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Guest Message by DevFuse

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The Knock sensor

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3 replies to this topic

#1 rverdoold


    Subaru Nut

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Posted 27 August 2007 - 05:18 AM

The knock sensor is a piezoelectric microphone placed on the engine to measure engine knocking (pinging). I actually measures early detonation of the explosion mixture in the chambers. It was designed for using low-octane fuel in high performance engines by SAAB (turbo in 1982). The system was named APC or automatic performance control.
On turbo's the wastegate is opened if knocking is detected lowering the turbo presure. On normal aspirated i think it changes firing point some. However i would suspect the system works in collaberation with the EGR (exhaust gas recirculation). Lowering the MPG but reducing knocking.

My question is, why does my European 91 legacy does not have a knocksensor. (or at least not on the location where i expect it, underneath the throttle body).
Is this because 92 octane is already hard to get. Usually i drive 95 and only 98 when its cheap to get. I have never heard any pinging in this engine.

(i would like to make a thread with good information about the knock sensor cause it often is asked. So images and ideas about location are welcome)

#2 porcupine73


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Posted 27 August 2007 - 08:06 AM

Not sure if that vehicle has a knock sensor or not.

Your area's octane rating method may differ from the U.S.'s average of the 'motor' and 'research' octane ([R+M]/2) rating.

Here's some Subaru knock sensor info to add to the thread:
The knock sensor is designed to sense knocking signals from each cylinder. The knock sensor is a piezo-electric element which converts knocking vibrations into electrical signals. The electrical signal is sent to the ECM, which changes the ignition timing to reduce the knock or ping. For this system to work correctly, the sensor must first hear engine ping. The driver of the vehicle may also hear a small engine ping.

A of approximately 1-2 seconds is normal, depending on the fuel quality, load, air temp, etc. At this time, the ECM will retard the timing. This function can be viewed on the Select Monitor RTRD mode. When the knock is eliminated, the timing is gradually advanced to the specified setting. engine ping is heard again this process is repeated. This will continue until knock sensor no longer hears the engine knock or ping.
Note: this is normal operation of the knock sensor. Do not try to repair it.

#3 OB99W


    Elite Master of the Subaru

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Posted 27 August 2007 - 08:46 AM

These may also be interesting:



#4 Reveeen


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Posted 27 August 2007 - 08:52 AM

why does my European 91 legacy does not have a knock sensor.

Probably, if you don't have one, Subaru considers users in Europe "bright" enough not to be stuffing 87 octane fuel in their turbo cars.

On turbo's the waste gate is opened if knocking is detected lowering the turbo pressure.

That is the Saab/Bosch system, far more intelligent than the Subaru system that simply retards the timing. My turbo Saab here makes boost at idle, topping out at 21psi, Subaru turbo systems do not come this way from the factory, employing lower pressures. The closer you run a system to the edge, the closer you have to monitor, and control it.

ideas about location are welcome

Standing at the front of the car, after locating the throttle body, look to the right of it, near enough straight down.

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