| "When the Z was new, the technology for listening to knock was not very good. Other noises in the engine sounded like knock, so Nissan did not allow the ECU to listen to knock after 3000 RPM. It was more or less a bad gas sensor for tip in/ low load knock." I'm curious on what information you are basing these statements. Is this something you've learned from nissan engineers directly, or something you've discovered from your own/others tuning experience? My own research into the OEM system (from analyzing disassembled ECU microcontroller code and data logging knock system variables), seems to indicate that it is actually quite sophisticated, well scienced, and capable of a lot more than what the currently available OEM ECU tuning tools allow you to tune/monitor.
For one, the OEM system detects knock on a per-cylinder basis to compensate for differences due to knock sensor placement (i.e. it has separate knock limit ROM tables for each cylinder). The ECU analyzes each individual combustion event separately to determine if knock has occured. It also has a tunable auto-advance/retard system that retards timing when knock is detected, and auto-advances timing when knock is not detected (the OEM rate for this is 0.101 degrees of timing advance per 100ms of knock-free operation, it drops timing at a rate of 1.51 degrees per knock event). From the factory this auto-advance system is restricted to only recovering timing that has been previously dropped due to knock detection. This auto-advance can be re-tuned to allow advance up to the point of knock, or to a hard limit, whichever comes first. This system is coupled with a high/low octane mode switching system (aka safety boost), which drops to low-octane mode after a certain amount of knock is detected over time. The system is capable of elevating back to high-octane mode if knock isn't detected, but again, from the factory this functionality is disabled. Something else I have come across while analyzing the ECU code is that it appears that knock detection actually works up to 4400rpm in high octane mode, although the factory tune is limited to using knock feedback only up to 3200rpm. The system itself adjusts ignition timing all the way up to redline, even in factory tune, based on knock feedback. So you will still be getting timing pulled at 6000rpm based on knock that occurred at 3000rpm.
I am currently developing some PC software that allows you to monitor and log the OEM ignition timing and knock system over the diagnostics port. I have been keeping somewhat quiet about this since it is still pre-beta, but I figure now is as good a time as any to open this big ol' can of worms up ;)
Here's a short video of this software in action, showing the ECU detecting knock and dropping into low octane mode (aka safety boost) Knockspy safety boost demo video
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