Click Here to Begin Slideshow
Getting the right clutch setup for a street-strip car isn’t exactly easy. On one hand, you must be able to harness the power available today – and as anyone reading this knows, power is pretty easy to find. On the other hand, you have to be able to come up with a clutch you can actually use on the street where stop and go traffic, stop signs at the top of a hill and all sorts of other obstacles can enter the equation.
Yours truly was (more like “is”) plagued with that exact quandary. Here I have this relatively large displacement big block going together in a small tire street machine. I’d looked at every available option - and then I paused and looked again. Given the time spent on the research, I’ve come up with a good chunk of info and I’ll share it right here:
The truth is, you can run something like an adjustable Long Style clutch with a sintered metallic disc (there are at least a half dozen of these setups available today from various different manufacturers). With this setup, you have to turn up the clutch base pressure on the street and then climb under the car and back it down for drag duty. This is all done through an access port in the bellhousing. Fair enough, but the iron “rag” which is a necessary part of the equation will still make everything a dirty mess (including virtually everything inside the bellhousing) and to be honest, it’s still not all that streetable. Basically, clutches with sintered iron discs are probably the least friendly of all the options.
Yes, there a few conventional single disc clutches out there that might work, but for something in the 750 or so HP range (or more) with say, 650-700-foot pounds of torque, they’re pretty much on the ragged edge of reliability. Needless to say, there are plenty of combinations out there with far more grunt, so that eliminates a single conventional disc for those applications.
That leaves dual disc clutches. Not that long ago, the only vehicles with dual disc clutch assemblies were heavy-duty trucks and the odd Corvette. As OEM engine horsepower levels dropped in the late seventies and almost all the way through the eighties, clutch assemblies for street-strip cars were pretty much in a stagnant development cycle, at least in North America. The hot street-strip ticket way back then was a huge pressure plate setup – something that taxed your left leg and bent clutch linkages. By the nineties, the odd race car saw the use of a dual disc clutch, but those were configured for racers, not street driven cars. Today, things have changed dramatically. Clutches with multiple discs are more common than you might have first thought.
Let’s back up a second and look at today’s racing clutch: Countless drag race, oval track and road race cars operate with itsy-bitsy clutch assemblies. These things are absolutely tiny – some with an outside diameter no larger than 5.50 inches (no typo – that’s half of what we’re used to). Some of the “larger” multi-disc setups that measure 7.00-8.00 inches in diameter see use too. For the most part, these tiny clutches and equally tiny pressure plates are based upon a diaphragm layout, but one of the real secrets is the use of multiple small clutch discs. In the racing world, this means two, three or more discs.
Several clutch specialty companies have been paying attention. Companies have been developing and eventually building high performance dual disc clutch assemblies for years, pretty much ever since GM discontinued their Borg-Warner-built dual disc clutch option in the very early seventies. The dilemma with those earlier dual disc setups has always been the need for some very special (and often expensive) components. For example, the old Chevy setup mandated a special flywheel. Some designs were, and still are, based upon modified steel clutch covers.
Fast forward to today. Enter Tilton Engineering, the hardcore race car clutch and component manufacturer. These folks are world renowned for their racing clutch performance and reliability, winning championships in nearly every form of motorsport worldwide, including F1, NASCAR, IMSA, IndyCar, SCCA, Formula Drift and SCORE. And they’re set to shake things up with their new ST-246 street-strip package. Here’s why:
Based upon a pair of 246 mm diameter (approximately 9.7 inch) clutch discs, a billet aluminum clutch cover, a proprietary pressure plate and floater plate material and aircraft hardware coupled with a very special billet steel flywheel, the entire clutch assembly is engineered to reduce the overall moment of inertia.
That’s a wrap for this issue. Next time around, we’ll dig deeper into the Tilton ST-246. You might be surprised at the technology contained inside it. After that we'll look over the dimensions involved, then wrap things up with an overview of how Tilton deals with flywheels. Meanwhile, for a preview of things to come, check out the accompanying photo slideshow:
My Nova street-strip sleeper project was progressing nicely, but one thing proved troubling: the clutch. The engine is slowly going together and it’s a relatively large displacement (540), naturally aspirated big block. The back tires are little P275x60’s. A manageable clutch posed the dilemma. It needs to hold power. It needs to be driveable. It needs to be reliable (plus a few more “needs”). What’s a body to do?
Hidden inside this bellhousing mock up is what I believe to be the answer:
It’s Tilton’s brand spanking new ST-246 dual disc clutch assembly.
The assembly consists of this lightweight flywheel. It’s carved from a chunk of billet chrome moly steel. We’ll take a closer look down the road.
Another part of the assembly puzzle is this clutch cover, precision machined from billet aluminum.
