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      12-25-2008, 09:28 PM   #8
Got Track!!!!!!
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Drives: 08 M3 coupe
Join Date: Aug 2008
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Originally Posted by msile View Post
No problems at all.

From ZeroG from another forum:

The ECU has a set fuel map or look up table for closed loop operation. It also has one for open loop operation. The ECU stores to trims STFT (Short Term Fuel Trim), and LTFT (Long Term Fuel Trim).

Basically it uses the feedback of the multiple O2 sensors in the exhaust track to try and maintain 14.7:1 AFR or Stoich. So the if the car is running either rich or lean it will send an offset or trim to the cars fuel map to get the car closer to 14.7. When reading the trims with an OBD Scan tool the car wants to be at 1.00 anything positive means that the ECU is adding fuel anything negative means the ECU is subtracting fuel. You want the car as close to 1.00 as possible meaning that there are no trims.

For example you are cruising down the highway and today it is abnormally cold and the car is running rich. The ECU will add an offset in the STFT (Short Term) to subtract some fuel and make the car 14.7. If lets say you actually moved to Alaska where it is cold all the time, the STFT will eventually become and LTFT and will finally replace a value in the fuel look up table with a new one. So in the long run the car is always learning and there is no way to just erase the fuel memory and start over. You can disconnect the battery cables for a period of time to wipe out the trims, but if a value has been replaced in the table then that is that. It will not change until the ECU thinks that is has found a better value for that cell in the table.

So what does that mean to you?

Well if you're just cruising around town nothing. It is at WOT (Wide Open Throttle) where the effect is most seen. So if you have say added larger fuel injectors. Without reprogramming the ECU the car will trim out the excess fuel that is being received by the injectors. So when you go WOT, and the car is no longer running in closed loop the trims still apply and the appropriate amount of fuel will be removed across the powerband.

I really hope that made some sense...

As far as knock and timing go to my knowledge they are not effected by fuel trimming they are basically determined by the amount of load placed on the engine. Under low load conditions timing can be advanced under high low it can be retarded. You can kind of manipulate the timing using a piggyback MAF scaler if you want to go down that route. Knock sensors are a safety mechanism as they sense knock and respond drastically not by retarding one or two degrees, but more like 20 just to be safe.
I dont know who you are but thanks! I have been trying to find out if you can reprogram the ecu to a target A/f, with the addition of using a MAF scaler to adjust timing.. Humm??