| The ZF6 clutch/flywheel/slave answer thread |
Subject: The ZF6 clutch/flywheel/slave answer thread by Josh on 2008/4/2 22:12:32 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. |
