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Knock sensor voltage safe voltage

EFI Tuning Fundamentals

Discussion and questions related to the course EFI Tuning Fundamentals

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Can you please tell me, what is the knock sensor voltage safe threshold value ?

Is there any general value ?

And how can i know it w/o destroying my engine ?

Can you please tell me which voltage value do you using as a threshold in your projects ?


Knock sensors output a voltage that is an analog of the audio signals being picked up. It functions like a microphone, which picks up audio, the output of this is then fed into knock detection circuitry in the ECU (Or in a standalone unit like the Plex Knock Monitor) which uses analog circuitry and DSP filtering to isolate the knock signal. This output is then used to determine if the engine is knocking.

The output of this signal processing is an dimensionless, arbitrary number, with a user defined threshold - above this threshold, the ECU retards ignition timing and updates it's short and long term timing trim maps.

So in short, you need to use an ECU that supports knock detection, or a unit like the Plex Knock Monitor, which can then in turn retard the engines ignition timing. You cannot measure the output of a knock sensor's voltage directly and use that to detect knock.

The safe voltage threshold will depend on the particular engine and the engine speed since the noise level the knock sensor detects increases with rpm. It's also more than just a voltage. What we really want to do is analyse the signal from the knock sensor and only focus on the noise frequencies where knock is known to occur. This helps reduce the effect of background engine noise.