A ported-shroud 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 an axially elongated ring. In the open position, the radially outer surface of the ring is spaced from a tapering inner surface of the air inlet so that air can flow in an annular passage between the tapering surface and the ring, and the ring opens the ported shroud. In the closed position, the ring abuts the tapering surface to close off the annular passage and to close off the ported shroud. Movement of the inlet-adjustment mechanism from the open position to the closed position is effective to shift the compressor's surge line to lower flow rates.