[Sean.Heihn]

Doesn't the MAS measure airflow by cooling rate of a heated wire suspended in the airflow? A WAI might decrease the cooling rate of the wire for the same airflow rate, so an ECU calibrated for a stock intake may think that means a decrease in airflow. And since a SG uses airflow to estimate FE, it might cause an increase in instrument MPG, while the exhaust O2 sensor keeps the FAR near stoich meaning real MPG never changes.

Edit: Hot wire MAS can still accurately measure air mass, not volume, despite changes in temp.