Impreza Group A Transmission
The Prodrive Impreza 555 Group A Rally car four wheel drive transmission is a Prodrive designed and built unit, manufactured by Hewland. The design is based on two units: a main unit comprising of the gearbox and gearbox case, transfer gear case, bellhousing, centre plate, front and centre differentials, centre diff housing and a separate remote rear differential unit.
The front diff is housed within the bellhousing and similar to the rear diff casing has side plates which allow access to the mechanical parts. The main unit and rear diff casings are both made from cast magnesium for lightness, despite this the weight of the main unit including the gear linkages and diff controls weighs in at approximately 80kg.
The main unit is a conventional H-pattern six speed gearbox using gear ratios set as per the current Homologation. A choice of four sets of transfer gears is available allowing for different gearing combinations for different events. Reverse and first through to sixth gear are fixed to the layshaft and the mainshaft gears are of dog clutch engagement type. These gears are straight cut and have no synchromesh so they can be changed at speed without using the clutch.
Within the main unit, an input shaft driven by a heavy duty AP Racing carbon / carbon triple plate clutch and splined to the layshaft of a longitudinal two shaft gearbox. At the rear of mainshaft is a pair of transfer gears, one of which powers the centre differential.
The centre diff works by dividing the torque between the front and rear diffs which is nominally 50 / 50 split. Passing out of the centre of the lower transfer gear is a quill shaft which powers the front diff and is splined to the front of the centre diff.
The rear diff is powered by the main steel or titanium two piece propshaft (dependent on use) which is bolted to and driven by the main unit. The rear diff is a plate type Limited Slip (LSD) type instead of a viscous coupling and is passive as opposed to front and centre diffs which are active. The casing has side plates which allow access to the mechanical parts and the drive to each wheel is by a steel driveshaft each with inner and outer C.V joints on each side thus powering each wheel independently.
All three diffs are plate type however, the faces of the plates in the front and centre diff are coated with a friction material which are then pressurised using hydraulic pistons controlled by electro-hydraulic servo valves. The hydraulic system operates at low pressure and a pump is run internally off the gearbox layshaft which then pressurises the gearbox oil, this is then cooled by an oil / water heat exchanger.
The active diffs are controlled by a Prodrive designed microprocessor based differential control unit that works by monitoring the wheel speed sensors and is interlinked with the engine and gearbox ecus. It can then communicate with the respective ecus and activate the necessary servo-valves according to the chosen programme. Information is collated from each of the ecus, updated 20 times per second and collated by the data acquisition system for future analysis by the teams.