Abstract: A device for varying the effective intake passage length of an internal combustion engine having a compact and simplified structure. A cylindrical rotor is rotatably mounted coaxially within a hollow cylindrical casing. A chamber formed within the rotor is communicated with the air filter of the engine, while a second chamber, defined between the inside of the casing and the outside of the rotor, is communicated with the intake chamber of the cylinders of the engine. A partition extends between the inner wall of the casing and the outer wall of the rotor. By turning the rotor, the effective air intake passage length is varied, thereby allowing the combustion efficiency to be maximized.