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< ECU Reset | | Connecting to your ECU >


If you have just reset your ECU or reflashed it with a new map you may know that this also resets your ECU learning data. Below is a little trick I found on a Subaru forum that has been shown to accelerate the factory ECU's ignition timing learning process. This procedure is commonly known as the Vishnu Reset after the company, Vishnu Performance, that wrote this on a Subaru forum.


There is something called Ignition Advance Multiplier (IAM). It represents, by some complicated algorithm, the average learned positive knock correction applied to the ignition maps. It's represented in 1/8th degrees increments. 1 being the lowest and 16 being the highest (1/8 to 2 degrees in absolute terms).

The "happier" (knock free) the car is, the higher the number will be. Conversely, the lower it is, the more knock prone it is. This number, after ECU reset or ECU swap defaults to 8 and usually creeps up to 16 (if well mapped) through normal driving. Depending on driving characteristics, this can happen within a few hours or a few weeks. Well, here's a way to make it happen in about 5 seconds ....

Fast learning procedure (Newage WRX)

  1. With the car fully warmed up, reset the ECU.
  2. Drive to a nice open road without traffic. Don't go on boost until you get there. Put the car into gear (3rd gear works the best) bring the revs up to 2600rpm.
  3. Push the accelerator pedal down slightly so boost stays around 2-4psi (~0.2 bar). You will need to MAINTAIN 2500rpm and 2-5psi for approx 5 seconds. You can do this by left-foot braking gently as to prevent acceleration. During these few seconds, the advance multiplier (which you can't see so you'll have to trust me) will go from 8, to 12 and then to 16. Works like a charm. And on our reset or reflashed ECU, is worth an immediate 10-20 BHP.

Fast learning procedure (Newage STi)

The procedure is slightly different for STi's.

  1. First you have to reset the ECU.
  2. Then let the car idle until it is fully warmed up in closed loop fuel control.
  3. Then hold approx 4-5psi (0.3 bar) of boost at 3500rpm for 3-4 seconds. Again left-foot braking will help you maintain a constant speed. This usually results in a increase of 20 BHP. This is something that you would probably see eventually over the course of a few days of driving. But this takes a few seconds and is great for those who are dyno testing/tuning or are just really impatient :-)

Warning: This little trick artificially speeds up the learning process. This is only a good thing if the re-mapped ECU is mapped properly. If there are trouble-spots where knock is present, this trick will make it even more present. So be careful!

So how does this work?

  1. There is only one ignition timing table and one knock correction table.
  2. The load axis for the fuel, ignition and knock correction tables is MAF, not MAP.
  3. The ignition timing map is very conservative and relies on anywhere from 0-12 degrees of additional advance which is provided by the active knock correction system.
  4. The knock correction tables resembles the ignition timing table with respect to having MAF and RPM as its X and Y axis. However, in its cells is maximum knock correction authority, not absolute base timing value. At low loads (off boost), knock correction authority is 0, meaning that no additional advance is allowed to be added upon the base ignition table values. Under boost (and especially around 5000rpm, knock correction reaches its maximum authority range (or around 10-12 degrees depending on ECU type). I can only presume that the engine calibrators at FHI determined this RPM range to be particularly "trouble-prone", requiring the "go ahead" from the knock sensor to add in the full amount of timing (learning to advance timing instead of simply assuming that is okay to do so). Naturally, the underlying timing values in the ignition timing tables are unusually low at this rpm/load range.
  5. Knock correction authorty range tapers off as RPM and Load goes up (above 5500rpm). I suspect this is because knock sensors tend to become inaccurate at higher engine speeds (unable to differenentiate between actual knock and normal engine noise).
  6. The above fast learning procedure simply keeps the engine operating in a rpm/load zone where base timing is very conservative and knock correction authority range is relatively large. When the engine assumes this rpm/load, the active knock correction system readily adds in maximum positive authority range due to an absense of knock and (I suspect, engine noise). I could make this special reset not work by simply advancing base timing too aggressively in his rpm/load area. This would induce knock prematurely and never all the ECU to add in the allowable knock correction. Needless to say, I keep these rpm/load zones purposely detuned to facilitate this accelerated learning technique.

Information source - NASIOC, author - Vishnu Performance

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Page last modified on January 14, 2006, at 10:07 AM
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