Ryan--

Talk to Carl and let him know what you have in mind. I'll speculate that he doesn't keep a lot of these on the shelf, so getting the early bolt patter for the stacked clutch on the later wheel with the timing ring wouldn't be tough.

I've noticed that the 6.x liter strokers seem to rev a lot faster than the stock GT/S4's, even though there is more rotating mass.

Mark's point about the flywheel weight not affecting engine dyno numbers is correct, but might be misleading. Lowering the inertia in rotating parts does affect road horsepower measured on DynoJet et al, similar to what you'd notice driving the car out on the road. Makes a difference accelerating the car to top speed, but once there the difference is negligible. I've been under the impression, right or wrong, that the stacked clutch setup is smaller diameter so the rotating inertia is less just from that. And last, I've driven cars with light flywheels, never had a problem with stalling or idle. Launches from a standstill take a little more finesse since you don't have the mass of the flywheel helping, but you'll learn about that quickly and will compensate. Carrera GT drivers know about this. Once rolling, the engine drops and gains revs more quickly, so your shifting technique will get a little adjustment. Your heel-and-toe blip on downshifts will get a lot more precise. I think it's a big net positive, but maybe an acquired taste. Some others who tried it just once or twice thought it was all too sensitive.

 

actually, the only difference you would notice in accelerating to top speed, in addition to what you saw on the dyno (which would be negligible in 4th gear and beyond), would be the effects of the now changed weight of the car.(also negligible, as 15lbs is like 1hp as well).
Inertial values only matter when it comes to rates of acceleration. as you say, once you get there, it really doesnt matter.

mk

as a rule of thumb, changing weights of rotating masses, like wheels and tires, has the same effect as if that weight was about double and sitting in the back seat (ie 1.5x) for flywheels, its a little more complicated but its something like 10lb changes are like 20hp in 1st, and 1hp in 4th . (even less in 5th). this depends on the diameter of the flywheel and the rate of RPM changes.

mk

 

Thanks for all the comments. Mark it sounds like you have the whole setup right there in your second to last post.

Interesting you and Bob mention HP increase. I am not doing this for any HP increase, but as you said Bob to change the feeling of the car somewhat. The lower weight and the fact that the clutch assembly is a smaller diameter will both contribute to this effect of the engine reving faster which is really where i was going in this idea. Again this is completely exploratory but if i find some of these things for sale on Ebay used I may give it a go and assemble the parts and throw it in and see what I think. I guess if it doesn't work as expected then I just pull it out and reinstall the original setup. I think It should only take 6-8 hours to remove and replace the entire clutch and flywheel so its not the end of the world if it doesn't work out.

I noticed Hacker, or I guess I need to say Enzo now mentioned something in another thread about the cost of the mid plates for the twin disc setup. I guess I need to check that out first.

Thanks Mark for the specifics, I will take a look at the PET and get a better idea of the layout as it does seem to encompass allot of different things. I don't know when Porsche got rid of the twin disc setup, I don't know much of the differences when it comes to earlier cars, Ive really only paid attention to S4 and later stuff, but I guess I need to crack into some books and familiarize myself with some of the older cars.

 

And as for calling Jim B, being that i don't have any budget for this at the moment, I figure I wouldn't waste his time.

Going to check the PET out now

 

the only change, might be in quick reviing you engine in neutral. the HP gains in gear are pretty minimal, especially in the taller gears, and certainly very little, almost no gains while approaching top speeds.

mk

 

The whole apparent-horsepower increase come from engine torque that isn't spent on accelerating the flywheel/clutch assy. As Mark suggests, the effect is multiplied by gear ratios and the rate of acceleration. Using his guestimates, getting an extra 15 apparent horsepower in first gear is a 5% improvement, and that diminishes both from the changes in gear ratio and the change in rate of engine acceleration. By the time you are approaching top speed the effect is undetectable.

There are good reasons to look at wheel and tire weights, the effects of larger/heavier rotating brake components (disks and hats), etc, beyond the obvious unsprung weight and the effects on suspension performance. All affect the apparent/available road horsepower numbers during acceleration (and decelleration/braking). Several years ago I had a chance to look at Toyota's off-road truck brakes. BIG aluminum hub and hat parts with steel only in the friction areas. There must be a way to get that kind of stuff stuffed into a 928 wheel...

 

There will soon be available a trick lightweight alloy fly wheel designed to use the Tilton triple disc clutch set up...even smaller diameter light weight PLUS much stronger. About 4-6 weeks for this setup which is what Mark Anderson has been using for some time now.

 

I am not sure how much "testing" you want... I have raced the aluminum flywheel with more than 450 ft lbs of torque for 3 consecutive years without a problem. And, I am not being kind to it with a stock pressure plate. I am using my own GT1 pressure plate with more than 2400 ft pounds of clamping force (adequate up to 750 ft pounds of torque) without a problem.

I have plenty of information for you about it here:
www.928clutch.com

We did not bring the aluminum flywheel to market until the 3rd year. I drove it for 2 years before listing it, and we used it a Pikes Peak also.

We have sold and shipped 11 of them now since I listed it in our catalogs.
Perhaps some of the owners can chime in and tell you what they have found.

In the first iteration, we found the taper at the back of the flywheel was a little too mild, and this caused a problem only with those late-model owners that were back-dating to the early twin-disk setup as the starter motor housing touched the flywheel slightly.

That adjsutment has been made, and I have a new batch of 10 sitting on my shelf that represents the newest version and fits all 928's without a problem.

You will find, as a result of storing less kinetic energy in the mass of the flywheel , that 1) the engine revs much faster, but 2) has less torque to assist you when starting from a dead stop. Just like any other car I ever drove when a alluminum flywheel.

You rev just a tad higher than you used to before letting out the clutch, and away you go. Its an adjustment that takes about 20 minutes to get used to, then it becomes automatic again. Given the amount of time a performance driver spends starting from a dead stop vs. the amount of time he spends driving - the trade-off is usually felt to be worth it.

 

Jim,

I am curious about this setup. I am wondering if this 3 plate clutch pack is a real viable option for a street car. If you say it is I believe it, but i have had doubts about previous racing components being used for street duty. I don't want to get away from the GT aspect of the car, I wouldn't want to compromise too much on the day to day driving aspect of the car. I tend to like this direction better then backdating as Carl put it. I think there is a better clutch design then the one porsche came up with in the 70's which is what the twin disc setup is really.

Can you PM me an idea on price for this setup? It sounds like, or at least I am hoping this new setup would eliminate the need to change the lower bellhousing, the slave cylinder and adding a different flywheel and a $900 mid plate thats close to NLA status. I would love to hear more about this, and if it in fact would be appropriate for street use.

Thanks

 

Perhaps something along the lines of this?

This twin disk setup with flywheel and clutch discs of a ceramic composition sells for $1200 I think. I am sure its for a Japanese car but I think this would be a nice modern update that would accomplish what i am looking for. And its less then the older clutch setup offered elsewhere. I guess you guys are taking care of the flywheel which would solve the mounting issues, but still i wonder if that type of clutch is useable for a street car.

From what i know which is only what ive heard from supposed ford engineers, I would like to see a 30-35 lb rotating assembly. I dont want to put on a 15lb or 20lb setup as i think that would just be too light and too much of a compromise.

Crap, I am trying to post a picture but i am out of space or something. (I also just noticed i am no longer a rennlist member, I think my membership lapsed. Is this why i cant post a pic? I am re-upping now)

 

Carl, im interested as well. mainly for the quick reving blips and the absolute weight saved.

one correction below, there is no torque gained from "starting from a dead stop" actually, quite the opposite, the stored energy of the entire rotating assembly is transfered to the driveline as you let the clutch out. there would be more energy available while you are letting out the clutch. after that, though , you are right, the lighter flywheel, IN FIRST GEAR (and a little bit in 2nd) would allow for more HP to be utilized do to the lower inertia.

Mk

 

very cool

 

That is excellent news. Just a word of caution. This is a race setup.
The tilton triple usually isn't recommended for street use.

 

Re-reading this thread, I also see a question about the pressure plates I did not address.

We start by purchasing a brand-new Sachs clutch from Porsche when building our pressure plates. So if you think they look stock form the outside - thats why.
Then the pressure plate is dis-assembled, and new springs and fulcrums are installed to raise the clamping force to the desired level.

This is far-and-away the most cost-effective way of producing a performance clutch that you know will be compatable with stock release forks and bell housings.

If you've installed one of our performance clutches, then you know there is no doubt of the difference inside. You first notice it on installation, and then again upon driving it.

I agree with John Venninger on his "street-able" comment above. A Tilton 3-stack will be a race-only application, as is our GT-1 twin disk setup. These clutches get pretty close to on/off in their feel.

We make a GT-3 and a GT-2 clutch pack that is stronger than stock, yet still street-able. They will handle anything your supercharged 928 can produce.

 

I am actually one to believe in making the clutch the weakest part of the drive line. I would rather burn up a clutch then put too much torque to the transaxle and destroy that.

Tilton makes a twin disc, I am curious to hear more from Jim B. I know the triple plate clutch weighs next to nothing, so i cant really see this setup with a flywheel being 30-35lbs. It would be too light and in my mind not desirable for a street car. But Ive never driven one. I would love to hear more on the market 928INTL is targeting with this setup, if there is a way of making a street version, or if this is for owners with track days in their future?