924/944/968 Frequently Asked Questions

9.0 Clutch & Transmission

9.1 When I release my clutch in neutral, I hear noises, is this bad?

Not usually. The transmission has several shafts that the gears move back and forth on. On the end of one of these shafts is the 5th "floating" gear. This gear rattles back and forth because it has a larger tolerance than the others. Porsche apparently stopped all warranty repairs for this noise because there is no real repair - the larger tolerance is normal. The noise is particularly evident because it vibrates the input shaft which is connected to the torque tube that runs up the middle of your car. This creates a megaphone type effect that amplifies the noise.

If you're unsure if your particularly "noise" is normal or not, taking it to a mechanic who has looked at a few 944's should clear the situation up for you. Some owners have expressed that using Swepco transmission oil quiets down the rattle somewhat.

9.2 I'm replacing my clutch, any helpful hints?

On cars with an hydraulic clutch, bleeding the clutch circuit can be quite an adventure. Using a positive pressure bleeding apparatus like the Gunson E-Z Bleed kit ($35 and uses compressed air from a tire) helps a lot. Another useful hint is to jack the back end of the car higher than the front while bleeding, this helps trapped air to escape.

Replace all the parts, not just the obviously broken ones. You should purchase and install a new friction disk, pressure plate, guide tube, throw-out bearing, and pilot bearing. It is a good idea to have the flywheel inspected, resurfaced, and balanced with the disc and pressure plate. The release arm bearings and lever should be inspected, cleaned, and lubricated prior to reassembly. Given the amount of time and effort required to service a 944's clutch, doing the job right the first time is very important.

The flywheel does not normally need to be removed and resurfaced. If it has deep or uneven grooving, such as is caused by a broken spring, you should consider resurfacing it. Check the spec book for minimum thickness after machining, and replace the flywheel if it is too thin.

9.3 Is there a complete clutch replacement procedure for the 944/951?

Michael Kehr has written an astoundingly detailed clutch replacement procedure for the 944/951. You can find a copy of his write-up right here.

His write-up applies by and large to the 924 as well. There is only one clutch disk design for the 924 (with the 2.0 engine), and this has the standard spring arrangement in the clutch disk. As Shrikumar discovered after weeks of weighing the relative merits and demerits of updating from rubber-center to spring-center, this entire discussion is moot in the case of the 924.

There are two types of friction disks used in 924S and 944 series cars. Normally-aspirated 944 and the 924S use a friction disk which has a rubber center, basically a large puck that’s fused to the disk and to a splined tube.

9.4 I have noticed a large amount of backlash in the drivetrain when engaging/releasing the clutch. What causes this?

There are two types of friction disks used in 924S and 944 series cars. Normally-aspirated cars use a friction disk which has a rubber center, basically a large puck that's fused to the disk and to a splined tube. The disk is sandwiched between the pressure plate and the flywheel, from which it is torqued by the running engine. Force travels through the rubber center to the splined tube, which is located on the drive shaft, which in turn transmits engine torque to the transmission at the rear of the car.

Instead of a rubber center, the turbocharged cars use a series of large and small coil springs within the friction disk. This design is able to tolerate higher torque loads than the rubber center. The disadvantage to the spring-centered design is that it transmits more mechanical shock to the transmission.

With age or abuse, the rubber center in normally-aspirated cars' friction disk can deteriorate then break. When it has deteriorated, chunks of rubber can separate and sometimes lodge themselves between the disk and the flywheel, inhibiting normal clutching. The missing rubber can also produce a noticable drivetrain vibration, typically aroung 3000 RPM.

If the rubber center breaks, all hope is not immediately lost. The friction disk has built into it a limp-home backup. There are mechanical stops which will keep the splined tube and disk connected enough for you to drive the car home or to the shop -- if you are gentle. You'll feel the backlash as you hit the stops in either direction: accelerating or decelerating. These stops are not terribly strong and can be destroyed if you drive aggresively on them.

Spring-centered clutches are not immune from failure. Their springs will break and, as with the rubber design, will lodge in a way which disable the clutch. The original 944 Turbo friction disk also had problem with its friction material separating from the backing plate. Porsche introduced an improved design which includes adhesive bonding in addition to riveting.

9.5 What transmission fluid should I use?

There are endless discussions of different transmission fluids on Internet forums. The advice, while well-intended, can be confusing and is often wrong. The topic of greatest contention seems to be whether a type GL 5 fluid may be safely used in our cars because until 1988, owner's manuals and factory literature call for a type GL 4 fluid. There are also published cautions about negative effects that additives used in type GL 5 fluids may have on brass synchronizers.

Porsche issued a service bulletin 3-8813 in 1988 requiring using of GL 5 fluids in all manual transmissions. My own experience, after twenty years' experience rebuilding 924/944/968 series transmissions, is that GL 5 fluids work fine and do not damage synchronizers.

The most important thing here is to change your fluid on a periodic basis, which can be every three years but will depend to some extent on how you use your car and where you live. The hypoid (also call palloid or Klingelnberg) design of the pinion gear puts great sliding stresses on the gear teeth, which makes good quality lubrication critical. All lubricants degrade with use, age, and contamination, and dirt is a poor lubricant.

There are many reputable brands that can be safely used in our transmissions. When picking a lubricant, read enough of the manufacturer's specification to be sure it is appropriate for your car. In particular, confirm that the fluid is specified for a hypoid pinion gear. For example, while Redline makes many fine products, their web spec sheet specifically states that "MTL, MT-85 & MT-90 are not for use in differentials with hypoid gears."

The recommended viscosity range for our transmissions is a 75W90 oil. It is possible to use lubricants with slightly different temperature characteristics, such as an 80W90. When doing so, pay attention to shifting smoothness and if you find you're having issues, reconsider the viscosity you are using. There is no good reason to use a single-weight lubricant, and there is no benefit to using mineral (dinosaur) oil rather than a synthetic. Among other things, synthetic lubricants have the wonderful benefit of greater viscosity stability across wide temperature ranges.

Mobil 1 LS 75W90 works well in 924/944/968 series transmissions in all climates and seasons. Owners who live in warmer areas or who are willing to carefully warm their transmissions report equally positive experiences with Swepco 201. My own experience living in the Northeastern US is that Swepco makes shifting stiff and difficult in cold weather. You do drive your Porsche year 'round, don't you?

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