An engine comprising an intake port which has a separating wall projecting downwardly from the upper wall of the intake port. The separating wall defines a helical portion, an inlet passage portion tangentially connected to the helical portion, and a bypass passage interconnecting the inlet passage portion and the helical portion. A rotary valve is arranged in the inlet portion of the bypass passage so that the axis of the rotary valve is substantially parallel to the axis of the stem of the intake valve. The rotary valve is actuated by a vacuum-operated diaphragm apparatus and is opened when the amount of air fed into the cylinder of the engine is increased beyond a predetermined value.