A tumble system for the intake manifold of an internal-combustion engine is provided with a number of cylinders. The tumble system is provided with at least one throttle valve for varying the section of passage of air through an intake pipe, which connects a cylinder with the intake manifold. The throttle valve has a throttle butterfly mounted so as to rotate under the thrust of an actuator device equipped with at least one actuator element, which is made of a shape-memory material that generates a torque by modifying its own geometry following upon application of an external stress of a physical nature.