Its something I picked up from Modshack in the states, makes perfect sense, to quote:
Increasing the inner diameter by only .15 of an inch increases the surface area by more than 11%.
When you increase the diameter, the velocity of the air running through it is reduced (even though you're getting more air volume.)
Since the ECU takes it's signals for fueling partially from the MAF sensor, you end up with a lower signal being interpreted by the ECU. ECU sees a lower signal, assumes less air, and reduces fuel.
The reduction in signal from the MAF is directly proportional to the increased surface area of the new diameter MAF housing. This is where the problem arises, but it is correctable.
The ECU will adapt to condidtions by taking downstream readings from the O2 sensors, but only in a very small range.
Increasing the airflow by 11% and reducing the Fuel 11% can only be compensated for for so long, then a lean code is thrown (Multiplicative adaptation exceeds +20%.)
This is where the 4 bar comes in... It squirts more fuel, the O2 sensors adapt to this new condition, and the Adaptation maps come into line somewhere around -4% (rich) which is about where you want them (0% being no correction).
Not tested it myself yet but I will be!
