Impeza RS, the pipes before and after DO have an affect on the MAF's ability to measure the air correctly. If in the stock setup the air is comming off of a 90 degree bend, that is also "flattened" it will have a far different velocity profile through the MAF than one that is straight through. Many cars "learn" a new calibration by refrencing the O2 sensor, and others are simply close enough that it doesn't matter.