An alternator of the present invention includes a housing, a stator assembly mounted stationary within the housing, and a rotor assembly mounted rotatably within the housing in functional engagement with the stator assembly. An air gap extends annularly around the alternator between the rotor assembly and the stator assembly. The rotor assembly has a first end and a second end, and the air gap has a non-uniform thickness that varies along the axial length of the rotor assembly between the first and second ends of the rotor assembly. This non-uniform thickness is designed to avoid interference between rotor outer surface and stator inner surface at high rotor spin speed, but minimize the averaged air gap thickness for high alternator performance.