The stock MAF can only measure flow up to 826Kg/h, this limit can be reached on a stock 850 with a 15g fairly easily. In order to maintain accurate fueling calculations past this rather low limit it is reccomended to upgrade the size of the MAF. Adding a larger MAF is a tricky task, it takes a lot of tuning and patience.
The ECU uses a lookup table to correlate MAF output voltage to airmass. This table is not available for tuning in the .xdf since it is a high/low byte table. It uses 16bit values broken into two 8bit tables. At the moment TunerPro does not accomodate such a table. Editing the values must be done manually in a hex editor. The high byte table is from EF2C-F02B, the low bytes at F02C-F12B. This table is also in the upper flash bank at 1EF2C-1F02B and 1F02C-1F12B.
As an alternative, the members have created a very simple but useful program that allows to EDIT the MAF Table contained inside the bin file. The latest and updated version can be downloaded here https://www.dropbox.com/s/40q1bfs009obszt/Motronic%204.4%20MAF%20Table%20Editor.exe
The easiest method to measure greater flow is to use the stock sensor in a larger housing.
I'have been comparing different Inner Diameter MAF housings with a stock Bosch sensor element installed, by mounting different ID MAF housing in front of the airbox with the stock MAF in it's normal position. That way I could establish the relationship between the voltage ouputs of several ID maf housings. It appeared that there is a linear relationship between housings of different diameters, meaning that the complete MAF table of a MAF housing with a bigger ID can be calculated by multiplying all the values of the stock MAF table by a single factor. For this the above mentioned MAF table editor comes in very handy! Just open the bin in the MAF editor and select the multiply function to get the new table. The factors needed are:
-2.5" ID 850/X70 : stock, no multiplier
-2.75" ID S90/960 : 1,34
-3.25" ID 540I : 1,94
-3.75" ID : 2,57
(use a comma instead of a dot as a decimal seperator in the MAF editor!!)
Depending on the regional settings in Windows, you might have to use a dot, not a comma
Our cars use the HFM2 Bosch sensor element. This sensor has been used in numerous other vehicles, some of which can measure higher flow. The high/low byte table can be grabbed from the vehicle matching the sensor. However, MAFs are sensitive to packaging considerations, therrfore, it is a difficult task to repurpose a MAF for another vehicle. Although, it is a good starting point.
The basic mehtodology of tuning the MAF requires a wideband. All fuel trims are set to 0 to eliminate narrowband feedback from skewing the values. The VE table is set completely to 1, a theoretical target of lambda 1.The actual AFR is then compared against the target AFR of 14.7. The difference between the two values is then used to adjust the MAF curve accordingly. This process must be done repeadtly until the deviation falls below 3%.
The following method is how the flow to voltage table was calculated for a 3.25" ID housing:
The curve for a 540i was used as the base value; the 540i M44 sensor shares the same 3.25" ID. This table was flashed as is and datalogged. The car was idled at operating temperature and the MAF airflow was averaged for a 2 minute time period. This value was compared to the airflow measured at idle using the stock MAF sensor (known flow rate). The differnence between the two was 41%. The entire 540i was multiplied by this increase of 41%.
Even though ID of the MAF in the 540i was the same as the sensor being tuned for there was a base difference of 41%, just from the difference in packaging!