Voith

Why would they change crank. Its a nice crank so why change it..

Also I dont understand what you mean with that correlation. Crank is self balancing so it really doesn't matter if you use light or heavy rods and pistons on it, since they balance each other out to some degree. The rest is up to balancer shafts.

Btw: my rods and pistons are 972g lighter than factory items.

67King

Honestly, the "drivability" aspect is more about its tendency to stall from a start. Think of that mass, which is inertia, as like a resivoir. You've got a lot of rotating inertia in there, it'll keep wanting to rotate when you let off the clutch pedal, even if you don't give it an ideal amount of throttle to overcome the mass of the car. If you make that rotating assembly lighter, it just becomes easier to stall, since there is less inertia that wants to keep things moving. Also, when the engine isn't loaded, and you give it a little throttle, it will rev more quickly. And that will tend to put heat into the clutch if you start slipping it at a higher RPM. It just is harder to get the throttle application right with the lighter rotating mass.

On the other end of the drivability spectrum is pedal effort. A multi-disk clutch is going to reduce pedal effort for a similar load capability. So that works the other way - this clutch will actually feel a whole lot better than other high load capability clutches out there.

So there is a tradeoff there. What would you rather deal with, a really high pedal effort, or an increased likelihood of stalling? Personal choice.

That said, the whole system on the Carrera GT weighed something ridiculous like 3 pounds. I don't play in that territory, but I don't recall hearing about issues with it.

Of course, once the clutch is engaged, the engine doesn't care where the load comes from, the mass of the car you are trying to accelerate, or the inertia of the engine you are trying to overcome. As long as you aren't traction limited, the engine will have the same load regardless of where the weight or inertia comes from. If there is less inertia to overcome, the car will accelerate more quickly, and in doing so, it will move more air through it in the same amount of time. So that will help you in every way possible.

But of course there is no free lunch, so the downside of that is that when you upshift, as you let off the throttle, the engine is going to lose more RPM than it would if it had more inertia. On downshifts, there is good and bad. It'll rev higher more quickly as you blip the throttle, but it'll also come back down more quickly. So it is easier to get the RPM where you need them on a downshift, but it makes timing more critical.

All that said, I've not gone to those extremes, so I'm extrapolating out from just changing to aluminum flywheels on a couple of cars, rather than a fully lightened assembly. But the physics are the same.

Personally, I switched to an aluminum flywheel on the race car, and was quite happy with it. I quickly adapted to it, and I odn't over-rev it or anything moving it around the pits. That said, the car is much lighter, so the reduced inertia wouldn't impact me as much as it would some. It also has a spec clutch, with which I'm very happy. I probably will keep an iron flywheel on the Turbo S, as it will remain a street car. Ironically, the street car would be more likley a candidate for this clutch than the race car, due to the power levels (i.e. race car limited by SP3 rules, street car will make a lot more power). But that gets back to the reduced pedal effort for a given clampload.

Thom

In my experience the rpm drop with a lightened stock flywheel works perfect with the usual short shift mechanism.

Would be curious to see how further this twin disc set up here would shorten the rpm drop...

Voith

There are some issues with CGT, to get it in motion, you have to leave the gas pedal alone and only lift the foot off the clutch, it electronically adjusts the rpm so it does not stall. Some snobs hate it for that, since it is not as easy to drive as some other high caliber cars.

I am building 944 turbo for fun sunday car, I want it to be rougher than stock (500lb springs, a/c delete, ps delete), just not so rough that I will suffer every second driving it on the road.. Some kind of middle ground between full race and road.

I was planing on fidanza or spec fw, aluminum pulleys, lighter pistons/rods & balancers.. Clutch will remain stock, but I will have the friction material changed to carbon/ceramic.

blade7

Exactly, so people that chop weight of the turbo crank assy aren't necessarily
doing the best thing for road use throttle response.

Thom

Many if not most modern engines have enough torque/inertia at idle to proceed this way. Sure is a good method against clutch wear.

gruhsy

With the 3.0L you don't really have an issue with throttle response on my setup...sure your driving style changes but to me it's no different....as an analogy....than say jumping from a truck to a car or a v-twin Ducatti to an inline GSXR....it's just different not undrive-able. As far as my cars original stock 2.5L to my 3.0 I like my 3.0 better with what I have done. Thats my experience others might have different experiences with their setup

michaelmount123

Since no one responded to this, I'm guessing that not many know the 968 crank is 5 lbs. lighter than an S2.

refresh951

As always, great info!

eman930

I ran a spec stage 2 clutch and spec lightweight flywheel in the 87 924S I had, it was a great clutch, pedal effort didn't change that much, Taking off from a dead stop was a little different just had to feather the gas a little more, no big deal, and the engine revs much faster but as noted its nothing you can't adapt to, downshifting wasn't much different. Spec is a great company, and as long as you follow their instructions for break in you shouldn't have a problem, This clutch shown is way more than Ill ever need in my 951 but it's great to have the option

Bob Rouleau

Guys, I've cleaned up the thread by removing the speculative declarations. That meant editing or removing some of your posts where robstah was quoted. I'll be watching this thread.
Best,

blade7

Have you weighed them both ?

Dubai944

I had both cranks available for my last rebuild. There was about a 2kg difference in weight when I weighed them, so I used the 968 crank.

I have had a Spec Stage 3 hybrid clutch and flywheel in my car for years. Great product.

blade7

No doubt. Is that a road or track car ? I remember someone posting a while back about the 968 crank being lighter but later said the S2 and 968 cranks were the same weight ?

Chris White

this is one of the oldest and silliest debates on this forum....
I really have to agree with the 'virgins talking about sex' analog, it really fits this one!
In the 'real' world, which of course means a road race track, the biggest benefit of a lighter rotating assembly is simply the ability to get the engine to pick up or drop RPM while not engaged to the drive train. Much better and quicker shifts without abusing the clutch or shocking the drive train. Its that simple. Does the rear wheels see more power? You couldn't measure the difference.

In the other 'real' world (the street) a significantly lighter rotating assembly along with a 'grabby' race type clutch/pressure plate is pretty annoying at stop lights.

The Lindsey solution is a great way to go for the track (and with the right set up is could be good on the street too). Its been done in the past but its a pain to engineer - hats off to Lindsey for offering this. I think I may use one on the next track car project and save myself a lot of time fabricating one!

One other thing mentioned in the tread - clutch ratings in torque - this is damn near meaningless. If you develop that kind of torque and the clutch is not fully engaged it will not hold. In other words if there is any clutch slippage while making significant power you can't really on the 'rated holding' power to work. So power shifting, drag starts or inept driving are not part of the rating! To this date I have only had one clutch/pressure plate system actually start slipping while under load and fully engaged and that turned out to be a faulty part. I have used the factory 'sport' clutch (or Cup clutch as some call it) with factory pressure plate on engines making 370 rwhp for multiple track season with no problems - as long as it was not abused!