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It's adjustments to your timing based off of feedback from the knock sensor.

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Water.....sometimes waves
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I am used to it as KR "knock retard." An engine management system retards the timing (less advance) when it hears knock from sonic sensors (microphones) in/on the engine. The sensors listen for specific waveforms (frequency, amplitude, phase, wavelength) created when knock (pre-ignition from a hot spot in the combustion chamber) occurs.

Less timing advance will stop the knock which is detrimental to an engine. Less advance also yields less power from the engine (typically.) KR is a safety system meant to limit harm to an engine not operating properly (knocking) by retarding timing advance and thus also cutting power.
 

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Æternum
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Knock control strategy is contingent on each brand's overall ECU strategy.

Feedback knock is a rapid response to knock sum (commulative knock).

Subaru uses it in conjunction with two other types of ignition retard strategies -- IAM (general octane "learning") and FLKC (learned knock in specific load cells).

The way in which knock sum propogates between FBKC, FLKC, and IAM is somewhat murky water -- it's Subaru IP, but there are some pretty good conjectures out there with solid supporting evidence.

What is known is that all three pull timing out of the Ignition Advance Tables (1 table for 16 bit ECUs with a bunch more in the new ECUs). The IA tables have different advance values depending on load and RPM, so it's a very flexible system. Overall, Subaru knock control strategy is pretty good.

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