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Knock Question - Plex Knock Monitor

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I recently purchased a Plex Knock Monitor. A customer with an Evo I tuned several years ago contacted me saying he was getting knock at low rpm when moving off from a standstill (the ROM in the ECU flashes the check light to indicate knock). Evos are known to show false knock at low rpm so I figured this was the issue and that this would be the perfect opportunity to test/learn the Plex.

I bolted the knock sensor to the cylinder head - there is a boss right above one of the intake manifold bolts. This is not the recommended place but I couldn't find a place on the block to take the sensor without drilling it out. Went on the road with the car. I couldn't repeat the issue the customer was having however at higher rpm at full load (4000 -5000 rpm). I could clearly hear what sounded like knock through the headset and the display on the knock monitor was showing the sensor activity in this area as being much higher than the threshold level (I was using the default threshold level). The ECU wasn't picking it up as knock however. I pulled 3 degrees of timing out of the MAP and richened it up some more (it was already quite rich at low 11s AFR) with no obvious change in the noise level. These changes did create a noticeable loss of power however.

It's a stock bore engine and the knock frequency was set as such. I even tried doubling the frequency as recommended for noisy engines and I could still clearly hear it. I was listening to the audio using the 10kHz high pass filter which may not have been the correct setting but I would expect if anything this would filter out some knock noise and not amplify it. This may be a wrong assumption.

My next thought is to purchase some high octane race gas and listen again. However, I am wondering if the location I placed the knock sensor at has anything to do with it - maybe it is picking up valve train or other noise?

I would be grateful if Andre or anyone else who has experience with the Plex could chime in on this. It has me feeling quite concerned that this car might have been happpily knocking away for some time, although I would expect the effect of the knock to have shown itself a long time ago if this were the case.


I use to bolt the knock sensor on the same position as the OEM one, but on the exhaust side on the 4g63 block. There is a M10x1,25 thread in the block.

There are alot of noises in the upper rpm which get's higher with more ignition advance.

I found it very difficult too, to distingush between opperation noise and knock.

Especially on E85 the noises are getting higher with advancing ignition and I feel often it's very difficult to find the knocklimit.

What are others experience on the 4g63?

I've just recently had a 4G63 on the dyno and was experiencing some knock (listening to the Plex Knock Monitor as well as showing it on datalogs) with very little total timing just at peak torque and peak boost, so it was very likely to knock for real.

Didn't touch the factory knock sensor, and my Plex Knock Sensor was mounted on the intake side of the block, were the support bracket bolts for the intake used to be.

This was a very frusting situation, as I've tuned the car a week prior and it didn't knock (not on the dyno, not on the Autobahn), then I got it back, because it behaved strange and ended up with this.

I am not able to do any further testing, as I had to return the car already (did my best to get rid of the knock), so hopefully I'll get a chance soon to test with some race gas.

On stockish engines I found the Plex tool very effective with the lowpass filters, you can clearly hear knock if you go crazy on the volume. Same with my old Phormula KS-4.

In comparison I have to say, that both tools very effective, you could distinguish clearly, but with the audio filters on the Plex Knock Monitor you really have an advantage.

Thanks for your responses. Adrian, I actually saw the boss you speak of but was afraid of the heat damaging my expensive to me investment. KG Performance this car still has the bracket unfortunately.

The thing is this car has been running on this same tune for over two years. I am going to try relocating the sensor to the block and if the noise is the same I will try the race gas. I really can't imagine the car would want any less timing however as it felt much slower after taking out the timing during my testing with no noticeable reduction in the noise.

I also use the Plex along with a 'Knock box' system and I mainly tune 4G63 motors.

As mentioned before, these motors are VERY noisy in the top end between the lifters, EV14 based injectors and E85 only makes it noisier once you have the timing cranked up.

I have found either the front M10 bolt, or the top of the intake manifold works well for me. I can clearly hear knock on both my units, before someone can audibly hear it outside the vehicle on the dyno, the Plex filtering didn't work for me at all when the sensor is on the head, simply to much background noise, and I actually found it more accurate / discernible for knock detection with the filter turned off when up around head area.

BENchmark, did you need to do anything special to protect the sensor and cable form the exhaust heat? (other than strapping it to prevent direct contact).

When you say the top of the manifold, do you mean on top of the plenum?

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