Monitor Knock Sensor Directly

[iamtall77]

This is more 2G specific. I know I've seen boxes and the LED. But can we monitor the knock sensor directly with a simple solution? The sensor itself has to put out some kind of signal. I really want to see what this thing is doing. Can you hook up an LED to the knock wire? What can be done to monitor this? I don't think I'm having any issues, the car is mostly stock, but I'd at least like to know.

[Carl Morris]

The signal from it is AC, with a lot of different frequency components as far as I know. You can't get anything from it unless you design a circuit that can process/interpret it. I guess if you listen to it as an audio signal your brain can process it fairly well, and you actually hear the knock...

[Jon Sisk]

Oooh! That's awesome Carl! Maybe a knock sensor piggyback with an FM transmitter. Then you can tune in the knock station when you are tuning and listen to your motor :) I bet Erik could build one :P

[DSMtuned]

From my discussions with Steve Wells years ago, it sounds like the knock signal is very sensitive and complex. This being the case, you don't want to deteriorate the signal at all by tapping wires into it.

Maybe you can add a second knock sensor and monitor that one. Or get a wideband...

-Craig

[rigby]

Yeah, there was actually a guy that posted on his website a raw wav file of the knock signal data he'd captured. Was several years back so I don't even know if it's still around. You might be able to google for it.

It looked like the waveform was suitable for frequency analysis, but just sounded like 'pink noise' when I played it as an audio signal.

IIRC from some of the AEM forum posts I read a long time ago, some guys
using thier EMS were effectively just using the normailized amplidude of the raw signal as their knock input (ie: how 'loud' it was). I don't think that'd be very robust though, because in the same breath, they'd talk about tuning for little or no knock anyway.

[cyberslug]

The knock sensor is just a condenser mic. The knock LED is a scam, it's just connected to the boost solinoid and tells you if the ECU is trying to pull boost (though usually a result of knock). If you really want to build your own "knock sensor" then you have to connect it to the mic and listen for 6400Hz right after the spark plug fires. Good luck with that dude... There is a couple of options though. I've seen knock/timing retard devices from 3rd parties (MSD?) or the ECU+ really tries hard to emulate what the ECU does to listen for knock through the stock sensor. I'm no electrical genius but I presume you would have to have some kind on DSP connected to the sensor and the tach signal, then do some major custom coding.

For Reference from http://www.streetrodstuff.com/Articles/Engine/Detonation/Page_2.php:

Detonation causes a very high, very sharp pressure spike in the combustion chamber but it is of a very short duration. If you look at a pressure trace of the combustion chamber process, you would see the normal burn as a normal pressure rise, then all of a sudden you would see a very sharp spike when the detonation occurred. That spike always occurs after the spark plug fires. The sharp spike in pressure creates a force in the combustion chamber. It causes the structure of the engine to ring, or resonate, much as if it were hit by a hammer. Resonance, which is characteristic of combustion detonation, occurs at about 6400 Hertz. So the pinging you hear is actually the structure of the engine reacting to the pressure spikes. This noise of detonation is commonly called spark knock. This noise changes only slightly between iron and aluminum. This noise or vibration is what a knock sensor picks up. The knock sensors are tuned to 6400 hertz and they will pick up that spark knock. Incidentally, the knocking or pinging sound is not the result of "two flame fronts meeting" as is often stated. Although this clash does generate a spike the noise you sense comes from the vibration of the engine structure reacting to the pressure spike.

[iamtall77]

Well, damn. I was hoping it was more simple than that. I just figured the knock sensor itself was a fairly simple device, so I thought maybe it was possible to monitor the signal coming from it. Kinda like reading the O2 sensor or something. I do know how the Knock LED works, but was looking for a more direct monitoring system. I have seen knock boxes, but haven't really looked at how they work. The stock ECU has to monitor knock somehow so it can adjust the timing or other factors for it. I thought maybe there was a way to read that part of the data signal outside of a logger. I kinda miss my 1G simply cause you can see this data from the ECU. :confused: