GregBBRD

I'm not sure what a stock single disc with the GT pressure plate will hold, for sure.

I know it won't hold down a very basic stroker, so I'd guess somewhere under 350 ft lbs.?

Lizard928

He's over 350rw ft lbs......

GregBBRD

Which is why it slips, I'd guess?

By the time people "think" a clutch slips...it slips a bunch! We were absolutely "floored" by the results of these dual disc carbon-carbon clutches. The cars are so much "faster", it is like we "found" another 50 ft. lbs.

dprantl

I don't understand this... usually it's pretty easy to tell when a clutch is slipping. Get on the highway, put the car in 4th or 5th gear, rev to torque peak, press the brake and floor it. Hold road speed constant and see if the RPM rises out of sync with road speed. Or did you find that the clutch slipped right after letting out the clutch after shifting gears?

Dan
'91 928GT S/C 475hp/460lb.ft

Rob Edwards

Door #2.

When the stroker was new, with stock exhaust on it and the reinforced DD clutch (with nearly new friction discs), I could do the torque-max thing in 4th gear through 4400 rpm without slippage. By 5000 miles in, with headers, more tuning, etc., I was beginning to get slip if I got on it in 4th. The marcel springs in the modded pressure plate were bent when I pulled the DD clutch, which might have been contributing to the increasing amounts of slip.

That said, I really think the DD clutch was slipping off the line, and during/just after the 1-2 shift. With as much torque as I have and a 2.73 box, jumping on it in first and second will just blow it right to the rev limiter, and it's really hard to tell (in that 2 or 3 second span ) what is wheelspin and what is clutch slip. But I can unequivocally say that after the carbon clutch install, the car launches significantly harder from a roll in second gear. There's certainly no less power now, so either I'm getting better at feathering the gas (unlikely since it's being crushed against the carpet...) or the clutch isn't slipping as much.

ptuomov

This is all very interesting. When the engine is "ready", I'll be looking for a better clutch and will keep these options in mind.

What's the right option for me given that the engine is making pretty much any torque number I want? I want it to hold no matter what I throw at it and still work in traffic like a modern manual-transmission Porsche.

Rob -- One possibility is that the take off is harder with the new clutch because it grabs hold better/harder/earlier. There's a lot of inertia in rotating engine parts and you may be experiencing that energy being released into acceleration. Or it could be something else, I am hardly the clutch expert.

Rob Edwards

Remember, medicine was my fall-back 'cuz math and engineering is too hard....

Yeah, I am struggling to explain why it's just 'better' now, but it just is. Wish you were on this coast, I'd hand you the keys so you could see what the clutch does with a measly 450-ish torques. (Forgive me father, for I have sinned, it's been 2.5 years since my last dyno session....)

GregBBRD

I'm not sure that I know how to explain the difference.

Once your foot is off the clutch pedal, the flywheel is absolutely/completely/100% locked to the transmission. Not the case, with anything else I've ever driven. I guess there's always a certain amount of slip, with other clutches, before they lock up.

It's a very different sensation.

jcorenman

We just installed Greg's twin-disc in our GT, and all I can say is that it is hands-down the best clutch I've ever driven. And there's been a few.

The history: This car came with a stock single-plate with a beefed-up pressure plate, which worked well-- engaged smoothly with a nice "bite"-- but was an absolute "beast" in terms of pedal pressure. Louie, in his ad for the car, wrote "The clutch in this car takes a good left leg". (It was Jim Hurtt's car before Louie, and apparently Phil Tong's a couple of owners before Jim).
Fixing the broken helper-spring made it better but at the expense of some of the nice "feel". Sue, having appropriated this car, was struggling a bit with the pedal pressure.
Torque was beyond a stock single-plate so I swapped it for a stock DD, which we had previously done for our S4. That worked fine, but was more "fiddly" than the S4's (requiring periodic manual adjustment), and the pedal travel-- while light-- was also quite long, not completely disengaging until the pedal was close to the floor.

It seemed silly to worry about short-throw gearshifters while putting up with a long-throw pedal on the clutch. And it is the same issue-- you want controls that have the right amount of pressure and travel for the job that needs doing.

What makes this clutch so great is its absolute precision. The pedal is lighter than a stock single-plate (with no helper-spring), yet has a relatively short but very smooth engagement-- and a very positive "bite". It is exactly what you want for a car like this.

Break-in was simple. Greg told us not to "baby" it, that the friction surfaces were fully "cooked" but needed to be smoothed a bit. And when we first fired it up, it did chatter some. So I backed it down the drive (a short but steep-ish 20% grade) and slowly drove back up to the shop in third gear at 3000 rpm, slipping the clutch like mad. That was all it took, it has been silky-smooth since, even in reverse.

And Sue loves it.

Thanks, Greg!

hacker-pschorr

That is awesome - just eliminating the stock slave cylinder had me sold.

The rest is just icing on the cake.

Ducman82

did your new flywheel come with the 60-2 ring? any pictures of a complete kit on the bench?

jcorenman

Yes, came complete with all the bits-- even loctite and tie-wraps. This is what a clutch should be.



.

Ducman82

That's so pretty I want to cry

oups59

hacker-pschorr

Two questions....

1. 78-86 Application?
2. Rough price estimation? One my cars is due for a clutch R&R, might as well aim for this instead.