WRC Drivetrain
The Subaru Impreza S13 Drivetrain
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Symmetrical All Wheel Drive drivetrain of Subaru Impreza 2007 |
The Subaru Impreza S13’s drive train in numbers
Where far more than expected meets the eye: the complexities of the gearboxes and differentials used in the Impreza WRC2007 and what goes into making them work are considerable. The Subaru World Rally Team produced over 100 WRC gearboxes since 1999 and almost 50 WRC differentials since 2004.
Gearbox Construction
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Subaru Impreza WRC sequential gearbox |
Each gearbox takes 85 hours to build and costs $100,000. Coupled with the rear differential, the transmission system contains over 700 individual components. Each differential takes 16 hours to build and costs $30,000. The front and rear differentials share components to reduce costs, whereas the centre differential is unique.
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Subaru Impreza WRC Gearbox |
Testing and Operation
Once a gearbox has been built, it takes three hours for the team’s dedicated dynamometer to complete the full test procedure before the unit is used on a rally.
Two special insulation panels are fitted to the side of the gearbox to shield it from exhaust heat, which runs alongside it. Each plate is made from titanium and ceramic-coated using a process designed for spacecraft.
The engine, gearbox, and differential are all individually oil-sealed, meaning each can be removed and replaced without fluid loss.
The gearbox lubrication system has a capacity of 4.5 litres and is fitted with a high-flow oil pump to keep temperatures down. The gear shift system operates at a pressure of 150 bar (2,500 psi) for near-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.
Gear Shifting
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Subaru Impreza WRC Gearbox with differential |
The gear shifter retains the road car’s H-pattern layout but uses a hydraulically and electronically controlled semi-automatic gear change system to shift each gear in fewer than 0.1 seconds.
Durability and Maintenance
Undergoing 3,500 gear shifts per rally, each unit is rebuilt after approximately 750 km of competitive use or two rallies. The oil is changed for every rally.
The gearbox and differential run at 100 degrees Celsius and require their own oil coolers to maintain that temperature. On hot rallies such as Rally Greece, operating temperatures can reach 120 degrees Celsius.
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Subaru Impreza WRX differential, STI spec C |
It takes between 10 and 12 minutes to replace a gearbox and rear differential. The fastest time on a rally was 10 minutes and 20 seconds, set during Rally Deutschland 2007.
The housings of both units are cast from magnesium for strength and reduced weight. A gearbox weighs 95 kg wet (full of oil) and a differential 25 kg wet.