A turbocharger configured to receive an exhaust gas stream from an engine configured to operate over a range of standard operating conditions, and to compress input air into a pressurized air stream, comprising: a housing including a turbine housing; and a rotor configured to rotate within the housing along an axis of rotor rotation, the rotor including an axial turbine wheel, a compressor wheel, and a shaft extending along the axis of rotor rotation and connecting the turbine wheel to the compressor wheel; wherein the turbine wheel is configured with a hub, and with a plurality of axial turbine blades configured to drive the rotor in rotation around the axis of rotor rotation when the turbocharger receives the exhaust gas stream from the engine, the blades having an axially upstream edge, an axially downstream edge, a hub end, and a tip end opposite the hub end; wherein the compressor wheel is configured to compress the input air into the pressurized air stream when the rotor is driven in rotation around the axis of rotor rotation by the turbine wheel; and wherein the turbine housing forms an inwardly spiraling turbine primary-scroll passageway which has a primary-scroll inlet port which has a centroid that is radially external to the axially upstream ends of the blades; wherein the engine exhaust gas stream has gas-specific attributes including a specific gas constant R sp and a Boltzmann constant k, and the axially upstream edges of the turbine wheel blades define a turbine wheel inlet, the turbine wheel inlet has an area; characterized in that the primary-scroll inlet port is characterized by an area (ai); the combined turbine housing and turbine wheel are characterized by a primary-scroll radius ratio (r r) defined as a radius of the hub at the axially upstream edge of the blade, divided by a radius of the centroid of the primary-scroll inlet port; the combined turbine housing and turbine wheel are further characterized by a corrected mass flow rate surface density (Dcr) at the primary-scroll inlet port when driven at a critical expansion ratio (E cr); the primary-scroll radius ratio (r r) and the primary-scroll inlet port area (ai) are sized such that the corrected mass flow rate surface density (Dcr) at the primary-scroll inlet port when driven at the critical expansion ratio (E cr) is greater than a critical corrected mass flow rate surface density (D cr); and the values of D cr and E cr are determined by the equations D cr = r r 10132 s 288 R sp 1 − k − 1 r r 2 k + 1 1 k − 1 2 k k + 1 and E cr = k + 1 2 k k − 1. a housing including a turbine housing; and a rotor configured to rotate within the housing along an axis of rotor rotation, the rotor including an axial turbine wheel, a compressor wheel, and a shaft extending along the axis of rotor rotation and connecting the turbine wheel to the compressor wheel; wherein the turbine wheel is configured with a hub, and with a plurality of axial turbine blades configured to drive the rotor in rotation around the axis of rotor rotation when the turbocharger receives the exhaust gas stream from the engine, the blades having an axially upstream edge, an axially downstream edge, a hub end, and a tip end opposite the hub end; wherein the compressor wheel is configured to compress the input air into the pressurized air stream when the rotor is driven in rotation around the axis of rotor rotation by the turbine wheel; and wherein the turbine housing forms an inwardly spiraling turbine primary-scroll passageway which has a primary-scroll inlet port which has a centroid that is radially external to the axially upstream ends of the blades; wherein the engine exhaust gas stream has gas-specific attributes including a specific gas constant R sp and a Boltzmann constant k, and the axially upstream edges of the turbine wheel blades define a turbine wheel inlet, the turbine wheel inlet has an area; characterized in that the primary-scroll inlet port is characterized by an area (ai); the combined turbine housing and turbine wheel are characterized by a primary-scroll radius ratio (r r) defined as a radius of the hub at the axially upstream edge of the blade, divided by a radius of the centroid of the primary-scroll inlet port; the combined turbine housing and turbine wheel are further characterized by a corrected mass flow rate surface density (Dcr) at the primary-scroll inlet port when driven at a critical expansion ratio (E cr); the primary-scroll radius ratio (r r) and the primary-scroll inlet port area (ai) are sized such that the corrected mass flow rate surface density (Dcr) at the primary-scroll inlet port when driven at the critical expansion ratio (E cr) is greater than a critical corrected mass flow rate surface density (D cr); and the values of D cr and E cr are determined by the equations D cr = r r 10132 s 288 R sp 1 − k − 1 r r 2 k + 1 1 k − 1 2 k k + 1 and E cr = k + 1 2 k k − 1.