Ok, so the parts are fitted, so i persuaded mark to try some redneck tuning!
The AEB/AJL ECU doesnt have any boost pressure monitoring, it bases everything on the Airflow reading alone. Changing the AFM for a larger unit, means that for a given airflow, the velocity thru the sensor is lower, so the voltage outputted is lower and the ECU "sees" less air, as a result, the ECU will push up the duty cycle on the N75 to attempt to achieve what it thinks is the correct amount of air. Obviously the big problem then is having the fuel to match, so i did some maths to work it out.
First some research:
Google provided the following data:
5v on S4 AFM (73mm ID) is 330g/s
5v on RS4 AFM (83mm ID) is 430g/s
430/330 = 30% difference in Airflow
73mm ID = 41.8cm^2 (3.14 * (7.3/2)^2)
83mm ID = 54.1cm^2 (3.14 * (8.3/2)^2)
54.1/41.8 = 29.4% difference in Area
This shows that peak peak airflow is directly related to area.
Apply these findings to Marks engine:
New AFM = 73mm ID = 41.8cm^2
Old AFM = 62mm ID = 30.2cm^2
That gives us
41.8/30.2 = 38.4% difference in Area
So for any given airflow figure the ECU is seeing, the engines actually swallowing 38.4% more air...
This is shown my looking at the adaption values, after a few short drives with the 3" AFM fitted the ECU was showing adaption fault codes and lambda multiplicative adaption is jammed at 25%...
Next we look at fuel:
Stock AEB injectors are 210cc/min@3bar, however the AEB runs a 4 bar pressure regulator, giving an actual output of 240cc/min (3->4 bar gives +15%)
Mark has a set of C20LET injectors which are rated at 340cc/min at 3bar.
Some more maths:
340/240 = 41.6%, So the LET injectors at 3bar are flowing 41.6% more fuel than the AEB injectors at 4bar.
Looks close enough for lambda correction to handle, so mark swapped in the C20LET injectors, and fitted a 3 bar fuel regulator inplace of the 4 bar item.
I'll leave mark to post his findings, as he's driven the car, but suffice it to say lambda adaption is now down to a friendly 3%.
lol
lol :icon_thumright:
