The Impreza WRC2007’s drivetrain in numbers
11 March 2008
The drivetrain of an Impreza WRC2007 - gearbox, shafts and differentials
Where far more than expected meets the eye, we look at the complexities of the gearboxes and differentials used in the Impreza WRC2007, and what goes into making them work:
The Subaru World Rally Team has produced over 100 WRC gearboxes since 1999. Almost 50 WRC differentials have been built since 2004.
Each gearbox takes 85 hours to build and costs £75,000. Coupled with the rear differential the transmission system contains over 700 individual components.
Each differential takes 16 hours to build, and costs £20,000. The front and rear differentials share components to reduce costs, whereas the centre differential is unique.
Once a gearbox has been built, it takes three hours for the team’s dedicated dyno to run through the complete test procedure before it is used on a rally.
There are two special insulation panels fitted to the side of the gearbox to shield it from the heat of the exhaust which runs alongside it. Each plate is made from titanium and ceramic coated, using a process designed for space craft.
The engine, gearbox and differential are all individually oil sealed, meaning that each can be removed and replaced without fluid loss.
The gearbox lubrication system has a capacity of 4.5l and is fitted with a high-flow oil pump to keep temperatures down. The gear shift system operates at a pressure of 150bar, or 2,500psi for almost instantaneous shifts.
Each unit uses over 40 specialised oil line connectors. Approximately half of these are built specifically for the Subaru World Rally Team, and all use aircraft grade materials.
The gearbox retains the road car’s H pattern but uses a hydraulically and electronically controlled semi-automatic gear change system to shift each gear in fewer than 0.1seconds.
Undergoing 3500 gear shifts on every rally, each unit is rebuilt after approximately 750km of competitive use or two rallies. The oil is changed for every rally.
The gearbox and differential run at 100 degrees Celsius, and need their own oil coolers to maintain that temperature. On hot rallies such as Greece, the operating temperatures can reach 120 degrees Celsius.
It takes between 10 – 12 minutes to replace a gearbox and rear differential. The fastest time on a rally was 10mins and 20 seconds set during Rally Deutschland 2007.
The housings of both units are cast from magnesium for strength and to reduce weight. A gearbox weights 95kg when wet (full of oil) and a differential 25kg wet.
