Forced ventilation for motor vehicles

The invention concerns a forced ventilation system (1) for equalizing the pressure between the passenger compartment and surroundings of a motor vehicle. The system comprises a hard plastics frame (2), delimiting an aperture (3), and a flat, elastomer ventilation flap (7) secured to the frame. A resilient restoring force counteracts the opening of the ventilation flap (7) from a closed position in which the ventilation flap (7) rests on the frame (2) and closes the aperture (3) therein. The ventilation flap (7) is injected onto the frame (2) from its free end and, in front of a region (8) in which it is contact with the frame (2) on its side facing the latter, comprises a groove (10) delimited by bevelled edges.