With drivers making an average of 3,500 gear changes on a typical WRC rally, the accuracy and speed with which the transmission system selects different ratios is absolutely vital to achieving a quick stage time. As engineers at the Subaru team HQ in England continually strive to make the car quicker and more efficient, a huge amount of effort is channelled into the design and operation of the transmission system found under the WRC car.

Unlike the Impreza road car with its standard gearbox and differentials, the Subaru Impreza WRC2004 has a six-speed, semi-automatic transmission system with electro-pneumatically 'active' differentials at the back, front and centre. The gearbox uses fly-by-wire technology so that Petter or Mikko can shift gear using a paddle located behind the steering wheel. Pulling it away from the dash moves the 'box up a gear, while pushing it away selects a lower ratio. The transmission system allows the WRC driver to change gear in 40 milliseconds, 10 times faster than on a road car.

Weighing in at 85 kilograms, some 10-15 kilograms lighter than its road car equivalent, the components in the Subaru Impreza WRC2004 gearbox are stronger, lighter and more durable to withstand the rigors of the harsh environments found in the World Rally Championship.

A team of five specialist technicians build, on average, 10 completely new gearboxes a year to maintain an operating stock of 16 units. These are used in the team's two rally cars on a rotation basis, with a complete re-build after each event (typically 500km). As Paul Roberts, transmission supervisor told us "Assembling a Subaru WRC gearbox is a highly skilled job that will take a technician approximately 80 hours to complete. There are 350 individual components in each gearbox, but many more in the whole transmission system. Taking parts and labour into account, each unit costs us around £75,000."

On event the team take two gearboxes per car and around £10,000 worth of spare parts for each unit. With no spare time to investigate any gearbox faults during a rally, at the first sign of trouble technicians will switch to the spare unit. WRC regulations permit unlimited gearbox swaps during each event, but as the units are sealed by scrutineers, repairs to the spare unit are limited to external components. For this reason many of the electronic and hydraulic components are externally mounted, to allow swift access.

With an average of four highly skilled technicians involved in the process, it takes just 15 minutes to change the gearbox. Paul explains the procedure "First the sump-guard is removed and the drive shafts are disconnected. Once disconnected the five bolts connecting the gearbox to the engine are removed and the hydraulic couplings disconnected. Next the exhaust and electrical connections are disconnected, followed by a stabiliser bar and finally the mounting bolts. Once that's done the box just drops down, we slide another under and repeat the process in reverse."

After an event, the gearboxes are completely stripped, tested and the 40-hour rebuilding process begins. Paul explains, "Before going into the cars the gearboxes are rig tested for two hours and go through a vigorous filtering process that last over an hour. Each individual component is inspected and the oil is checked under a microscope to make sure there are no large particles that could damage the shift control system."