A turbocharger, comprising: a turbine housing and a turbine wheel mounted in the turbine housing and connected to a rotatable shaft for rotation therewith, the turbine housing receiving exhaust gas and supplying the exhaust gas to the turbine wheel; a centrifugal compressor assembly comprising a compressor housing and a compressor wheel mounted in the compressor housing and connected to the rotatable shaft for rotation therewith, the compressor housing defining an air inlet for leading air generally axially into the compressor wheel, the compressor housing further defining a volute for receiving compressed air discharged generally radially outwardly from the compressor wheel, the air inlet having an inner surface a portion of which forms a sliding surface that extends for an axial length along a downstream axial direction, the sliding surface being followed by a tapering surface that extends for an axial length along the downstream axial direction, the tapering surface becoming smaller in diameter in the downstream axial direction, wherein the compressor housing defines a flow recirculation system comprising a bleed port located adjacent the inducer portion of the compressor wheel for allowing air to pass therethrough, a recirculation passage connected with the bleed port and extending upstream from the bleed port, and an injection port leading from the recirculation passage into the air inlet upstream from the inducer portion, whereby air may flow through the recirculation system in either direction between the air inlet and the inducer portion; and a compressor inlet-adjustment mechanism disposed in the air inlet of the compressor housing and movable between an open position and a closed position, the inlet-adjustment mechanism comprising an axially elongated ring, the ring being arranged so that when the inlet-adjustment mechanism is in the open position the ring is spaced from the tapering surface of the air inlet such that there is an annular passage between the tapering surface and the ring for a flow of air therethrough and the ring opens the injection port to allow air flow through the recirculation system, and when the inlet-adjustment mechanism is in the closed position the ring abuts the tapering surface to eliminate said annular passage and the ring closes the injection port and prevents air flow through the recirculation system;. wherein the ring in the closed position extends in the downstream direction substantially to an inducer portion of the compressor wheel such that an effective diameter of the air inlet at the inducer portion is determined by an inner diameter of the ring; and characterized in that: the ring comprises a tubular wall, the tubular wall at a trailing edge thereof defining an inside diameter d r that is between 0.45 and 0.98 times d 1S, where d 1S is a diameter of the inducer portion of the compressor wheel. a turbine housing and a turbine wheel mounted in the turbine housing and connected to a rotatable shaft for rotation therewith, the turbine housing receiving exhaust gas and supplying the exhaust gas to the turbine wheel; a centrifugal compressor assembly comprising a compressor housing and a compressor wheel mounted in the compressor housing and connected to the rotatable shaft for rotation therewith, the compressor housing defining an air inlet for leading air generally axially into the compressor wheel, the compressor housing further defining a volute for receiving compressed air discharged generally radially outwardly from the compressor wheel, the air inlet having an inner surface a portion of which forms a sliding surface that extends for an axial length along a downstream axial direction, the sliding surface being followed by a tapering surface that extends for an axial length along the downstream axial direction, the tapering surface becoming smaller in diameter in the downstream axial direction, wherein the compressor housing defines a flow recirculation system comprising a bleed port located adjacent the inducer portion of the compressor wheel for allowing air to pass therethrough, a recirculation passage connected with the bleed port and extending upstream from the bleed port, and an injection port leading from the recirculation passage into the air inlet upstream from the inducer portion, whereby air may flow through the recirculation system in either direction between the air inlet and the inducer portion; and a compressor inlet-adjustment mechanism disposed in the air inlet of the compressor housing and movable between an open position and a closed position, the inlet-adjustment mechanism comprising an axially elongated ring, the ring being arranged so that when the inlet-adjustment mechanism is in the open position the ring is spaced from the tapering surface of the air inlet such that there is an annular passage between the tapering surface and the ring for a flow of air therethrough and the ring opens the injection port to allow air flow through the recirculation system, and when the inlet-adjustment mechanism is in the closed position the ring abuts the tapering surface to eliminate said annular passage and the ring closes the injection port and prevents air flow through the recirculation system;.