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· Registered
765 Posts
Discussion Starter · #1 ·
I have a 94 Supra Turbo, im just wondering if its a common problem or if anyone else has had this problem. I have trouble starting the car sometimes, i noticed that sometimes i have to press real hard on the clutch pedal before the car would start up. Do i need to replace the switch that the clutch pedal triggers to start the car? Wondering if that is a common problem or not.

· Lagmeister
2,867 Posts
quoted from -carchitect-

"Ok. This is the best way to explain this. On a car with a standard transmission the clutch is released by applying pressure to the pressure plate. Most cars use a push clutch but the mk4 is the only supra that uses a pull clutch.

This means exactly what it says. The clutch release bearing pulls the fingers of the pressure plate away from the flywheel. What this causes in the long run?

Well Toyota had engineered the clutch to work with the power of the vehicle. So the pressure was moderate enough to normally prevent the crank from moving. Now adding power is our thirst . This causes the clutch to slip since the pressure plate clamping force is now too weak to hold the power. So when we upgrade the clutch there are multiple options. Of course all of us would like to have a clutch that feels like it gives us some feel back but don't want one which will endup adding meat to the thigh. A lot of manufacturers would change the pressure plate pivot to relieve the pressure but this gives you more travel. They further reduce the marcel in between the clutch disc lining to reduce travel. Some have even worked with other composites like kevlar to give better coefficient of friction and increase clutch life as well. But all boils down to how much friction surface area do you have to work with? An 800 hp clutch will be tough to use. We all endup with a clutch which has a lot of pedal force to release the clutch. This all ends up tearing up the thrust bearing.

So what is the thrust bearing? What does it do?

This bearing prevents the crank from moving front to back. Its supposed to have a marginal amount of movement to allow for the oil to lubricate it. But when the clutch is engaged this bearing is receiving wear. There are two halves to this bearing. But unlike the main or rod bearings, these bearings are only halves and are only used in the upper halves of the block. These bearings are only used in one if the main bearing areas isntead of all seven. Since the Mk4 is a pull clutch it will only have its wear on the rear thrust bearing. Other cars will have its wear on the front thrust bearing.

So how can I prevent this?

If you have a vehicle that has a T-78/ T04r/ T51R you have a lot of power. This requires you to use a very strong clutch. A single disc clutch will endup with a stiffer pedal an cause rather rapid wear on these bearings. A multi-disc like the TRD and RPS twin disc that uses a pull type clutch. The HKS double and tripple disc, I believe, use a push clutch and this may be the reason why you need to use parts of the Mk3 to use this clutch. I'm not so sure if the blitz, Os giken or Tilton carbon fiber clutches are push or pull configurations. Any pull clutch will wear out the rear thrust bearing and the push clutch will wear out the front thrust bearing.

Multi-disc will allow the clutch to have more surface area. By adding multiple discs the friction surface is multiplied so the clamping force doesn't need to be increased as much. This will endup giving you a lot more life out of your thrust bearings. Also the clutch hydraulics will last a lot longer.

So I really can't prevent this from happening?

No, but you can minimise this. You can bypass the clutch start switch to allow the oil pressure to buildup before you use your clutch. This will allow the bearing a lot more lubrication instead of using the clutch while the oil pressure is zero psi while the engine is started early morning.

You could also have the engine gone thru and have the insides of the crankcase machined to accept additional thrust bearings. This may or may not be possible w/o crank modification or a having a custom crank built.

You could also replace the thrust bearings every so many miles to prevent any wear on the engine but it envolves a lot of work.

By the way the auto tranny guys really don't have this kind of problem.

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