It's all about SCALING.

No one puts it better than williaty did on his post on MAF scaling on nasioc.com:

"One of the most common mods people do to our cars is to change the intake. One of the most common results of changing an intake is to change the relationship between the amount of air flowing past the MAF sensor and the total amount of air entering the system. Without adjusting the MAF scaling to reflect the changes to the intake, the ECU will constantly mis-calculate the amount of air entering the engine. This will affect nearly everything the ECU does from fueling, to timing, to the Closed-Loop to Open-Loop Transition. Re-scaling the MAF is a process of altering the programmed relationship between MAF sensor voltage and the grams per second of air entering the engine." (see the full post at: http://forums.nasioc.com/forums/showthread.php?t=1427448)

Fuel injector scaling works in the same way. When one swaps the stock fuel injectors for another set, the injector scale table must also be re-calibrated.

If you have read my earlier post, I've changed both my fuel injectors (to STi 550cc) and intake (to APS 70mm CAI). So both my MAF and injector tables needs to be re-calibrated.

Actually changing both at the same time is not the right way to do it as you can NEVER scale both MAF and Injectors at the SAME time (both tables affect each other). The proper way to do it is:

1) Install new injectors with a factory intake. Scale injector size to minimize fuel trims. Check your fuel table against logs and see how accurate it is. It should be close even in open loop and really high load with the stock intake.

2) Install aftermarket intake. Scale the whole MAF scaling table.

In my case, I'm lucky coz the ecu which I have got (I swapped my ecutek enabled stock WRX ecu) is an STi 03 WRX (see screenshot below from romraider):


The injector flow table is already set to its correct value (see screenshot of injector flow table from romraider):






I'm assuming Subaru's calibration is correct (it should be since the engineers in the factory have spent so many hours calibrating it) so, the only scaling I need to do is my MAF Scaling. I'm glad it has turned out this way or else it would have been difficult to do both at the same time. :-)

I hope to talk more about the different methods of MAF scaling I've tried on my next post.