7.25" clutch for occasional street?

All comes down to personal preference. Myself? Would not do it.

Window of engagement is tiny and friction material is aggressive. The reason organic is not available is it would not hold up to any power in this diameter. (well maybe in a double or triple disc configuration)

You will have to be on your game to drive this in traffic. (I've seen some people really not able to drive these at all).

 

Agreed, for track only it's fine. On the street, not so much.

 

What's the purpose of the car? Stock power?

 

There are several reasons. One as you mention is less rotating mass. And yes, how aggressive the clutch material is though not an issue with organic.

You still probably have no marcel spring on the discs which makes the engagement window smaller and the hubs are not sprung, both make driving in traffic a bit more work.

What are you trying to achieve by changing this from the original design?

 

For my 550 Maranello and another turbocharged hotrod that I own, I've replaced their organic full face clutches with the smaller, multidisc, metal lined variety, and I've regretted it both times.

They are nearly impossible to launch smoothly, and are miserable to drive in traffic, and are very easy to stall, so I'll be retrofitting full face organics back into both of these cars.

Going forward, I would never put anything but a full faced organic or its equivalent into any street car, unless I intended to launch and drive it like a racecar.

 

Bottom Line: Caught in stop and go traffic? Say OH F it. Pullover,park it,and find a nearby venue for a cold adult beverage.
GTS Bruce

 

Here's what you want, it's butter smooth and has ZERO issue with pulling from a dead stop. They have made the entire package from flywheel to TO bearing, clutch setup and adjustment was spot on to factory spec, something that's really difficult with the quater master or tilton as I always needed to shim out the bearing.
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The OS clutch is near the same size as stock as I recall, that really helps with inertial weight in getting off the line. They do have an option I want them to build for us that is a smaller multi-disk pack that looks similar to a differential clutch pack, lighter and handles very high tq.

from what I've experienced the small 'track' clutches are great for the track but miserable on the road, no way around it. Look at it from an engineering view, the tq moment arm is much much shorter so to offset that we need to increase the kinetic energy of the system, i.e. higher rev's this in turn introduces the 'slip factor' or 'shock' in letting the clutch out fast at high rpm to get going, i.e. the 'launch' all great for the track but horrid for the street. A good street clutch is heavy and takes longer to spin up but has the advantage of more kinetic energy at a lower rpm, softer launch and easier on the drivetrain. Finding a good balance between them is tricky, if you need to make up time and it's not from the dead stop/start on the track then getting more power down low is going to help, a lighter clutch package that also sports a tq arm the same length as stock would be ideal. I'd have to dig up the numbers, I think I have them but the OS clutch package is a bit lighter then stock.

it may also be time to run some numbers, dyno your engine or even use one of the many great android/iphone apps to get a good tq curve from your chassis, with that then produce a thrust scalar tq map, it'll tell you exactly the best shift points to stay in the power band of your engine. I have an excel file that covers just about every gearbox iteration and the calcs needed if you want, all you'd need to do is drop in the hp or tq for each rpm point. It seems 'nerdy' but knowing the right time to shift to avoid lag or having to shift again will make you faster, it's all about maintaining speed smoothly...

 

I think there are data logging apps for racing, given that smart phones today sport a plethora of sensors not to mention a std. 3-axis accelerometer the capability is there. I do know there are also 'dyno' apps that use the vehicle weight and Cd to give a good est of power, I've got one that also data logs and I can read the output file on the PC. In anycase having a close Tq curve over the RPM would enable you to put together a 'thrust scalar' map that would give you the best shift points to stay in the power band. You're right about the gear ratio's though, they are goofy, but they are also best suited to high RPM shifting and higher over-all speeds vs auto-X that favors lower gears and low RPM tq, something the 308 just doesn't have enough of.

If you're looking to really make up time I'd suggest data logging your runs, it may surprise you were you can pick up time, most standalone ECU's will record telemetry for braking and accelerator input and that can be very helpful in shaving time. I used to race with a buddy who would brake far far too soon and not enough, once he got past the mental aspect of late hard braking he started shaving time. Not saying that's what your doing but oddly enough the ability stop faster will make up more time then trying to go faster.

Lighter rotational mass will effect the spin up/down of the engine it doesn't net more power. So it can gain time in the sense of not waiting for the 'shift' points to match up but it's going to be fractional depending on the shift point and mass involved. I.e. a shift point at 6k is very different from one at 9k.

New vehicles are going to be next to impossible to beat around the track, esp club events, in a 30yr old one. The sheer amount of electronic assist and anywhere from 6~9 gears puts you at a massive disadvantage. Oh and the power band of the most basic grocery getter has more tq down low then a 3x8 does. I'd imagine a VW TDi with DSG would sweep an auto-x event without much effort, brutal tq and a butt-load of instant gear changes.

 

Scott, FWIW, I have a triple disc OS Giken in the 550 and it's miserable to drive on the street. I even changed the master cylinder to mitigate the on/off nature of it, but it's still difficult for me to launch smoothly, and dam near impossible for anyone else to drive the car at all.

 

STR or R series, I'd expect the R series to behave that way as it's a full on race clutch, the STR shouldn't though.

 

Interesting thread. Porsche Carrera GT has a 6.25'' clutch so even smaller than we're talking about here. It does come with a anti stall feature (put ZERO throttle until you get rolling...any amount of throttle automatically overrides the system and you're launching on your own). Depending what ECU you're running (specifically Crib), it may be worth looking to...

 

Scott, mine is the STR-3C, but it has the unspring hubs for its 215mm (8 1/2" discs). See pic below.

The Giken people were of little help, and advised me that if I found or developed a solution to make the clutch more street friendly, to please inform them so they could incorporate it!
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Interesting, well the lack of sprung hubs is going to be a problem. They don't list the STR that way though, is this an older unit? I wonder if it's a packaging constraint. Fortunately the 308 does have a sprung hub and does not have the linear on/off nature of a race clutch.

Across the board clutch co's tend to be a real PITA to deal with on small run built to order stuff as it is.

 

The clutch was installed in 2010 right after I bought the car.

I did have misgivings back then about putting in a triple disc metallic, but had always heard the OS Giken's were the most streetable of the multiplate clutches, and I felt the V12 with all its torque and moment of inertia could probably smooth out the engagement, but I was wrong.

When I get to the point in my project where I need the clutch, I'll see about having the discs replaced or relined with something more street friendly, and perhaps OS Giken will have made some advancements by then. They were working on a carbon/carbon version last time I was in touch with them in early 2015, but it wasn't ready for prime time yet.

 

Not really a similar application, but I've been running a Tilton 7.25" ceremetallic twin plate in my 450whp Evo for about 6 months now. I had a single plate 6 puck clutch before and this is light years better in every way. It's actually very nice once you get used to it, and you can slip it if required (within reason). While I've not had any experience with the latter myself, I am told that the ceremetallic "rally drive" plates are night and day in terms of streetability compared to the sintered metal plates.
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the 308 gearbox ratios SUCK... I mean really really suck...