thanks to FUJI, I found this in the manual in fuel injection section 2-7 p13
Control Of Ignition Timing
(Australia TURBO Only)
The turbochargers functioning causes the compressing pressure to increase in the vicinity of full load, resulting in the knocking being prone to occur.. To prevent the intensive knocking which may damage the engine, the knocking control system is designed for detecting its occurence and performing automatic retardation of ignition timing.
Furthermore, the distributor of * positive pressure retard type is adopted which performs the timing retardation against an excessive supercharging pressure from the turbocharger.Thereby optimum ignition timing is maintained to enhance the power and the fuel efficiency as well.
* Good authority told me IN PLAIN ENGLISH
this dizzies vac advance unit can retard beyond normal vacuum advance rest or neutral position on BOOST. clever dicks weren't they ? Guess we dub it the vacuum advance/ boost retard cannister
Knocking Control System
(Australia TURBO Only)
The knocking control system consists of a knock sensor, knock control unit, and distributor with built-in IC igniter. It optimises the ignition timing so that the engine may develop the maximum output with minmal fuel consumption but without causing a knock. When a knock occurs in the engine, the knock sensor detects its vibration and sends a signal to the knock control unit which determines the intencity and sends a timing retard signal to the IC igniter in the distributor. Thus, the ignition timing is retarded at the moment just before the knocking occurs.
KNOCK CONTROL UNIT
The KCU processes the knocking signalsent from the knock sensor and transmits retarding signals to the HIC igniter built into the distributor. On occurences of the knocking, the momentary retardationis performed. If there appears no knocking, the advance is perforemed. If the knockingis encountered in the course of advance, the retardation is made as in the former case
KNOCKING OCCURS > RETARD >KNOCKING ILLIMINATED > ADVANCE GRADUALLY > KNOCKING OCCURS and so lfe goes
In this fashion occurence of the knocking is supervised at all times to provide the optimum ignition timing against any changein the driving condition, gasoline characteristics (OOPS) and environment