an apparatus for pneumatic control of supercharging pressure in a turbocompressed engine , characterized by the fact that the spring of a pneumatic actuator opens a dump valve , and that a collection pipe receives the pressure in the intake pipe above the butterfly valve and below the compressor and transmits the pressure to two parallel tubes , one leading to the closing chamber of the actuator , and the other leading through a throttling constriction to the opening chamber of this actuator , which chamber communicates through a tube and through a pressure valve with the atmosphere , while a third tube supplies the control chamber of the pressure valve .