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Old 11-04-2010, 02:25 PM   #1
Geek Guy
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Default Knock sensor frequency?

Hi folks,

Looking to see if anyone had any solid details on how the 02 WRX knock sensor functions? Does it hone in around 6kHz or 13kHz (or somewhere in between)?

Trying to setup an external sensor, and am looking to make sure I get the settings "right".
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Old 11-06-2010, 06:30 PM   #2
2milehi
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Here is an in depth look at a 2.5 subie. The knock will be at a higher frequency than the 5.9 kHz I found.

http://www.msextra.com/forums/viewto...p?f=67&t=36089
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Old 11-07-2010, 01:38 AM   #3
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So, calculated frequency for at stock-bore EJ20 is 6.2kHz. I'm using the Vi-PEC digital knock rig which allows for an adjustable narrow-band filter.

Right now, I've got it centered on 6kHz (+/- 1kHz at the corners), so the whole detectable range is 5kHz to 7kHz.

But the documentation mentions something about v7 WRXes onwards using a 13kHz knock sensor. (which would make sense, as it's closer to the 2nd harmonic).

Does anyone know if the stock sensor is a 1st or 2nd harmonic unit? I guess I could hook up the laptop and try to induce knock (while driving around).

Thoughts?
-E
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Old 11-07-2010, 01:59 AM   #4
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That is your best bet, capture some knocks and look at the spectrum of the signal. Then build the circuit accordingly.
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Old 11-07-2010, 01:02 AM   #5
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Well, nothing to build - it's a pretty slick entirely integrated (adjustable filter) box:

http://www.vi-pec.com/techdata/Vipec...kAmplifier.pdf
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Old 11-07-2010, 11:28 AM   #6
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Slick little unit. Looks like it has features that I have yet to come up with on the one I have been designing from scratch. The frequency spread seems a little wide for a narrow band filter. It looks more like a notch filter. Curious, how much did that unit set you back?
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Old 11-07-2010, 11:43 AM   #7
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I think the whole unit was something like $400? Bought it rolled into the price of my ECU (also a Vi-PEC). Went with this unit as it as a window in/out so the ECU pulls the knock data from the sensor when it's ready.
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Old 11-07-2010, 11:46 AM   #8
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Do you think it's better to be high or low on the notch filtering in this case? eg, 5-7kHz (0.8 kHz above, 1.2kHz below) or 6-8kHz (0.2kHz below, 1.8kHz above) for headroom?

Most knock instances have a noise range that STARTS at the calculated value, right? or does it bleed below? I can't tell from the graphs you posted in the other thread?
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Old 11-07-2010, 01:26 PM   #9
2milehi
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Better to have the calculated frequency in the center of your filter. Temperature does affect the knock frequency so better to have some room for drift. So I would pick the 5-7 kHz filter.

As for your second question from what I have seen the knock frequency starts at approximately the calculated frequency. In the below pic, I recorded detonation from my knock sensor. Each lighter colored vertical band represents a knock event. From what I observed there was always a signal at 5.9 kHz. Because of the placement of the knock sensor, each cylinder has a different knock sound to it.


Here is the spectrum of the engine with no knock.


Here is the spectrum of the engine with a small knock event.


Here is the spectrum of the engine with large knock event.


It was pretty obvious for me to go after 5.9 kHz.

Last edited by 2milehi; 11-07-2010 at 01:33 PM.
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Old 12-14-2010, 07:08 PM   #10
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Quote:
Originally Posted by 2milehi View Post
That is your best bet, capture some knocks and look at the spectrum of the signal. Then build the circuit accordingly.
Nice work. What freq analysis software did you use to log the data, and how did you connect to get the data? Thx
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Old 12-15-2010, 04:12 PM   #11
2milehi
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I recorded the signal from the knock detector via the microphone on my laptop. I'll chime in later with more info on how to do this.
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Old 12-15-2010, 04:17 PM   #12
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Quote:
Originally Posted by 2milehi View Post
Better to have the calculated frequency in the center of your filter. Temperature does affect the knock frequency so better to have some room for drift. So I would pick the 5-7 kHz filter.

As for your second question from what I have seen the knock frequency starts at approximately the calculated frequency. In the below pic, I recorded detonation from my knock sensor. Each lighter colored vertical band represents a knock event. From what I observed there was always a signal at 5.9 kHz. Because of the placement of the knock sensor, each cylinder has a different knock sound to it.


Here is the spectrum of the engine with no knock.


Here is the spectrum of the engine with a small knock event.


Here is the spectrum of the engine with large knock event.


It was pretty obvious for me to go after 5.9 kHz.

Very nice info!

Great post
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Old 12-15-2010, 09:42 PM   #13
2milehi
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Here is the spectrum analyzer
http://www.electronics-lab.com/downl...003/index.html

Here is a *.wav analyzer that generated the smaller spectrums
http://www.music-editor.net/download/index.htm

How to build the interface circuit.
http://www.electronic-engineering.ch...n/connect.html
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