I have been using a Vipec knock ear on the dyno for a few years now. They have an algorithm that filters out background noise quite accurately. When used with a matching Vipec ECU, they have a similar system to Motec in that they listen to a tuned frequency within a specified window. It takes a fair bit of time to set either system up but it is the only way to set up accurate individual cylinder knock control.
The new little KnockLink is made by Link who also manufacture the Vipec ECUs. This great little light is really simple. You just need to mount a suitable knock sensor (or use the existing ones on 993 or later engines) and fasten the light with the supplied mount. The wiring is simple too. Two wires go to the sensor plus 12V power and earth. It self calibrates on engine start up and will flash to warn if knock is present. It won’t adjust the timing but at least you will see that you have a problem!
Very handy for early model 911s that aren’t running fuel injection or good quality fuel!
I have a few of these in stock now. The RRP is $275. I can also supply a suitable knock sensor.