A centrifugal compressor for a turbocharger includes an inlet-adjustment mechanism in an air inlet for the compressor, operable to move between an open position and a closed position in the air inlet. The inlet-adjustment mechanism includes a variable-geometry orifice actuated by a rotatable actuator ring. In the closed position, the orifice's inner diameter is smaller than that of a shroud surface of the compressor housing and therefore dictates the effective inlet diameter for the compressor. In the open position the orifice no longer forms an inner diameter smaller than the shroud surface, so that the effective inlet diameter is determined by the shroud surface.