A brake booster assembly comprising: a rack moveable along a first axis, at least one sensor coupled with said rack for sensing force and axial displacement of said rack and outputting a signal proportional to the sensed force and the sensed displacement, a clutch subassembly disposed adjacent and coupled to said rack and including a drum and a hub in a spaced relationship with said drum, a motor disposed adjacent said rack and coupled to said clutch subassembly for rotating said drum, a magnetorheological fluid disposed between said drum and said hub, an electromagnet disposed about said clutch subassembly for generating an electromagnetic field to affect the viscosity of said magnetorheological fluid, and a controller electrically connected to said electromagnet and to said sensor and to said motor for controlling said motor and for controlling the electromagnetic field of said electromagnet, and thus controlling movement of said rack in response to the signal from said sensor, wherein said drum of said clutch subassembly extends along and is rotatable about a second axis transverse to said first axis and has an outside surface defining a plurality of drum teeth extending radially from said outside surface, wherein said motor includes an axle rotatable about and extending along a third axis generally parallel to the second axis and attached to a pinion gear having a plurality of pinion teeth meshed with said drum teeth for rotating said drum about said second axis, wherein said rack has a plurality of rack teeth extending from said rack and said hub of said clutch subassembly includes a sleeve having an exterior defining a plurality of hub teeth extending radially from said exterior and meshed with said rack teeth for moving said rack along said first axis in response to rotation of said clutch subassembly, wherein said drum teeth extend radially to a first diameter and said plurality of hub teeth extend radially to a second diameter being smaller than said first diameter for providing an increase in torque from said drum to said hub, and said drum defines an opening disposed centrally and extending therethrough along said second axis and said sleeve of said hub is annularly disposed about said second axis and extends through said hub and through said opening of said drum. a rack moveable along a first axis, at least one sensor coupled with said rack for sensing force and axial displacement of said rack and outputting a signal proportional to the sensed force and the sensed displacement, a clutch subassembly disposed adjacent and coupled to said rack and including a drum and a hub in a spaced relationship with said drum, a motor disposed adjacent said rack and coupled to said clutch subassembly for rotating said drum, a magnetorheological fluid disposed between said drum and said hub, an electromagnet disposed about said clutch subassembly for generating an electromagnetic field to affect the viscosity of said magnetorheological fluid, and a controller electrically connected to said electromagnet and to said sensor and to said motor for controlling said motor and for controlling the electromagnetic field of said electromagnet, and thus controlling movement of said rack in response to the signal from said sensor, wherein said drum of said clutch subassembly extends along and is rotatable about a second axis transverse to said first axis and has an outside surface defining a plurality of drum teeth extending radially from said outside surface, wherein said motor includes an axle rotatable about and extending along a third axis generally parallel to the second axis and attached to a pinion gear having a plurality of pinion teeth meshed with said drum teeth for rotating said drum about said second axis, wherein said rack has a plurality of rack teeth extending from said rack and said hub of said clutch subassembly includes a sleeve having an exterior defining a plurality of hub teeth extending radially from said exterior and meshed with said rack teeth for moving said rack along said first axis in response to rotation of said clutch subassembly, wherein said drum teeth extend radially to a first diameter and said plurality of hub teeth extend radially to a second diameter being smaller than said first diameter for providing an increase in torque from said drum to said hub, and said drum defines an opening disposed centrally and extending therethrough along said second axis and said sleeve of said hub is annularly disposed about said second axis and extends through said hub and through said opening of said drum.