The trick is to disconnect the battery and either press the brake pedal to completely drain any charge in the system or leave over night before reconnecting. THEN you go out and drive the car in the manner you want the ECU to work with. Which for most of us means thrashing the nuts off it! Also make sure you are running whatever fuel you plan to run the most - Optimax is probably the one everyone would most recommend.
I did it with my R33 and it made a small but noticable difference.
You can reset the ecu by disconnecting the battery and then pressing the brake pedal in for a minute or so, doing this tries to light up the brake lights and removes all of the voltage from the ecu, then connect it back up and go for a drive. Make sure the front wheels are straight when you do this or else the hicass can get a bit screwed up because it thinks the wheels are turning slightly.
The ECU has the mapping for the fuelling this wont change but the timing and airflow is monitored all of the time and adjusted according to conditions if you have been using low octane fuel or carried out recent mods then you'll benifit from the ECU reset as it relearns the optimum settings and changes the timing in very large increments initially then as it gets closer to being right it narrows the adjustments down.
The ignition timing is advanced until knock is detected and then pulled back in narrowing swings above and below ideal until it has narrowed it down to being ok. Once settled it almost stops learning and will use tiny adjustments each time you go out and take a lot longer to get to its ideal each time.
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