Maybe we can get this stickied, or turned into a tech tip or something.

Here's everything I know about the clutch options out there for ZF6 cars. This is all just my opinion, but I try pretty hard to be fair and only offer opinions on what I have experienced. Here we go:

The stock unit: The stock clutch, with working hydraulics, and in fairly new condition should support 350 RWHP with relative ease. Launching the car aggressively on drag radials or slicks will shorten the life of the clutch, but that's the price you pay for drag racing the car on a sticky tire.

The dual mass flywheel: The popular rumor is that GM used the dual mass flywheel to help quiet down the noisy ZF6 transmission. I don't know if this is true or not, but it does make sense. Another benefit of the dual mass is the amount that it slips when dropping the clutch to launch the car. The dual mass will turn probably half a rotation under power, which cushions the transmission, universal joints, and rear end from the violent hit of trying to launch the car. The downside to the dual mass is the weight (which eats up power), and that it can not be machined if you decide to replace the stock clutch. Some people have gotten away with machining the flywheel, but in my opinion they have just gotten lucky. There is no way to chuck up the flywheel in a lathe or cutting device and be sure that they flywheel is being cut true. The nature of the dual mass prevents this.

Slight upgrades from stock: In my opinion, for those making ~400 RWHP/RWTQ a stock pressure plate and flywheel with a hardened stock style disk is adequate. Again, aggressive drag racing will shorten the life of the unit, but you have to pay to play. Another option at this level is the stage 2 clutch from www.carolinaclutch.com. I haven't personally used this clutch, but a friend of mine has it in his 400 RWHP 1995 C4 with good results. He doesn't drag race the car, and the car is actually his daily driver so I can't speak on how the clutch will hold up under drag strip abuse. The stock set-up with a hardened disk is something have personal experience with (1992 C4, 380 RWHP/380 RWTQ) and have no complaints. It held up to probably 150 launches at the drag strip.

More clutch: In my opinion, the next step up is a Center Force Dual friction. I have one of these in my 91 and have no complaints. It does eliminate the dual mass flywheel, so it should theoretically make the transmission noise worse (in my car, it made no difference, but I'm the minority there). It's held up great to drag race abuse, but will start to give up if you are making 500+ RWHP/RWTQ and launch the car aggressively.

Super clutch: The McLeod twin. What a great clutch. It drives almost like the stocker, with silky smooth engagement, and a nice wide pedal area of slipping. However, at the drag strip, you'd better be packing a solid rear or you'll break things. The clutch hit is violent beyond belief. There is ZERO give with this unit. I guess that is the price you pay for a clutch that will withstand 1000+ HP. You will lose the dual mass, so expect more noise. I suppose if you have the self restraint and control to slip the clutch on the launch every single time this clutch would be great for you; but I don't have that kind of control. I was breaking things on an average of every 4 passes at the track with this clutch. Still though, I don't have anything negative to say about the clutch. Drove great, girl friend could drive the car without issues, and it bit like freakin' Godzilla.

Other options: The F-body flywheel. Some people have taken the flywheel from an LT1 Camaro or Firebird and made it work with the stock clutch disk and pressure plate. To do this you must machine .090 off the face of the flywheel. This is well beyond normal tolerances, and not what GM intended for the flywheel. I've never heard of one of these units failing, but the fear of it has been enough for me to shy away from running this combo in a car. You also lose the benefits of the dual mass, so expect a bit more noise and a harsher hit to the drive train. The reason behind this swap is that the dual mass is expensive, more than $500 most of the time, and used LT1 flywheels can be had for as little as $35.

Notes: Make sure your hydraulics are working. If your hydraulics aren't at 100%, you'll be killing the insides of your ZF6 every time you rip a gear. This is cheap insurance considering the ZF6 is over $1200. There was a pretty big quality control issue with the slave cylinders from GM for a long time; the seal inside the cylinder was actually being installed backwards which allowed the cylinder to leak down under pressure and fail. The masters aren't much better, and feeling the carpet up behind the clutch pedal for moisture from time to time is a good idea. If you ever notice the fluid is low, find out where it's going. Don't just top it off and assume that everything will be fine.

Good replacement parts are hard to come by. There are some cast metal slave cylinders out there (the stockers are usually cheap plastic), I was able to find a cast one at a local Car Quest store. I've been told that not all Car Quest stores carry the cast metal slave, so don't count on being able to find one there. Once you do find a cast slave, take it apart and make sure there aren't metal filings in the bore. Every single new slave I've taken apart had some form of debris inside the bore, just waiting to cut up the seal. Taking it apart is easy, cleaning it out is easy, and not being stranded on the side of the road because your slave cylinder seal is still working properly is cool. This is also a great time to check that the seal is installed in the proper direction. McLeod makes a really nice metal master cylinder, but again quality control issues surfaced with it. CNC was their manufacturer and they were installing the wrong size piston for quite awhile. In mine, the wrong piston was not only installed, but the spring was installed backwards and was cutting into the cup seal. This created a situation in where the master would pass the initial quality check because it functioned, albeit at a lesser capacity due to the wrong piston size, but it did function. After a month or so, and a few thousand clutch presses, the spring finally tore the cup seal and I was stranded on the side of road. After much hoopla, I was finally able to get McLeod and CNC to correct the problem on a large level. If you get one of these masters from McLeod now, it should be correct. It wouldn't hurt to pop it apart though and double check it. An ounce of prevention is worth a pound of cure. I've also heard that there are parts store replacement master cylinders that are cast, as opposed to the cheesy plastic stock style ones. I've never seen one, but that doesn't mean they don't exist. If you do find a cast metal master at a parts store, snap some pictures and post them up.

Keep the fluid fresh, especially if you abuse the car at some kind of racing venue. Most people use a high quality brake fluid, but I like to use the GM clutch fluid. The GM clutch fluid has some additives to help condition seals, and the main benefit that I've found when using it is that the clutch hydraulics are less prone to squeaking when you push in the pedal.

I'm sure there is a ton more, but that should help most of the new people looking to replace their ZF6 clutch.

Thanks! Before we write it in stone though, I would add a few things.

First off... the clutch/flywheel info applies only to 1-pc rear main cars. The 84/85 and other 2-pc seal crank flanges won't bolt up to those flywheels (obviously), and the clutch options are severly limited due to less clutch hub clearance in the flywheel.

Second... I would add my 2 cents on the clutch slave cylinders, but I'm still not 100% sure on some of the info. Hopefully someone else can fill in the details.

The DOM (smooth steel) slave cylinders by Raybestos, AC-Delco, and others were assembled with backwards seals. This problem was addressed recently, but old stock may still be on the shelves, so check them anyways.

The cast slave cylinders do not have a seal issue, but are not available at most parts stores. They are manufactured by PBR, p/n:W0133-1621927, and Wagner(?), p/n: ?

The 89-91 cast (or both?) slave cylinder is discontinued. The 92+ slave will bolt in its place except with the bleeder pointing downward.



Eventually we're gonna have to morph bogus' FAQ file into something that can hold all this info. Also, I'll add slave and flywheel to your title so it comes up better in searches.

I get ahead of myself sometimes and don't think about things from a complete new person's point of view. I took for granted that everyone knew a ZF6 was from 1989 forward, good catch.

Well, obviously it is (well to most people), but my point was more that there are plenty of 84-88 cars, and maybe even some C3s and earlier cars with ZF conversions.

Lots of great information Josh! Thanks!

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At some point the FAQ will need some serious organization. Most other car forums use a system where subjects are broken down into links that split off to other threads, or new posts in the thread. I think that method makes the information much more usable so a person doesn't have to sift through many paragraphs to get at what they want..

I just received a new clutch slave (92-96), made by Rhino Pac from RockAuto- it is the cast design, FWIW.

According to Chris May @ Superior, GM has discontinued the 92-96 slave now.

One thing.

On carolinaclutch.com it says that their stage 1 and stage 2 has to be used with a 1 piece flywheel

Your writeup says stage 2 is good for the DM

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Stock unit survives just fine with 600 RWHP.

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I made some changes to his FAQ on CF a few months back, and added some info I dug up and info you submitted to me.

Here is some of the info I compiled:

http://members.cisdi.com/~anesthes/zf6/


-- Joe

My stock units slipped on my 85 with drag radials, but that was with the non-ZR1 plate and non-ZR1 pivot. The stock pressure plate is too shallow, and moves the fork out against the slave in such a way that the slave cylinder preloads it. I believe this reduces the clampling force of the clutch and is also why the clutch engages/disengages so far up, and makes the car a little tricker to drive on the street. I found myself backing the clutch out and waiting, waiting, for that engagement point to finally happen, which is hard to get used to. Other people would find it awkward when driving the car for the first time.

I haven't been back to the strip on my properly working clutch setup.

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That is why I opted to use the ZR1 pivot stud you sent me, to fix the geometry. My clutch grabs right off the floor like it should with no slip.

-- Joe

And that may ultimately prove to be the better solution if the ZR1 plate becomes hard to purchase. A pivot stud will last much longer than a pressure plate. I'd love to drive your car and compare. There's so many butchered conversions out there.

In fact, I'd consider with few exceptions, that every single 84-86 ZF6 conversion out there is done wrong, and mine used to be one of em.

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Next time you are in Boston give me a shout, take it for a spin.

Like I said before, I still hate the way the ZF shifts - and it shifts like every other one I've been in. I realize it's the shifter.. I should have changed it while the trans was out but. No shop now, no lifts so that will have to wait until I build a garage next year.

The only thing rattling in the back of my mind (other than the trans) is I didn't put locktite on the pivot stud. The ZR1 BH is thicker, so you can put the "cap" on the end screwed into the pivot stud to jam it. Not possible with the L98/LT1 BH.. I should have used locktite. I hope that thing doesn't ever vibrate loose. If that contacts the spinning pressure plate it will come through the BH like a rocket. Luckily the motor turns clockwise so it should spin it down to the ground..

-- Joe

The LT1s came with a screw cap for the pivot stud eventually.

I thought I included that with what I sent?

Or are you saying it doesn't work because the ZR1 stud threads are too long to use the cap with the LT1 bell?

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The threads are too long, and if it wasn't for the fact that there is an indent in the casting on the trans I'd have had to grind them down as they stick out about 1/4" beyond the bell housing.


-- Joe

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Really, if that's the case I'll stick with my stocker for my blower build. I was going to get a dual disk setup.

Though a friend of mine fried his cluth with a 383 LT1, this was on slicks BTW.

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415 CI LT5, stock drive train with drag radials. 3:73 gears.


http://www.pnwzr1.com/images/BG_08/Movies/MVI_1332.avi

&

http://www.pnwzr1.com/images/BG_08/Movies/MVI_1341.avi

Congrats, powerful car, hauling the mail for sure.

I'm still not sure what you are trying to convey though. One and a half passes at Bowling Green doesn't really tell me/the forum much about the car.

If 11.49 is the best the car has gone, that sort of proves my point about the clutch lasting as long as you don't launch the car. 11.49 is dang fast, no doubt about it, but to do it at 132 MPH means that you aren't exactly rotating the Earth on the launch. I would bet that run was, at best, a 1.9 short time. Unfortunately, I don't have enough information to really form an opinion (other than "That car kicks ass!")

By way of comparison, my 11.99 is only 1/2 a second slower in ET, but literally 20 MPH slower on the big end. I cut a 1.70 on the 11.99 pass. Jonecap went 11.12 a couple weeks ago at 127. 1.75 or so. Obviously, I'm not rotating the Earth on my launches either, but I'm coming out harder than the car in the video. Maybe that is why you are able to use the stock clutch and I'm not. I'd love to be able to muscle my way to an awesome time like the car in the video, but my car just doesn't make that kind of power. To get mine into the 11's I have to leave pretty hard (relatively speaking).

On the second video, the launch looks better, but I don't have any actual data from which to form an opinion. It also sounds to me like the driver slipped the clutch a bit on the launch, snatched second (with no real discernible RPM drop - which might indicate clutch slip, I dunno though) then missed third, which could also be indicative of the heat in the clutch from the slip through first and second. Again though, I don't have enough information to really form an opinion.

The car is fast though, no doubt. It's powerful too, pushing a full weight C4 to 132 is no joke.

I realize you must be busy, your time is valuable, and that posting here is probably the last thing on your 'to do' list; but I'm asking/begging, for those of us like myself who are too stupid to understand what you mean, could you go into a little more detail?

The questions burning in my mind are:

Is the 11.49 the best the car has gone? If not, what is the best time, and what is the typical time.

How many passes per week/month/year does the car see? Miles on the clutch?

What kind of 60 foot time does the car normally turn?

What do you attribute the longevity of your stock clutch to? Luck of the draw, driver skill, or ???

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The clutch discs are exactly the same.


And the pressure plates are almost the same, except the ZR1 has a thicker plate and a thicker frame to match, but the spring diaphram looks exactly the same.

Any replacement clutch is going to have the same holding pressure. It has to be within the specs of the hydraulic system. The difference is the friction materials. I'm using a dual friction from fort wayne. I've used his clutches for many years now, on some pretty fast builds.

-- Joe

By dual friction do you mean a single disc with different materials on each side; i.e. ceramic on one side and kevlar on the other?

Or do you mean a pressure plate with centrifugal weights that are supposed to affect holding power, like the Center Force?