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Clutches for Dummies

Iin the first part of this writeup, we will review how your basic hydraulicly-assisted clutch works. Then we will address some very common clutch system woes that plague the LS1 community.

The flywheel is connected to the engine, and the clutch plate is connected to the transmission. When your foot is off the pedal, the springs in the pressure plate push the pressure plate against the clutch disc, which in turn presses against the flywheel. This locks the engine to the transmission input shaft, causing them to spin at the same speed. The amount of force the clutch can hold depends on the friction between the clutch plate and the flywheel, and how much force the springs put on the pressure plate.

When the clutch pedal is pressed, it engages the master cylinder, which draws fluid from the clutch fluid reservoir and pushes it via the hydraulic line to the slave cylinder. The slave cylinder then engages a hydraulic piston that pushes on the release fork, which in turn presses the throw-out bearing against the middle of the diaphragm spring.

As the middle of the diaphragm spring is pushed in, a series of pins near the outside of the spring causes the spring to pull the pressure plate away from the clutch disc (see below). This releases the clutch from the spinning engine. Note the springs in the clutch plate. These springs help to isolate the transmission from the shock of the clutch engaging.

COMMON PROBLEMS

Problem: I have not upgraded my clutch or my supporting components. The clutch smells very bad and there is a noticable slip.(A quick way to check if your clutch is slipping is to place it in 6th gear at about 2000 rpms and floor it, if the RPM rise quicker than the car accelerates then it is in fact slipping.)

Solution: The most common problem with clutches is that the friction material on the disc wears out. The friction material on a clutch disc is very similar to the friction material on the pads of a disc brake, or the shoes of a drum brake -- after a while, it wears away. When most or all of the friction material is gone, the clutch will start to slip and eventually it won't transmit any power from the engine to the wheels. The clutch only wears while the clutch disc and the flywheel are spinning at different speeds. When they are locked together, the friction material is held tightly against the flywheel, and they spin in sync. It is only when the clutch disc is slipping against the flywheel that wearing occurs.

So if you are the type of driver who slips the clutch a lot, you will wear out your clutch a lot faster. If your clutch is slipping this much then it is time for a new clutch disk and flywheel. It is at this time that most people chose to upgrade their whole clutch system (a good idea).

Problem: My clutch will not engage at all. I push the clutch pedal in but it will not go into gear.

Solution: There are a few reasons for this. Most often you have a bad throwout bearing or related clutch assembly part. For this particular trouble it is time to replace you clutch assembly and it is also a good idea to either replace your flywheel or have it resurfaced as it will not cost you more to have these parts put in at the same time.

Problem: Under normal driving my clutch works fine, but at wide open throttle my clutch sticks to the floor in 3rd gear and I cannot get it to shift into 4th.

Solution: This is a very common problem with LS1 f-bodies. You do not have a fried clutch, the problem 90% of the time happens after you got you car tuned and/or raised your rpm limiter. GM put a restriction in the hydraulic line between the master cylinder and the slave. This restriction lets only a measured amount of clutch fluid flow from one to the other. This restriction was placed in the line to make shifts feel more smooth and to give the pedal a softer feel. The solution to this line restriction is a simple procedure referred to as the drill mod.

In short, it involves removing the line, using a drill to bore out the restriction and then replacing the line. For instruction on doing this yourself, go to www.installuniversity.com

Problem: Under normal driving my clutch feels soft but works fine. Under wide open throttle there are many times I can?t get the car to shift at any given gear until the RPMs come down. I have already done the drill mod (or not). What could it be?

Solution: The answer to this problem is simple. However, the reason the problem exists is not so simple. This symptom almost always points to air in your hydraulic lines. The reason this causes problems is because, unlike the clutch fluid that is meant to occupy that space and is very incompressible, the air that now resides in the line can be easily compressed. This creates that "squishy" feeling you get in the clutch pedal. Also (and more importantly) the fact that the air compresses means that the fluid is not pushing the pressure plate to its full disengaged state. Under wide open throttle, where the clutch requires even more pressure to fully disengage the pressure plate, the compression of said air is often severe enough to not allow the pressure plate to disengage at all, making it nearly impossible to get into the next gear up. The fix is to bleed down your clutch hydraulics very carefully until all the air bubbles have been removed. There is a write up on how to bleed your clutch hydraulics on www.installuniversity.com, or you can go to any shop and have it done for a very nominal fee.

Now the bad news: If you never have this problem again (lets say after 3 months), then you are fine and the air probably got there when you performed the drill mod. However, if you have either not done the drill mod, or the symptom returns, then you have a bigger problem. There is most likely a leak in your hydraulic system.

The first culprit to look at is your master / slave line. Inspect it for kinks or pin holes. Next is your slave cylinder. If you have gone through the trouble to remove it go ahead and replace it.

Problem: I just replaced my worn out clutch with a brand new clutch. The clutch grabs strong and does not slip at all, but the clutch disengagement happens only 1-2 inches from the floor board and in wide open throttle at higher RPMs it will not shift into the next gears.

Solution: Often, people replace their tired clutch with the latest /greatest clutch, but forget to upgrade the supporting pieces. Because the brand new clutch and flywheel that you just installed have MUCH more material than your old worn-out piece, it takes more pressure to completely disengage the clutch disc from the new flywheel. The first step in fixing this problem is to do the drill mod discussed before. If that does not cure your trouble then it means that your clutch requires much more fluid pressure in order to operate properly. Your stock master--even after the drill mod--can only push a very set amount of fluid. If this set amount is not enough to push the pressure plate then it is time to step up to an adjustable master cylinder. An adjustable master cylinder will allow you to increase the fluid pressure enough to operate your new clutch. It will also allow you to adjust exactly where in your pedal travel your clutch engages and disengages.

There is one company that attempts a more simplistic fix known as a slave shim. This effectively shortens the space that the fluid resides in, therefore putting a "preload" on your fluid pressure. I don?t like this fix for two reasons: 1-it is not adjustable, making it impossible to correct your pressure settings as your clutch pad and flywheel wears, which makes it necessary to replace both of them sooner than you would have to if you could adjust the correct amount of pressure; and 2-because it is not adjustable, it does not account for those whose master cylinder puts out a little more fluid than it was built to. For these people, they will notice that their clutch engages and disengages in the very first 1-2 inches of clutch pedal travel. If this is the case, there is a very good chance that
your clutch is never fully engaging, which will result in a broken or worn out clutch disk very early in the new clutch?s life, and a very displeased customer who thinks this part was junk.

In closing, what we have learned is that the vast majority of clutch problems are not really problems with the clutch itself. Rather, most of the time the trouble lies in the supporting components. Many people will say that when they replaced their clutch that it fixed a trouble similar to yours; however, if they took a step back and looked they would see that a few good clutch manufacturers instruct you to upgrade the pieces mentioned before. Those pieces, not the clutch, may have been what fixed their problem. So, a word to the wise: check the simple, less expensive supporting components before you drop $500-$1200 on that new clutch.
 
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