Dog-box transmissions are about rapid shifts with minimal pause in power production— the exact opposite of how what you’d expect from a synchronized box. That’s fun on the track, but it’s typically going to necessitate the use of a clutch that will be a nightmare on the street. The combination of a dog box and a cerametallic clutch is fairly binary — it’s either engaged or disengaged with very little slipping between the two states. This is of particular concern for me because the driver, as opposed to GCU, is responsible for shifting from neutral into first or reverse. That doesn’t happen much on the track, but it’s constant on the street.
Carbon clutches are more slippable and I looked at one from Tilton, but it wouldn’t fit the bellhousing. After talking with Agile Automotive we decided to purchase a triple carbon/carbon clutch from RPS. Agile worked directly with RPS to develop their proven carbon/carbon clutch to work in this application. This involved tweaking the geometry to fit the ST6-M’s bellhousing and adding a shim between the piston on the slave cylinder and the throw-out bearing. Many carbon clutch packages utilize metal-on-metal friction surfaces on the flywheel, floater disc and pressure plate. The RPS package utilizes carbon on all of the friction surfaces (a F1 trickle-down technology), hence the carbon/carbon moniker. The package has many benefits for the street and track with the only downside being cost:
Greater than 1,200 ft/lbs torque capability
Smooth slippable engagement
Minimal pedal effort compared to cerametallic clutches of the same capacity
Massively improved lifespan over cerametallic clutches with the same number of discs
Improved reliability due to all friction surfaces being carbon eliminating glazing, warpage, and cracking common with other clutch types
Very light weight
Fully rebuildable
On pump gas my engine produces 860 ft/lbs at 2,000 RPMs and over a thousand at 4k RPMs, so when running E85 I’ll push the torque threshold.
The complete unit including the flywheel, pressure plate, clutch and mounting hardware weighs 26 lbs. All of the weight is in the flywheel which will make driving on the street more enjoyable. In comparison the flywheel for my Ricardo is 14.3 lbs and the pressure plate and clutch are 31.6 lbs so it’s >46 lbs when the bolts are added.
Going forward Agile will recommend this package for their SL-C and Aero builds for street, road racing and endurance racing. Plans for future development include a longer piston for the slave cylinder to eliminate the shim and have an ultra-light flywheel for pure race applications.