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Old 01-23-2003, 02:18 PM   #62
Jon [in CT]
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Member#: 2992
Join Date: Nov 2000
Chapter/Region: NESIC
Location: Connecticut, USA
02 WRX Sedan


Originally posted by Vishnu Performance
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 "vishnu reset" simply keeps in 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.

Thanks for replying. I urge you to print out and read the following US patents, all filed by the same two guys at FHI at around the same time and all titled Method for controlling ignition timing of an internal combustion engine:
The first one lays out the basic strategy while the other two add embelishments to that basic strategy. These patents might alter your interpretation of the ignition and correction maps which, in turn might alter your tuning strategy.

Here are the essentials.

There is a base ignition timing table. Each entry (IGB) in that table represents a basic ignition timing advance and is indexed by load and RPM. The ECU will never retard ignition advance for a load/rpm point below the IGB value. Each value in this table represents the timing advance that produces maximum safe torque (i.e. without knock) when operating on the lowest supported octane fuel. I believe this table corresponds to what you call the "ignition timing table" and might explain why it seems so conservative -- FHI might have devloped this table using 87 octane gas.

There is a maximum advance table. Each entry (MBT) in that table represents a maximum value that can added to the basic ignition timing advance and is also indexed by load and RPM. The ECU will never add, for any load/RPM point, more than the MBT value to the basic ignition (IGB). Each value in this table represents, when added to its corresponding base table entry, the timing advance that produces maximum safe torque (i.e. without knock) when operating on the highest suppported octane fuel. I believe this table corresponds to what you call the "knock correction table" and the reason that its entries are zero in the low-load and low RPM regions is that the low octane fuel can successfully support the ideal (i.e. maximum brake torque) timing in those regions without knock and increasing octane in those regions won't change the ideal timing. And you mentioned that the "knock correction table" entries had the largest values in the high-load 5K RPM area. This is likely the area most prone to knock and therefore the area that benfits most from higher octane fuel.

I believe the "Ignition Advance Multiplier" which you mentioned in your first post in this thread is used to determine what the patents call a "rough" (i.e. coarse) correcting quantity. Think of the IAM as an indicator of percentage of the MBT entry to add to the IGB entry, with its lowest value meaning "don't add any advance to the base table entry and its highest value meaning "add 100 percent of the maximum advance to the base table entry." This concept is described in US patent 5,000,149.

Not mentioned in your description is what the patents call the "fine correction" table. It's a table stored in volatile memory (i.e. it's reset when the ECU is reset) with the same dimensions as the base ignition timing table and the maximum advance table. It contains advance corrections for each load point, to fine tune the advance after the coarse correction has been applied.
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