Q4 is an all-wheel drive system which has been installed on Alfa Romeo cars since 1994. It improves the vehicle’s dynamics and off-road capabilities. It also increases its stability on slippery surfaces.
How Q4 works
At first, only full-time all-wheel-drive versions were produced, where the engine torque was transmitted to all four wheels at the same time. A Ferguson-type viscous coupling or a Torsen self-locking differential was responsible for torque distribution between the axles.
Later, versions with part-time AWD were released. Under normal driving conditions, the engine torque is supplied to the rear axle only, but if the rear wheels slip, the front axle is engaged and receives up to 50% of torque. An electronically-controlled multi-plate friction clutch is used for this.
Q4 features
| Drive type | Distinctive features of the differentials | Alfa Romeo models equipped with this drive type |
| Full-time AWD | The centre differential is a planetary gear with a viscous coupling. The front differential is of open type, the rear one is a self-locking Torsen differential. | 155 167. |
| A Torsen central differential distributes torque between the axles in a 57:43 ratio. In case of wheel slip, this ratio can change from 72:28 to 22:78, providing more power to the wheels with better traction. | 159 939; Brera 939; Spider 939. | |
| The centre differential is equipped with a viscous coupling and electronic lockup. The rear differential is self-locking. | 164. | |
| Part-time AWD | A Torsen self-locking or an electronically-controlled open differential can be used on the rear axle. The active transfer case (ATC) is characterised by its quick actuation and connects the front axle immediately. | Giulia 952; Stelvio 949. |
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