Pro Mod and Pro Stock manual gearboxes
Pro Mod and Pro Stock drag racing classes still see manual transmissions used in competition, and they range from two-speed units on short, eighth-of-a-mile courses to five speeds on quarter-miles. In Pro Mod the use of manuals is optional; in Pro Stock they are mandatory. Given these cars’ prodigious power outputs, the transmissions they use are rather different from those found in other, less potent race machinery.
One of the most popular set-ups is a transmission using a planetary gear design, relying on a central ‘sun’ gear to provide a 1:1 direct-drive ratio once it has shifted from the launch ratio. To achieve underdrive in first gear, ‘planet’ gears rotate around the sun gear, with each set of sun and planet gears providing two speeds – a low ratio and a direct, 1:1 locked-up drive.
To create multiple speeds, the system is modular, with extra segments being attached together to provide up to five forward speeds. This layout means multiple underdrive ratios are available, as each segment of the system contains a direct and an underdriven ratio. This modular design can make life complicated though when it comes to creating a shift system, as each box requires a dedicated shift lever, but this can be simplified by adding a pneumatic shift system that removes the need for complex linkages.
In addition, automatic shift systems are available that use a control box and pneumatic system to select each gear automatically. The set-up of the system is exceptionally simple compared to many automatic-manual shifters found in race or production cars. It uses an rpm activation box that is controlled by interchangeable ‘chips’ that govern the shift point. To set a shift point for each gear the relevant chip is simply inserted into the box, and at a predetermined rpm the activation box triggers a shift. The system is not particularly smooth, but when running a 6.25 s, it doesn’t need to be.
The internal action of these planetary boxes is such that when the transmission is in first gear the dedicated planet gear assembly provides the launch ratio. The planet gears are driven by an encompassing ring gear, and when the transmission is shifted a clutch pack compresses the rotating sun gear assembly to the direct-drive central sun gear, achieving a 1:1 ratio. Each case is shifted in succession, with its particular underdrive ratio determining the immediate drive ratio. Once all cases have been shifted then the entire transmission assembly is locked in the final 1:1 direct-drive ratio.
Another popular manual set-up uses a twin gear cluster arrangement as opposed to a planetary gear system. The idea behind this is to halve the load on the gear clusters, with each having to deal with only half of the input power from the engine. That means each gear has twice the number of effective teeth as a single-shaft set-up, with double the number of support bearings, increasing the torque capacity. Another benefit is that each rotating assembly can be made lighter than a similar single-shaft set-up, producing a lighter overall design. Once the power is applied on this system, each gearshift automatically overrides the previous gear, disengaging it and providing a near-seamless shift.
Both these transmission solutions are very different from those found anywhere else in racing, and highlight the lateral thinking needed to harness the power of engines capable of producing more than 2500 bhp.
Written by Lawrence Butcher