A control device for the throughput in a conduit portion a is provided. The device comprises a valve or a flap composed of a body mounted on a rotary support shaft and being displaceable between a position of maximum closure and a position of minimum closure by pivoting about said shaft. The device is characterised in that at least one shape or structure (4,4') which is at least partially obstructive, by projecting into the internal passage of said conduit portion (1) and locally reducing the passage section thereof, is present in the region of the internal face of the wall of said conduit portion. The device includes at least one cooperating surface zone (5) which intimately mates with the shape of at least one part of the imaginary surface swept by at least one segment of the external peripheral edge (6) of the body (2) of the valve or flap when pivoted.