Technical talk - Semi-automatic gearboxes.

The Subaru Impreza WRC2004 is fitted with a semi-automatic six-speed gearbox that represents the latest in rally technology. As Subaru team principal David Lapworth explains, gearboxes have evolved considerably since 1993 when the Impreza made its WRC debut...

"Ten years ago, F1 and World Rally cars used a gearbox known as a 'dog box' that was based on a non-synchromesh, manual-change transmission with lightweight spur gears. In contrast to a typical road car gearbox, which is designed for ease of use and smooth operation, the emphasis with the dog box was on minimum power loss and minimum weight, but they required a considerable degree of driver skill to be used effectively.

"The semi-automatic gearbox was designed to combine the mechanical benefits of a lightweight racing gearbox with an electronic / hydraulic control system to make shifting simpler and more effective. The on-board computer systems combine data from the engine, transmission and gear lever to control the throttle, clutch and gearshift during each change. The driver stays on the throttle during the shift and merely pulls the lever to go up a gear or pushes it to go down, the clutch pedal is not needed except for pulling away from rest.

"By making the unit electronically controlled, we can ensure the driver makes the perfect shift, time after time. The system eliminates driver error during gearshifts, which also extends the life of the gearbox and engine. As a result the semi-automatic gearbox is lightweight with a very fast, smooth gearshift.

"Shift time for a typical road car gearbox: 0.5 -1.0 secs.

"Shift time for a typical manual racing gearbox: 0.1 - 0.2 secs.

"Shift time for a Subaru Impreza WRC2004 gearbox: Less than 0.05 secs."