| Harry (ztunerz) had sparked my interest in the knock system a while back so I started doing some testing with it and my software. I had come to the conclusion that the stock knock system wasn't the greatest. It was good seeing Greg from SZ post about the J&S knock system. In his OP I had post a link to some info on how it connects to the ECU. Looking at the diagram It was interesting in how it was able to detect knock in individual cylinders so I decided to do some research on how this system works. First a little about the stock knock system. The TT ECU has a daughterboard which as everyone probably knows is part of the knock system. The board contains the analog bandpass filtering circuit which is made to help distinguish frequencies related to actual knock from other engine noises. The knock sensor (which is really just a piezo microphone) listens for a certain frequency of sound and is only somewhat reliable at lower RPMs. At higher RPMs it is unable to distinguish sounds due to all the noise and limits are set to disable it. Not good in high HP racing applications. After the year 2000 Nissan started using digital signal processing of the knock waveform in their ECUs but there is still the problem higher RPM noise. The J&S system uses what's called Ionisation Current monitoring. The best thing to do to understand this thoroughly is to read the links I've included. Basically, when knock occurs the action causes a voltage current which can be detected back through the ignition wiring to the J&S processing unit. When it detects these current changes it concludes knock has been detected. This is how it's able to tell which cylinder is having knock. Here are a few pictures of my crude but effective test setup. I've also added the picture of how the J&S system connects. 
J&S Knock System
 Ionisation Current links.
[ http://linuxfan.org/~ipdown/mybrick/automotive/gas_ignics.pdf ] [ http://pid.sagepub.com/content/211/6/499.abstract ] [ http://www.herningg.com/singh/Ionization%20current%20analysis.pdf ] [ http://www.google.com/url?sa=t&rct=j&q=ionisation%20current&source=web&cd=2&ved=0CCkQFjAB&url=http%3A%2F%2Fciteseerx.ist.psu.edu%2Fviewdoc%2Fdownload%3Fdoi%3D10.1.1.140.8040%26rep%3Drep1%26type%3Dpdf&ei=9eTqTobCJcTj0QGI4oGpCQ&usg=AFQjCNFfbuVXt6kByTp_aLkiR4y3ly351Q ]
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Some info on how the J&S knock system works. - 
