| This write-up serves to explain the knock control system employed in our beloved Z32s and to outline the differences between the 8-bit NA and TT versions. Both the NA and TT implement the same knock control system in the ECU hardware and software, but are tuned to behave slightly differently.
Knock Detection
The ECU listens to the knock sensor for an interval of each cylinder's combustion stroke. Knock is detected when a sharp spike in knock frequencies is heard over the background noise. If knock is detected, the ignition timing is immediately retarded by an amount relative to the number of knocking cylinders per engine cycle. Conversely, when knock is not detected, the ECU auto-advances the timing every 100ms up to a set limit. This basic behaviour maintains the engine close to its optimal ignition timing angle, just shy of the point at which knock occurs.
To determine the allowable 'spikes' in knock frequencies for a given fuel octane, the ECU contains knock threshold tables, one table per cylinder for both high (98RON) and low (91RON) octane fuels. The TT tables indicate that knock detection is available between 1200-4000rpm in low octane mode, and 1200-4400rpm in high octane mode. Knock detection occurs for all six cylinders.
 The NA tables are noticably different - the ECU only uses certain cylinders at certain RPMs for knock detection
 Ignition Maps and Knock Regions The ECU contains ignition timing maps for both high and low octane fuels. These maps define base ignition timing values used for the entire operating range of RPM and Engine Load. The ignition maps are logically divided into four regions as used by the knock control system. At low engine loads knock control is not required and it is completely disabled (white parts of the maps in the screenshots). At higher engine loads where knock control is required, the ignition map is split in three distinct knock control zones: regions A (yellow), B (green), and C (red). When operating within one of these three knock control regions, the ignition timing is further adjusted by a knock timing value calculated based on engine knock within that region. Regions A/B (yellow/green) Regions A and B are used at low and medium RPM where knock sensor feedback is available. The ECU maintains an average knock timing value for each region, which is updated once per second while the engine remains operating within that region. The ECU uses region B exclusively for octane mode switching by comparing the region B knock timing value to upper and lower mode-switch limits defined in ROM. If in high-octane mode, and region B timing hits -8 degrees, the ECU will switch to low-octane mode. If in low-octane mode and region B timing hits +10 degrees, the ECU will switch to high-octane mode. Region C (red) Region C is used for high-rpm operation, where engine noise and vibration are too high for accurate knock sensor readings, but knock control is still required. Region C is calculated as the weighted sum of regions A and B, the weighting values being defined by ROM constants. TT high octane ignition map
 NA high octane ignition map
 TT Knock system constants
 NA Knock system constants
Octane Modes and the factory knock control "bug"
The OEM ECU is designed to handle japanese market regular (91RON) and super (98RON) fuels, and contains logic to switch between high and low-octane operating modes based upon detected knock. When the vehicle is started, the ECU defaults to high-octane mode, and switches to low-octane mode after detecting a certain amount of knock over time. This same system is capable of promoting back to high-octane mode when zero knock is detected over time, however, this feature is disabled from the factory.
There is a bit of a catch with how this system works. I am hesitant to call it a bug because it is actually designed to work this way. Knock detection is octane dependent, but the ECU can't tell the difference between knock caused by low fuel octane and knock caused for other reasons, such as weather conditions or heat soak. If you are running high-octane fuel and get enough knock to switch to low-octane mode, the ECU starts using low-octane knock threshold tables, which are more sensitive than high-octane tables. This results in overly retarded timing when you run high octane fuel in low octane mode. Apart from reduced performance from running sub-optimal timing, overly retarded timing can result in elevated EGTs which have a detrimental effect on engine longevity. This also explains why the ECU starts in high-octane mode and drops to low-octane as needed, rather than the other way around. (it does seem a bit counter-intuitive that starting in high-octane mode can actually be safer for the engine in the long run!)
Knock sensor failsafe mode
If the ECU detects a problem with the knock sensor circuit (i.e. code 34), it switches the knock control system into a failsafe mode. This causes the system to go full retard and permanently run at -8 degrees of knock timing which is the proper, safe thing to do. Fooling the system and bypassing the knock sensor with a resistor stops the ECU from going into this failsafe mode with potentially disastrous consequences.
Issues with knock control in aftermarket ECU upgrades
Ok, here's the kicker. What if I told you that many aftermarket ECU upgrades actually contain serious errors in how the knock control system is tuned. Specifically, the knock control system in these chips isn't properly re-tuned for upgraded injectors. To maintain high-rpm knock control when the injector size is changed, knock control Region C *must* be re-scaled along with the other commonly modified parameters in the ECU, and it is obvious that many tuners don't even know this exists, let alone how to properly tune it. Region C is used for high-rpm knock control, and without it you run the risk of knocking and getting some serious engine damage any time you go above 3200rpm! Personally, I would be very concerned if my car's knock-control system was broken like it is in these ECU upgrades, and so should you! Region C (the red zone) in this aftermarket 740cc tune is practically non-existent compared to the stock TT map above. This car is effectively missing out on all knock control from 3200rpm up to redline... SCARY! Do you really think this motor is going to last 200,000miles before needing a rebuild??
Stay tuned for a post in the General forum regarding availability of KnockSpy Beta, so you can find out for yourself what your ECU is or (more disturbingly) is *not* doing about knock control.
Sam
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Z32 Knock Control - Do you know your A,B,C's? - 
