A system for controlling the aerodynamic drag of a motor vehicle by causing detachment of the boundary layer at a predetermined area of a part of the motor-vehicle body, said system comprising at least one component of the structure of the motor vehicle constituting a separate element rigidly connected to said structure,
- wherein said component defines an outer wall of the motor vehicle,
- wherein said outer wall is made porous, with an orderly distribution of pores or holes, and
- wherein said outer porous wall closes externally a chamber obtained within said component of the structure of the motor vehicle in such a way that, when the motor vehicle is travelling, a first part of said pores or holes situated in an area of the aerodynamic field of the vehicle with higher relative pressure enables part of the flow of air that laps the vehicle to enter said chamber, whilst a part of said pores or holes situated in an area of the aerodynamic field of the vehicle with lower relative pressure functions as outlet for the flow of air from the aforesaid chamber towards the outside of the vehicle in such a way as to generate jets of air that cause detachment of the boundary layer,
- the position of said pores or holes on said outer porous wall situated in an area of the aerodynamic field of the vehicle with lower relative pressure being chosen so as to determine detachment of the boundary layer in a desired area.