There's not much information on the OSG twin available on-line so I thought some might find this information useful. I put some info in the build thread for Vlad but I've been waiting to post an in-depth review of this clutch till I had the chance to put some decent mileage on it. Now that it's got 5,000 miles on it I think I can give a better evaluation. The need for an upgraded clutch became pretty obvious to me when after about 8,000 miles the new factory JCW clutch started slipping in third on Map C. Did a couple of other upgrades at the same time as the clutch. Put in a Wavetrac LSD, CravenSpeed short shifter, new slave cylinder, ARP rod bolts, RX catch can and flushed the clutch and brake lines with RT700 fluid. There's probably something I'm forgetting to add to the list... Getting old sucks.

First up a little car ****. The pictures could be better but hey I enjoy building and tuning Mini's. I'm not a photographer so no complaining







The factory dual mass weighs in at 36 lbs. on the nose and the twin hits the scales at 28.3 lbs. A pretty impressive achievement for a clutch that's rated at 570 ft lbs of torque. The normal STR clutch is rated at 285 ft lbs and there are two versions of the GT clutch. The soft pedal GTS version rated at 280 ft lbs and the GT rated at 315 ft lbs.

One might ask what in the hell do you need that much clutch for. The short answer is I have plans for Vlad since he also serves as my development test bed and it might come in handy. Besides inquiring minds want to know and sharing is what NAM is all about. So I said why not. Worst case scenario since Vlad is also my DD would be I can't stand the damn thing and have to swap it out. At least then we'd all know.

Installation is pretty straight forward. However, don't try it unless you have a spline alignment tool. Unlike a single disc where the disc just has to be centered, the splines on both discs must be perfectly aligned or the transmission input shaft will not go in. Anyway, here's my thoughts after living with the OS Giken twin for a bit.

• Break-in Process - Well truthfully there's not much of one. It's a full face metallic race clutch designed to rock and roll out of the box. Most clutch manufacturers recommend between 200 and 500 miles so I resisted the urge till I couldn't take it anymore.

• Pedal effort - Having owned a big block with 12" McLeod clutch and mechanical clutch linkage I wasn't too concerned about a heavier pedal feel. But I'm happy to report pedal feel is very close to stock.

• Clutch engagement - Other twins I've driven have been a bit on the temperamental side acting more like an on/off switch than a clutch. OSG obviously did their homework because that's not the case with this one. It is very easy to feather for smooth take offs especially when cold. Once up to operating temperature the engagement windows shortens a little bit but it is still by no means difficult to drive. We recently relocated to Sunrise FL so I do a lot more stop and go driving than I used to. I did manage to stall the vehicle once but I just finished running a 3K. (That's my story and I'm sticking to it)

• Gear Noise - One thing I would change if redesigning this clutch to create the ultimate street/race clutch would be to use sprung hub discs. OSG chose otherwise and went with solid hubs. It truly is a no compromise design with a focus on durability under repeated abuse. With any single mass flywheel / solid hub clutch combo gear noise is inevitable. The Getrag is not immune to this and will produce gear noise under load at lower RPM's once up to operating temps. Cruising at low RPM doesn't produce it but acceleration when under 2000 RPM will produce it in higher gears. I also noted gear noise when decelerating in 6th gear from about 70 down to 55 MPH. To be honest I rarely notice it unless listening for it with the windows up and the radio off. Just to be thorough I mixed one quart of Torco SGO 75W-90 with a quart of SGO 75W-140 and replaced the Motul 300 to see if it would make any difference. There was a small improvement and the slightly thicker viscosity at temp made no difference in shifting.

• Clutch noise/chatter - This is the one area that would turn your average driver off. If your not comfortable with sacrificing a little comfort or stock like drive ability for performance then a twin disc clutch is not for you. One would expect a twin to be a bit talkative and it is. There are three occasions where this clutch will make a bit of noise. The first is common to all twins and occurs when the vehicle is at a stop light with the clutch pedal depressed. Imagine metal plates spinning in opposite directions and very lightly touching. That is the sound produced. It is not very noticeable inside the vehicle unless once again the windows are down and radio off but its there and may cause a few strange looks at a stoplight from the uninformed. I just put it in neutral at a light which is easier on the throw-out bearing anyways. The other two times the clutch makes noise is directly attributable to the solid hub design. Sometimes when gently taking off from a dead stop you will note some clutch chatter since there are no springs to absorb the individual pulses from the engine. I've found two solutions for this. This first would be to stop fiddle farting around and just go Sometimes easier said than done in stop and go traffic however. The other solution is to rev slightly higher ~1500 RPM and release the clutch as the RPM's are dropping (blip the throttle). The other time you can make the clutch chatter is if you shift very lazily or don't rev match when down shifting. This for me was kind of a wash since it forced me to be a better driver. It probably took about a week or so before it became second nature. Either way a sprung hub would be nice on a daily driver.

• Lighter clutch weight - The eight pound difference in weight made a noticeable difference in how quickly the engine revs, the speed with which you can rev match and also made a considerable difference when it comes to speed shifting at high RPM. The engine immediately felt more responsive and resulted in quicker acceleration and I noted slightly improved engine braking. Shifting into third at 6,800 with the dual mass would occasionally result in the not so friendly result of grinding gears. Not so with the OSG twin as it loves to be driven in a spirited manner. In fact the harder you drive it, the happier it gets.

• Gripping power - Hmm... what can one say about a clutch rated at 570 ft lbs by a company that is known for under rating it's clutches? Quite simply it is a beast. The transmission or crank will probably fail before this clutch will slip. Imagine the grip of an STR, then multiple it by two and that's pretty much where your at.

• Durability - The STR has pretty conclusively demonstrated the wear characteristics of OSG's full face metallic disc. In this case we have twice as much abradable surface area and material. Time will tell but I'll be very surprised if this clutch doesn't outlast my transmission.

I'll try and capture video/audio to better put the descriptions above in context. Overall I'm extremely happy with the twin from a performance perspective and pleasantly surprised by its docile driving characteristics. Could it be improved on? Yep, I think sprung hubs would tame this monster a bit resulting in the end all of performance clutches on the Mini. As it is I'd say it's reserved for those that want the absolute best performance and don't mind sacrificing a little bit to get it.

A note on the RT700 brake fluid. Vlad already had SS brake lines and ATA Superblue fluid so the clutch and brake pedal feel was pretty good. After flushing and bleeding the system with RT700 however I noted almost zero dead space at the top of the clutch pedal and a noticeable improvement in brake pedal feel and feedback. It's not cheap but is well worth the investment in my opinion.

 

Thanks for posting this detailed guide! I'm looking forward to hearing audio, especially about the chatter. I'm on stage 2-ish and I have dreams of bigger turbo and meth injection... could do with a beefier clutch too. The noise interests me as I daily drive my car.

 

Sweet! We are installing the clutch as well as we speak... what did you use for spline alignment tool??? Looked online already but can't find one for the R56 GP2

 

Congratulations. No more sissy clutch lol.

Here is the tool you need.
http://www.clutchtools.com/index.php?productID=42778

I still need to post a video of the clutch. It quieted down a bit more by the 10,000 mile mark. Either that or I've just gotten better at learning how to quietly slip it. When stuck in stop and go traffic, keeping the RPM down as you start to slip it seems to be the key. Other than that, let her rip and enjoy!

 

I've had this clutch for over a year now (previous owner installed, as well as OS LSD). I have put about 20k km on it. I'd say my experience has been similar. At first I thought it was an on-off switch and I'd stall CONSTANTLY. I had to drive like a Honda fanboy with multiple throttle blips. Turns out... it was the tune that's shitty. Without the tune, the car drives relatively smooth for take-offs, but it's easier to stall due to less rotating mass. Gonna get a better tune next year after I do my timing chain to take advantage of the FrankenTurbo anyway

And yes, all my car friends (initially) think I have a bad t.o. bearing.

 

Thanks for the update! Really appreciated!

 

Lol yeah it's not a clutch for grandma to drive. Way better than a Tilton Carbon but it's still a high performance race part. I let my local BMW SA drive it after the Stage 3 but gave him a heads up on the clutch. He had zero issues but after that he would always drive it in and out of the shop not trusting the techs or valets to drive it. Last time it was in, the service tech gave him crap about it saying "I've been driving sticks for 25 years blah, blah, blah." So he let him pull it out. Stalled it right off the bat haha. He was a bit embarrassed but after the ego check he did fine.

 

We ordered the part today. Hope it won’t take long before it arrives in Holland.... than we can build back my toy.

 

The Stage 3 tune I'm running now has the warm idle raised from 750 to 900 RPM. The same change is going into the new tune I'm writing for Vlad now. The idea was to help smooth out the idle when the head gets ported and the cams go in. A side effect beside the overall smoother idle however is that it is easier to slip the clutch without stalling. When you get ready for your tune I'd run that past your tuner as it's fairly easy to implement.

 

I'm looking at giken clutch options currently online it seems they are rated as follows

280
285
310
Then this 550+ beast, seems a very big jump from the 1st 3

Can you explain why the others are so close together they may as well be the same ?
Am I missing something of important between gt and str options at this lower rating

Is there an upgrade path from baby to daddy str,
I'm worried I might be compromising daily driver feel to accommodate a engine upgrades 3 years down the line

 

 

I definitely wouldn't consider the STR as a baby clutch. OSG is notorious for underrating their clutches. OBW ran a single disc STR at 350 HP with no issues. The difference between the STR and GT series is the GT series retains the floating pressure plate from the multidisc design. The helps reduce chatter which you will get with any full metallic clutch. All of the singles use a sprung hub but the twin uses solid hubs. The STR single is fairly quiet while the twin is as chattery as a 10 year old girl. All of the STR's and the GTS have a soft diaphragm in the pressure plate so the clutch pedal feels stock but the straight GT1 uses a stronger diaphragm for increased clamping and load handling while still being quiet. There's always a big jump when going from a single to a twin as your basically doubling the clamping surface area.

280 GTS - Soft Pedal
285 STR - Soft Pedal
315 GT1 - Firmer Pedal
570 STR2 - Soft Pedal

I'd think of them this way:

Fast, street friendly with the occasional track day. ~300 HP
Streetable but very race capable. ~350 HP
Ultra high performance, racing but still street friendly. ~400 HP
You're crazy & better learn rev matching. ~Nobody knows yet...