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Old 02-27-2006, 11:03 AM   #1
tmarcel
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Default Want to build a det can w/extra sensor Q's

I've got an extra knock sensor and am wondering how to build a det can device with it. It's a stock WRX sensor and IIRC it's only one wire.

My questions are:
1.) how would I go about hooking this up power (ground?), and
2.) could I just use a small tweeter type speaker wired on the other end to listen for knock.
3.) I guess it would just sound like distortion or something and would elevate as knock occurs?

I want to do this temporarily only so I can dial in the knock sensitivity on my UTEC. The engine is noisier than the average 2.5L stock engine and also this is a WRX UTEC so I need some direction. I'm thinking that if this idea would work I could just velcro the speaker to the dash or something and yank it out when I finish.

Thanks in advance for any help.

Todd
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Old 02-27-2006, 12:39 PM   #2
cdvma
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You are going to have to amplify the knock sensor. Use a tape player (walkman), computer, or a headphone amp ect. If the knock sensor has two wires, one is for the channels and the other is ground. If its just one use the chassis as ground. Best way I can describe knock is that it sounds like a little bird chirp. For best results you may want to use a filter to drop out low frequency noises so that there isn't as much background noise that will flood the headphones as the engine goes up. Without one, it sounds much like tv static.

Edit: there are a few threads on this around here, search for det can or similar and you will find some homebrew stuff.
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Old 02-27-2006, 01:52 PM   #3
Jon [in CT]
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Click here to find out how to build what you need. I'd recommend the MIC input option, either to a casette recorder or your laptop. That way you can, essentially, log the sensor and review it later, either by ear or (better) via a spectrum analyzer like gram 5.0. If you want to 'log' RPMs at the same time then use the MIC input to a video camera focused on your tachometer.
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Old 02-27-2006, 08:29 PM   #4
tmarcel
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Guys, thanks for the info. I will read up on this.
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Old 02-27-2006, 08:41 PM   #5
tmarcel
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I looked through the first link that Jon [in CT] through up. That tape recorder deal looks pretty easy to put together.

I'm wondering if there's a way to use my extra sensor instead of the one that the ECU uses? I really don't want to tap into the harness if I don't have to. Any recommendations on how I can power the extra sensor without tapping the ECU pin?
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Old 02-28-2006, 02:42 PM   #6
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Quote:
Originally Posted by tmarcel
The engine is noisier than the average 2.5L stock engine and also this is a WRX UTEC
Are you using the STi knock params? I wasn't at first and I got ALL sorts of false knock. The STi params are much lower than that of the WRX.

About tapping, I just tapped into the harness. I mean, you don't even need to cut it, just strip a small section and solder. It's 10 min job.

When you get the Gram50 working be sure to post your logs so I can compare.
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Old 02-28-2006, 02:48 PM   #7
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Quote:
Originally Posted by tmarcel
Any recommendations on how I can power the extra sensor without tapping the ECU pin?
A knock sensor doesn't use power.
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Old 02-28-2006, 02:53 PM   #8
tmarcel
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NOOO! Mine are way lower than the STI params. On mine, sensitivity starts about 85 to 2k rpms then 60 then 55 then 50 then back to 60. I couldn't even rev in neutral without the UTEC picking false knock. The engine (bottom end) sounds great except for loosely shimmed lifters, which on the other hand may may be casuing the false knock. I'm not sure about your motor but mine is just so different from a stock 2.5 that I can't trust what the UTEC is telling me until I can listen for myself and then dial in the correct knock sensitivity.
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Old 02-28-2006, 02:55 PM   #9
tmarcel
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Quote:
Originally Posted by Jon [in CT]
A knock sensor doesn't use power.
So how would one go about hooking it up or does it need the ECU after all, i.e. I won't be able to use the extra sensor afterall and just tap the existing one?
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Old 02-28-2006, 06:10 PM   #10
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Think of the knock sensor as a microphone... that should help you figure out how to hook it up...
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Old 02-28-2006, 11:27 PM   #11
cdvma
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Quote:
Originally Posted by tmarcel
So how would one go about hooking it up or does it need the ECU after all, i.e. I won't be able to use the extra sensor afterall and just tap the existing one?
Read my first post...
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Old 03-01-2006, 08:01 AM   #12
tmarcel
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Quote:
Originally Posted by cdvma
If the knock sensor has two wires, one is for the channels and the other is ground. If its just one use the chassis as ground.
Sorry man Since it was the #2 post I forgot to go back Maybe it's just me (advisment: I suck at wiring and electrical) but if there's only one wire, I don't see how it would ground then. Are you saying that the block is the ground, just by being bolted to it?

Also, I still am not understanding the way the extra sensor (not the one in the stock location) is going to be powered. Is it simply a 12v power source or will it need to go back through to the ECU pin?
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Old 03-01-2006, 10:26 AM   #13
Jon [in CT]
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Quote:
Originally Posted by tmarcel
I suck at wiring and electrical
You really do.

A knock sensor doesn't need power, just like a thermocouple EGT sensor doesn't need power. The piezoelectric ceramic in the knock sensor will generate a voltage in response to vibrations.
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Old 03-01-2006, 10:47 AM   #14
tmarcel
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Quote:
Originally Posted by Jon [in CT]
You really do.

A knock sensor doesn't need power, just like a thermocouple EGT sensor doesn't need power. The piezoelectric ceramic in the knock sensor will generate a voltage in response to vibrations.
Now come on...was there really a need to restate what I already said I could probably help you out with a lesson in diplomacy too

Anyhow, I'm getting closer then. Thanks again for the info everyone.
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Old 03-01-2006, 11:59 AM   #15
cdvma
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Quote:
Originally Posted by tmarcel
but if there's only one wire, I don't see how it would ground then. Are you saying that the block is the ground, just by being bolted to it?
Yep!
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Old 03-01-2006, 12:43 PM   #16
tmarcel
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Okay, here's a little MS Paint action I think this is it! Note: I only referenced the stock knock sensor for location as to where the other one will go.

Last edited by tmarcel; 03-01-2006 at 01:48 PM.
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Old 03-01-2006, 01:37 PM   #17
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Quote:
Originally Posted by tmarcel
NOOO! Mine are way lower than the STI params. On mine, sensitivity starts about 85 to 2k rpms then 60 then 55 then 50 then back to 60. I couldn't even rev in neutral without the UTEC picking false knock. The engine (bottom end) sounds great except for loosely shimmed lifters, which on the other hand may may be casuing the false knock. I'm not sure about your motor but mine is just so different from a stock 2.5 that I can't trust what the UTEC is telling me until I can listen for myself and then dial in the correct knock sensitivity.
My results are from stock pistons in a hybrid. I'm dropping in forged ones this weekend, so I'll put my results when I have them.
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Old 03-01-2006, 01:46 PM   #18
tmarcel
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Quote:
Originally Posted by TypeC
My results are from stock pistons in a hybrid. I'm dropping in forged ones this weekend, so I'll put my results when I have them.

Please do. I bet you'll find that there's a bit of difference.
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