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PLEX knock monitor - false knock?

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Hello everyone,

So I did some road tuning for an Audi UrS6, 5 cylinder engine, AAN. 9.3 CR (stock).

With the following mods:


AR size .59

Compressor wheel:

✔ Inducer 56 mm

✔ Exducer 78 mm

Turbine wheel:

✔ Inducer 76 mm

✔ Exducer 60 mm

Custom exhaust manifold and other supporting mods.


We did a custom mount for the plex knock sensor on the engine block (we used just 1), located on the 3rd cylinder close to the TDC.

I have used the trigger from the 1st cylinder to get the knock per cylinder reading.

I will also outline the settings set for the Plex knock monitor software

1. Knock freq - 7270 (80mm) - the engine has a 81mm bore

2. Bandpass 7Khz

I have set the gain so that I have the max level reading between 80-90 for each cylinders.

Car is running 99 octane petrol.

As you can see, I've set a very safe ignition map and I was also targeting about 0.88 lambda for the transition between vacuum and boost and 0.76 lambda for the full boost.

We did a couple of WOT runs, started with 1.5 bar of boost and raising to 2 bars , as you can see it's not a big turbo and the ignition map should make it more than safe, all was fine.

While I could hear some distorted sound at the high rpm's, it just didn't make sense for me, no-knock did I hear up until 5000 rpms.

Considering lambda value of .76 , ignition angle in our case knock can not occur at 5000+ rpms, also, remember the small turbo and .AR 59 that we got.

You will also see on the log file attached that we had about 6-7 ign angle at 1.9 bars of boost.

So I had the final values from the plex knock monitor

Cylinder 1 - Max value 85

Cylinder 2 - Max value 80

Cylinder 3 - Max value 140

Cylinder 4 - Max value 125

Cylinder 5 - Max value 78

On the graph I had erratic spikes from cylinders 3 and 4 and I could also audibly hear the knock (distortion?)

I did add more fuel to cylinders (10%) to be exact for the sake of the experiment, to the cylinders 3 and 4 and also cut 2 degrees for each of them and did another run, though not a big difference even after doing this.

My questions are:

Is it possible to get knock considering the statements above?

If it is a false knock, should I lower the gain for the knock monitor, then I will get significantly lower signal from the other cylinders that had the max level at approx 80.

Sorry I do not have a dyno, so how could I ensure that I actually don't get false knock, what rpm and map range should I target to make a test run to actually hear the knock, considering it's a turbo engine with 9.3 CR.

Looking forward for any advices on how to correctly work with Plex.

I have watched dozens of videos, heard samples of knock sounds but I am not still 100% sure when the real knock occurs.

Attached Files

What filter setting do you have in the configuration under point 5.2? I prefer a high pass 5 kHz and have no problem whatsoever to hear knock.

Thanks for your reply,

I was using 7 khz as the filter frequency is recommended to be close to the main frequency as far as I know.

I am not sure if changing the filter would drastically change the result, but I may be wrong.

For me it was very odd to see the signal to be around 80 (at max level) for 3 cylinders and 140 for the other 2.

If the knock sensor is located at the 3rd cylinder, it is normal to have a higher output signal on that. On most standalones, you can adjust the gain to read equally. Just use your audio output to verify it is not knocking or do a run with pulled timing to compare noise levels.

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