| The stock system is calibrated to detect knock at a max RPM of 4400 in high-octane mode (4000rpm in low octane). The thresholds for knock are stored in 16 entry tables (i.e. one entry per 400rpm segment of the powerband) with one table for each cylinder. Bare in mind that these tables were calibrated from the factory using cylinder pressure sensors on an engine dyno, and ideally would require the same to properly re-calibrate. The OEM system uses analog circuitry to rectify, band-pass filter and integrate (average) the knock sensor signal over an interval of the combustion stroke, so no digital sampling is involved. More modern knock systems do use DSPs which discretely sample and are far cheaper to implement. Its a bit like comparing recording audio to cassette tape vs. CD, same thing really! For each engine revolution the ECU reads this integrated knock signal for each cylinder and checks it against the knock threshold level tables and a background noise level that it maintains. If a spike is found over the background noise, then knock has been detected and the timing is retarded. The hardware itself is entirely capable of checking for knock at 6000rpm, but Nissan determined through testing that 4400rpm was the max reliable RPM for detection before noise became an issue. For anything over 4400rpm it uses a feed-forward calculation to adjust timing, so you still do have decent knock protection at higher RPM assuming your ignition tables are well balanced. I am curious about what the max RPM that the J&S unit can reliably detect knock at, because at some point engine noise is going to become a problem for any knock-sensor based detection system.
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