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Knock sensing, whats the crack with that then?? 
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LQFP144 - On Top Of The Game

Joined: Sun Jul 13, 2008 2:58 pm
Posts: 331
Location: South Wales, UK
OK, now I know I just blew my engine up but thats no need to take the piss! OK?

What I would like to know is what do normal OEM knock sensors produce as output?

I know they are essentially piezo transducers tuned to knock frequencies, but what does that equate to in terms of voltage and frequency?

Would I be looking at making a simple headphone amplifier to capture these obnoxious sounds for live tuning purposes or something a bit more specialised?

I realise that I'll need to interface this the EMS if I want closed loop control but for human intervention tuning to connect to headphones would a simple headphone amp do the trick???

The only reason I ask is £120 for a head gasket and 18hours of my time to change the bitch! multiplied by the number of times I manage to Fuck this up. = Grrrr!

93'BMW 325is M50B25TU, Rebuilt 06/06, JE10.5:1, polish&port. Scorpion BB, K&N CAI, TEJ21 WBO2, '07 M3 Evo 18" 225F, 255R, EBC Kevlar, Bilstien Sprint, Polyflex. Head rebuild Oct'08, OEM+FSE FPR, MS2v3.0_DJB Custom, Extra 2.0.1

Sat Oct 18, 2008 12:40 am
1N4001 - Signed up
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Joined: Tue Jun 10, 2008 5:17 pm
Posts: 1607
First remember to plane the head and make sure it's flat, ect. A couple thousandths is a huge deal in terms of keeping the head gasket properly compressed. I'm sure you'll do it right, you seem to prefer it that way.

I believe the signals vary. Many will produce a DC output, something that is simply a ping, no ping indicator. I believe this is more commonly found on the low end vehicles and older vehicles. Some vehicles will route the audio path(s) back to the ECU for processing, some others will have a dedicated processor at the device w/ digital signal(s) that go back to the EMS.

Frequencies vary based on the engines physical design, I seem to recall it's around 6 kHz ish +/- ?. Also the Pin should be placed relative to the spark. Listening for both can usually displace other noises like bearings injector knock, ect. You could find the resonant frequencies by connecting the audio path to your computer and recording it when it pings.

I seem to recall a thread there somewhere that had good reading about it. Do you have a knock sensor in mind that you might want to start using / prototyping with?

Sat Oct 18, 2008 1:08 am
1N4001 - Signed up
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Joined: Tue Jun 10, 2008 5:17 pm
Posts: 1607
Appears you posted the article I was thinking of, any how I found it in this thread.


So I guess that means you already know about it. Hi hi. Also under the article submissions, there is this thread.


I'd be willing to bet you lucked out with the head planing thing. Sounds like you caught it before excessive heating got a chance to work it's magic.

EDIT] also don't forget about this thread


And this thread notes a PSoC glass break sensor, that could be made into a knock sensor.


Sat Oct 18, 2008 1:30 am
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Joined: Tue Jan 15, 2008 2:31 pm
Posts: 15030
Location: Home sweet home!
Dave, start simple KISS :-)

If you have a line level in on your car audio, just hook it to that. If not, you just need a normal audio preamp to drive some headphones. Otherwise, you can do it without the sensor by creating a paper cup and string type arrangement. You do that by flattening one end of a piece of metal tube, plugging the other end and fitting 1 or 2 tubes to it that feed back to the cabin where they are plumbed to each side of a pair of ear muffs/ear defenders modified to have a path from the tube to your ear. The nice aspect of that method is that they are designed to block out noise, so you get a fairly pure engine sound that should be dead obvious to pick knock from.

There are lots of good links around, I'll let you google them up :-)


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Sat Oct 18, 2008 9:36 am
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