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Plex Knock monitor v2 - choosing the right settings.

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To anyone who has experience with the Plex knock monitor.

Hello everyone,

I am dealing with a 5 cylinder, 2.5 stroker engine with mechanical flat tappets.

The engine is quite noisy by itself, but all the mechanical tuning probably made it even more difficult for a novice to detect the knock.

So, considering I've got a diesel block (it's a stroker engine and we've used a diesel) AEL Audi block (81mm).

So I've started with setting the trigger to the 1st coil to get the benefit of seeing which cylinder is knocking.

I've started with 7270Hz freq, adjusted the gain so that I have the peak number between 80-100 and chosen the 7Khz filtering bandpass (tried with 10Khz as suggested in tutorials but didn't get better). I did some revving at idle and It was quite noisy, difficult to distinguish anything at high rpm, then I've decided to go with the 2nd harmonic frequency which is 14360hz. This was perfect, a nice and clean background noise which would easily help me distinguish the knock.

I've run about 14 degrees of advance at 0.45 positive boost (0.45 bar) and I was listening to the awful sound to me which maybe sounds like knock (I'm not 100% sure). Theoretically, this is quite impossible to me tbh, considering the low boost and as this engine is mechanically ready for 600+ hp and I could only blame this on the wrong settings chosen by me in the Plex monitor configuration.

I have attached the audio file of the so-called knock I've heard, sorry for the sound quality, I've recorded this using Goldwave.

So, I've gone back to the 7270Hz frequency, adjusted the gain, same 7Khz bandpass and the nasty distortion/noises disappeared at the same rpm range. I did raise the boost a bit, just under 1 bar, and again what sounded like knocking came up at 5-6k rpm, considering the ignition angle was about 6-8 degrees, this got me confused again.

Now I am not sure what else I could try in order to make sure that I'm not getting the false knock.

The problem with 7270Hz is that audibly it's quite difficult (especially at high rpm) to hear the knock, the Plex Realtime monitoring with individual cylinder knock detection helps for sure, but I would like to rely on my ears first.

I was thinking maybe to go back to the 2nd frequency, and lower the gain until I start to hear the real knock. Also I'm 99% sure this is just distorted sound because of the wrong bypass filter/gain adjustment which caused this distorted sound in the recording. Although as I've mentioned before, sensor gain was set just to meet the 80-100 peak value.

P.S. I've read the instructions and also watched Andre's (hp academy) webinar on plex, I do know how to adjust the curve and how to use the device (probably).

* CR 9.5

* AEL engine has a 81mm bore

* Was running 99 RON fuel

* Knock sensor located on the block, middle, 25 nM

* AFR 0.76 (11.3) when on boost

I am afraid to lower the gain to a degree when I could miss the real knock, is this possible?

I would be very grateful if I could talk to someone via whatsapp,facebook,skype or anything who would be happy to help me with this.

I can provide live audio from the plex monitor I'm hearing to make it easier for both of us.

https://1drv.ms/u/s!Akcze73BmaiP3wLHunJ-5gWYw5VK

Attached Files

It is a little hard to tell you definitively if you're hearing knock in that sample or if it's distortion. Unfortunately audio knock detection isn't always completely black and white and there is a little skill involved in getting a reliable result. In general you've done exactly what I'd suggest which is to try the 2nd order frequency as this usually gives a better signal to noise ratio. The other aspect I suggest to ensure you're able to accurately detect knock is to purposely induce some knock at light load and low rpm so you can notice the sound it makes. In most engines this is relatively straight forward to achieve.

Conversely if you've got a sound like you're noticing that may be distortion or a mechanical noise, I'd try aggressively retarding the timing in this area (pull 6-8 degrees) and see if that makes any difference to the sound. If it's still there and hasn't changed then it almost certainly isn't knock.