Again, excuse the tach signal pick-up error (which when plotted over RPM simply gets eliminated instead of showing a spike), but here are the same two torque curves at 25 PSI plotted over RPM. The area under the curve is so much better with the current setup.
In this graph you can not only see the 600 RPM improvement in spool-up, but the actual gains. Look at the cursor—how’s a 150-plus lb-ft gain in torque at 4700 RPM!
It’s also neat to see the difference in flow compared to our previous setup. Whereas the WHP vs Torque figure used to be about a 20% difference (i.e. the WHP being 20 % more), now it’s a 26% difference, thanks to the improved flow of the new components. One could argue that a laggy turbo can make a lot of horsepower vs torque as well. This is true, but as you can see we’re also comparing to what originally was a much laggier turbo than what we’ve got today to begin with. The engine is spooling quicker while at the same time making much better power without too much cylinder pressure (torque), which is a testament to the performance of all of the flow-related parts on the car (cylinder head, turbo, intake manifold, exhaust manifold, intercooler, etc).
You’re only as strong as your weakest link. In the case of torque, the engine can make all the lb-ft it wants, but if the clutch cannot handle it, you’re out of luck. For the past five years, Project Supra has been running flawlessly with an RPS BC2 clutch, which is rated at 750 lb-ft of torque to the wheels.
My primary motive for the move to this clutch came as a result of a mishap on the dyno. You see, I was running a traditional “strap” clutch, which can snap if enough twist is seen on it when you get off the throttle. On my first attempt at an 8000-RPM rip on the dyno, the strap broke the moment of throttle-lift and the strap—now sticking slightly outward like a box cutter knife—sliced through the entire transmission housing. Thankfully I was able to get the front piece of the transmission housing separately from Champion Toyota of Houston or I would have been out $6k for a new transmission!
This RPS clutch is the BC2, which stands for Billet Carbon twin disc. Here’s a shot of the strapless pressure plate. No more straps slicing through transmission housings!
Here’s a close up of one of the carbon discs. I’ve driven other twin and triple-discs before, and most of them not only had an incredibly hard pedal, but they also produced a really harsh engagement. Surprisingly, this clutch is very docile, and I would have no trouble parallel parking it on a sloped, San Francisco street if I had to. In fact, my minivan-driving wife has driven this Supra many times (and broken the tires loose at speed on occasion).
The Supra’s Getrag six-speed manual gearbox has two windows to loosen pressure plate bolts prior to a tranny removal. It’s also nice to be able to somewhat view the condition of the clutch itself.
As previously mentioned, the clutch drives nice, and we’ve never had a problem with it. But we’re about to crank up the boost to around 30 PSI, so we’re hoping it lives up to RPS’ torque rating! Will it hold? Will it slip? Will it explode like our previous clutch? Find out on Page 7!