A control system for a variable displacement internal combustion engine determines effective flow cross sectional area as a predetermined function of a cross sectional area given against a throttle position. The effective flow cross sectional area is used to determine a volumetric airflow ratio. A control unit is connected with cylinder operator means for deactivating and reactivating at least some of engine cylinders and varying strokes in a cycle. The control unit modifies the predetermined function in response to the number of cylinders being activated and the number of strokes in a current cycle.