock”).

Relevant Monitors

• Dynamic Advance Multiplier (DAM): This value represents a global adjustment to ignition timing. In general this value is determined by historical detonation/knock as well as the default mapping value. For the 2.5L / 2L DIT ECU, DAM is stated as a decimal ranging from 0 to 1. For the 2.0L EJ ECU DAM is stated as an integer between 0 and 16. DAM will tend to vary the most immediately following a reflash or ECU Reset procedure.

• Feedback Knock Correction: This value represents a current realtime minor timing correction made by the ECU in response to a perceived noise. Feedback Knock Correction is the default correction used by the ECU. The ECU will “respond” with this monitor by immediately removing timing and then slowly decaying the value back to zero assuming no further noise is detected.

• Fine Knock Learning: This value represents minor learned corrections currently being applied by the ECU as a product of historical noise. These values are reset if the DAM value is changed; once DAM has stabilized, any learned corrections will be intermittently evaluated for sanity when certain thresholds/conditions are met. Fine Knock Learning is learned (and thus applied) for specific ranges defined by load and RPM; once a correction value has been learned into a particular range, it will be applied whenever the engine is operated within that range.

• Knock Sum (where applicable): This is a somewhat arbitrary value that should only be analyzed under the specific conditions you wish to evaluate, such as during wide-open throttle (WOT) operation over a set RPM range. Some later ECUs can report this on a percylinder level, some only on a global basis. If a noise is perceived, this value will increment at all times, regardless of if the knock detection system is deemed to be accurate or not, such as idle and very low RPM.

Knock Per Cylinder

The ECU detects which cylinder is knocking based on the premise that knock occurs in a window of crankshaft rotation during which cylinder pressure is rising post ignition event. If the knock sensor generates sufficient voltage, and a cylinder is within that knock window, the ECU increments the knock sum for that cylinder. In addition, if the knock voltage is sufficient enough to surpass the calculated background noise under current engine conditions, and the engine is operating within the RPM and load range over which feedback or fine knock learning may occur, the ECU will react to the knock event by pulling timing, in steps based on the processed knock data. While the ECU can use this method to take an educated guess as to which cylinder is likely knocking, the knock response of pulling timing affects all cylinders. Subaru does not perform per cylinder knock based timing correction on the STI, though that may be something they add in the future.

Whether the knock sum you're experiencing is false or not you can't say based on it occurring in a single cylinder. Keep in mind the knock sensor is a piezo element, a contact microphone, so any noise occurring within the knock window which is within the frequency range the sensor is tuned to, and of sufficient amplitude, can be registered as knock by the ECU. As just one example of how you can get false knock on a single cylinder, excessive valve lash on a single bucket could cause excessive valvetrain noise which happens to occur within the knock window for a cylinder. The noise may not even be occurring in the same cylinder the knock is being detected...it may just happen to occur within the knock monitoring window for cylinder 1.

Using high quality, highly knock resistant fuel is one way to start narrowing things down. If you're running pump gas, retuning for e85 to see if the knock events continue being detected is a common path forward.

Subaru Knock Monitoring

Subaru Knock Monitoring - Everything You Need To Know – DRAFT

Applicable Vehicle:

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