Knock sensor for an N/A.

joel w · 11-16-2005, 12:34 AM

i wish i could help but i did steal this off another site, it was interesting to me.. specially the second paragraph..

Knock sensors are nothing but heat resistant microphones. The signal from a knock sensor is an audio signal recording engine noise. The knock detect circuit in an ECU first of all looks at that audio signal ONLY during about 5 deg ATDC to about 70 deg ATDC, where knock is most likely. The characteristic audio signal for knock has a specific frequency (pitch) that the knock detect circuitry filters out. This pitch (some frequency between 5 kHz and 20 kHz) is dependent on bore size, average gas temp in the chamber and comb. chamber shape. The knock detect circuit is factory tuned for that. On many OEM ECU's the knock circuit is switched off above ~4000 RPM, because the knock signature signal gets buried in other engine noise.
Engine mods like boring out, turbo's added, different boost, different fuel and so on, can change the characteristic knock frequency with the result that the ECU will listen to the wrong frequency and will not detect knock at all with it's existing sensor. I can also pick up other engine noise as knock.
There are products out there that attempt to detect knock from existing knock sensors. They typically require you to program a threshhold amplitude level for the knock signal by RPM, without regard to the characteristic knock frequency of the engine. That works only very marginally, because if the knock sound level is above the engine noise level at higher RPMs, you can hear it already with your ears before that circuit can detect it and you might already have engine damage.

Regards,
Klaus

found this too, maybe its a common problem..
http://www.supras.com/06/techcenter/display.php?QID=69

11-16-2005, 12:34 AM	#2
joel w Lexus & 550's Join Date: Aug 2005 Location: In the shadows Posts: 506	i wish i could help but i did steal this off another site, it was interesting to me.. specially the second paragraph.. Knock sensors are nothing but heat resistant microphones. The signal from a knock sensor is an audio signal recording engine noise. The knock detect circuit in an ECU first of all looks at that audio signal ONLY during about 5 deg ATDC to about 70 deg ATDC, where knock is most likely. The characteristic audio signal for knock has a specific frequency (pitch) that the knock detect circuitry filters out. This pitch (some frequency between 5 kHz and 20 kHz) is dependent on bore size, average gas temp in the chamber and comb. chamber shape. The knock detect circuit is factory tuned for that. On many OEM ECU's the knock circuit is switched off above ~4000 RPM, because the knock signature signal gets buried in other engine noise. Engine mods like boring out, turbo's added, different boost, different fuel and so on, can change the characteristic knock frequency with the result that the ECU will listen to the wrong frequency and will not detect knock at all with it's existing sensor. I can also pick up other engine noise as knock. There are products out there that attempt to detect knock from existing knock sensors. They typically require you to program a threshhold amplitude level for the knock signal by RPM, without regard to the characteristic knock frequency of the engine. That works only very marginally, because if the knock sound level is above the engine noise level at higher RPMs, you can hear it already with your ears before that circuit can detect it and you might already have engine damage. Regards, Klaus found this too, maybe its a common problem.. http://www.supras.com/06/techcenter/display.php?QID=69 Last edited by joel williams; 11-16-2005 at 12:52 AM.