Parts Catalog Accessories Catalog Tech Info Tech Forums
 
  Search our site:    
View Recent Cars  |   Cart  | Project List | Order Status | Help    
 >  >
Replacing Porsche 911 Carrera Limited Slip Differential Carrier Bearings and Seals
 
Bookmark and Share

Pelican Technical Article:

Replacing Porsche 911 Carrera Limited Slip Differential Carrier Bearings and Seals

Time:

8 hours8 hrs

Tab:

$2500

Talent:

*****

Tools:

Torx set, slide hammer, gear puller

Applicable Models:

Porsche 996 Carrera models (1999-05)
Porsche 996 Turbo, GT2, GT3 (2001-05)
Porsche 997 Carrera models (2005-12)
Porsche 997 Turbo, GT2, GT3 (2007-13)

Parts Required:

LSD, differential carrier bearings, seals

Hot Tip:

It's best to perform this installation with the transmission out of the car, during a clutch job

Performance Gain:

Better traction and performance

Complementary Modification:

Clutch replacement

While there certainly isn't enough space in this book to cover a complete transmission rebuild, there are a few tasks that can be performed to upgrade and restore the differential portion of the transmission.

Transmission Internals: If your transmission is leaking from the driveshaft area, your differential seals are probably shot. The photo array in this project shows you how to pull your axles and replace these seals. At the same time, you can replace your differential carrier bearings. These are the bearings that support the output flanges in the transmission. Sometimes when you have a grinding noise or high-pitched whine that you cannot locate, it can be your differential carrier bearings. Often when you've replaced your wheel bearings (Pelican Technical Article: Replacing Wheel Bearings on your Porsche 911 Carrera) and your CV joints (Pelican Technical Article: Replacing CV Joints on your Porsche 911 Carrera) and you still have a whining noise, it's the carrier bearings that are worn.

The best time to perform this work on your transmission is when you have it out of the car for a clutch job or engine work. You can perform these tasks with the transmission still installed in the car, but it makes life much more difficult if you do so.

When you install a new differential into your transmission, you need to make sure that you have the proper shims for the differential carrier bearings. When you install your new differential, you will need to provide a measurement to the shop that provided the differential so that they can provide you with the appropriate shim set.

Limited Slip Differentials: Gears in the differential allow the gears to rotate at different speeds, but supply torque (rotational force) to each axle equally. If one wheel is on ice and another wheel is mounted firmly on pavement, the wheel on ice will spin at twice the speed of the ring gear, while the wheel on the ground will not spin at all. Each wheel gets the same amount of torque, and since the wheel on the ice requires very little torque to spin, the wheel on the ground also receives very little torque. Likewise, in performance driving, when turning around a corner, the weight shift due to cornering forces may increase or reduce the effective weight placed on each drive wheel. If, for example, during cornering, the inside drive wheel comes completely off the ground (unlikely, but let's assume it for demonstration purposes), then the situation becomes very similar to the case where one wheel was on ice and the other was on the ground. The differential will supply less torque (power) to the outside wheel as the inside wheel begins to slip. This is the primary argument for using a limited slip differential. It's important to note that under normal, everyday street driving, you will almost never encounter this situation. Thus, the installation of an LSD is often overkill for street-only cars.

An LSD contains small plates inside called clutches that limit and constrain the movement of the side gears. Springs or spring plates inside the LSD force the gears outward against the clutch plates, which in turn forces them outward against the differential housing. The friction between the plates causes the side gears and housing to rotate at the same speed. However, the springs and clutch plates are not strong enough to prevent normal differential rotation of the wheels on curves. When one wheel loses traction, however, the clutches will limit the "slip" and provide some additional torque to the non-spinning wheel. The amount of torque provided is determined by the clutch plates and the springs and is called the torque bias.

Torque bias indicates the ratio of the torque that can be transmitted to the high-torque (high-grip or ground) axle, divided by the low-torque (low-grip or ice) axle. A standard open differential often has a built-in torque bias ratio of about 1 to 1.3. A limited slip differential can provide almost any torque-bias level depending upon the arrangement of the clutch discs and strength of the springs inside. Stronger springs means a higher torque bias.

A torque bias of about 1.4 (40 percent) is best suitable for mid-performance street cars that will inhabit the occasional autocross or cars that will be driven on the track with stock engines and suspension. A more aggressive bias of 1.6 (60 percent) is best for modified street/track cars that have stiffer suspension and perhaps an upgraded or larger engine. Track-only cars that are not going to see any street time often run LSDs with torque bias levels of 1.8 (80 percent).

Clutch-type LSDs provide excellent lock-up on both acceleration and deceleration, and the units can be customized by changing the sequence of the internal clutch plates and springs. Differential lock-up on deceleration allows for late braking and very aggressive driving into high-speed turns.

You also need to make sure that you fill your transmission with fluid that is compatible with your differential. Typically a manufacturer will have some recommendations for transmission gear oil that works well with their particular differential. If you use oil that is too slippery, you may reduce your torque bias and render the LSD less effective. If you use oil that is not slippery enough, you may increase the bias and encourage premature wear of the clutch discs inside of the LSD.

As stated previously, limited slip differentials are not necessarily ideal for street driving. The clutch-pack limited slip can have a nasty habit of locking up the rear differential at inopportune times, like when you are cornering a road in the rain or on slick surfaces like ice. The reason for this is that sometimes these surfaces don't provide enough friction to provide for the normal differential action that allows slip between the two wheels. Limited slip differentials also have other drawbacks. They tend to be noisier than open differentials, the clutch discs wear out because they are friction components, you need to use special transmission lubricants, tire wear is increased, and overall fuel economy is reduced. They also exhibit a slight time lag between when the clutch springs compress and when the torque is transferred. For these reasons, I primarily recommend that people avoid traditional limited slip differentials in street cars.

If you're looking for an alternative to an LSD for your street car, then I recommend looking at what is known as a torque biasing differential (TSB). These are differentials that are similar to conventional open differentials, but can lock up if a torque imbalance occurs. Through a complex arrangement of gears, the TSB units provide some biasing of torque toward an unloaded axle, but only if that particular axle remains planted firmly on the ground. TSB differentials are a good choice for street cars because they act mostly like an open differential, except when cornering begins to skew the traction between both wheels. TSB units, such as the differentials manufactured by Quaife, only provide lock-up on acceleration though, which makes these units better suited for slower-speed turns like you would find during an autocross. Using a high-bias clutch-type LSD in an autocross would likely cause a significant amount of unwanted understeer.

Transmission Gear Ratios: Another thought to consider is your choice of transmission gears. A poorly matched transmission can make the most powerful engine seem sluggish. Nearly all of the 996 and 997 Carrera engines have a somewhat high-rpm powerband (like the early 911 S). Because of this, you will probably want a transmission with very close ratio gears. This will allow you to maintain your optimum powerband and maximize the power output to the wheels. The six-speed Carrera transmission is ideal for this purpose. It's not uncommon to find Porsche race cars specifically designed for long tracks and rolling starts that have a "tall" first gear. This basically allows the racers to use first gear for actual track use, which effectively creates a true six-speed transmission for racing. Such a car would be very difficult to drive on the street, because "off-the-line" performance would be quite sluggish. However, on the track in the narrow powerband is where the drivetrain would shine, delivering peak power in a powerband closely matched to the transmission and the type of racetrack. For more information on choosing gear ratios, see chapter nine in the book Gearing and Differentials in Race Car Engineering & Mechanics by Paul Van Valkenburgh.

The first step is to remove the half shafts from the transmission.
Figure 1

The first step is to remove the half shafts from the transmission. Begin by removing the center bolt that fastens the half shaft to the transmission. To pull out the half shaft, I used a slide hammer, combined with an old CV joint, as shown. Place the end of the slide hammer shaft against the half-shaft flange and then fasten it down with two CV bolts. Tap the hammer along the shaft and the half shaft should slide out of the transmission. Another method you can use involves placing two bolts into the half-shaft flange and then using them to wedge the half shaft out of the transmission (inset photo). This is the method documented in the Porsche factory manuals. The photos for this particular project are from a Boxster transmission, which is extremely similar to the Carrera transmission. Some of the photos have been flipped around and modified to display similarity to the Carrera transmission housing.

Here's a photo of the half shaft after it has been pulled out of the transmission.
Figure 2

Here's a photo of the half shaft after it has been pulled out of the transmission. Although it's more difficult in the tight space, you can remove the half shaft while the transmission is still in the car. For clarity in the photos, these tasks were performed on a transmission that was out of the car and on my bench.

After you have the half shafts removed, you can replace the differential shaft seals.
Figure 3

After you have the half shafts removed, you can replace the differential shaft seals. There is one on each side of the transmission, and these seal the driveshaft flanges to the transmission case. If they are old and leaking, then you will see transmission fluid leaking around your axles. Pull out the old seal, and then gently tap in the new one.

With the half shafts removed, you can then pull off the differential cover.
Figure 4

With the half shafts removed, you can then pull off the differential cover. Remove the screws on the outside of the cover (inset photo). Removing the cover will expose the differential inside the transmission. Be sure that you have emptied all of the transmission fluid out of the unit before you remove the cover--otherwise you will have a big mess on your hands. Be prepared for some residual fluid to leak out when you remove the cover. The six-speed transmissions have a large O-ring on the differential cover (not shown on this transmission) that I recommend replacing when you reseal it.

With the cover removed, you should be able simply to pull out the differential.
Figure 5

With the cover removed, you should be able simply to pull out the differential. This is what an open differential looks like. It has planetary gears that distribute and provide equal torque to each wheel. This type of differential allows for both wheels to rotate and spin at different rates of speed, such as when the car is going around a corner or turn.

Here's the view inside the transmission case.
Figure 6

Here's the view inside the transmission case. The curved gear on the right (yellow arrow) is attached to the pinion shaft and mates with the ring gear that is attached to the differential. On some transmissions there is a magnet in the case that attracts debris and metallic parts that have worn in the transmission (blue arrow). Take a paper towel and thoroughly clean this magnet, removing any grit or grime attached to it.

If you are replacing the differential carrier bearings, then use a bearing puller to remove the old ones off of the transmission.
Figure 7

If you are replacing the differential carrier bearings, then use a bearing puller to remove the old ones off of the transmission. If they are difficult to pull off, then you might try lightly heating the bearing with a propane torch to loosen it up.

New bearings need to be pressed on in a similar manner.
Figure 8

New bearings need to be pressed on in a similar manner. If you heat them in an oven or on a hot plate beforehand, it can make their installation much easier (obviously don't pick them up with your bare hands as shown in this photo if they are hot). The open differential is shown here with new carrier bearings installed. Don't forget the spacer and any shims that you may have taken off when you disassembled the unit. If you are installing a new differential, then you will need to obtain new shims that are matched to your transmission.

Shown here is a limited slip differential from Guard Transmission.
Figure 9

Shown here is a limited slip differential from Guard Transmission. GT is one of the leading providers of LDS differentials to the Porsche market, having earned their stripes designing race transmission components for the cars that competed in the GT class of the American Le Mans series. In addition, GT is an OEM supplier to Porsche AG with components used in the factory race cars on a regular basis. The unit I chose here is a street/track version with 60/40 biasing, which is ideal for a 911 Carrera with a stiffer suspension.

Here is the GT LSD installed onto the ring gear and fitted with new bearings.
Figure 10

Here is the GT LSD installed onto the ring gear and fitted with new bearings. As mentioned previously, be sure to confirm that you install the correctly sized shims with the new differential, as there have been some changes over the years.

Here's a side shot of the GT LSD installed back into the transmission.
Figure 11

Here's a side shot of the GT LSD installed back into the transmission. The inset photo shows how the half shafts sit inside the differential (shown without the differential cover installed).

When reinstalling your half shafts, be sure to use a new circlip on the end as mentioned in the Porsche factory manuals.
Figure 12

When reinstalling your half shafts, be sure to use a new circlip on the end as mentioned in the Porsche factory manuals. The part number for all Carreras (1998-2005) is Picture 332:-Picture 00: and 2005- 2008 is Picture 332:-Picture 01:

Shown here is a Quaife Automatic Torque Biasing Differential.
Figure 13

Shown here is a Quaife Automatic Torque Biasing Differential. This is a slightly different type of limited slip differential that is often a good compromise between street and track use for everyday drivers. Geared LSDs, such as the Quaife units, provide friction through the gears and their supports rather than the clutch plates found in typical LSDs. However, both output shafts must be loaded in order to maintain the proper torque distribution within the unit. If one shaft of the differential becomes free, then no torque is transmitted to that shaft, and the differential behaves very much like an open differential at that point.

Bookmark and Share
Comments and Suggestions:
son of krypton Comments: What is the part number for the two circlips on the half shafts from figure 12 996?
April 20, 2016
  Followup from the Pelican Staff: I’m not the best with part numbers.

Give our parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
 
selsel Comments: Do you know the weight of the open diff? I have also had a Guard lsd installed, and would like to know the weight difference.
February 17, 2016
  Followup from the Pelican Staff: I don't. I opened a post in our forums. A Pelican community member may be able to answer your question.
- Nick at Pelican Parts
 
Sanderz Comments: how much oil needs to be filled?
where you can read this off
manny thanks!
May 5, 2015
  Followup from the Pelican Staff: If it is a 996 C2 they usually take 3-3.5 liters. - Casey at Pelican Parts  
Kone Killer Comments: I have a 1999 996, I have called a couple shops regarding putting a LSD in and they all have told me there is some machining of the falnges to properly fit the GT or a OS GIKEN unit. Your thoughts.

V/r

Bob Hallett
May 20, 2013
  Followup from the Pelican Staff: The factory 996 LSD obviously fits no problem. As far as a retrofit or aftermarket, you'll have to trust your installer. it depends on the model of the LSD.

I opened a post in our forums, a community member may be able to better answer your question. - Nick at Pelican Parts
 

  Search our site:    

View Cart & CheckOut | Project List | Order Status |  Help    

 

[Home] [Customer Service] [Shopping Cart] [Project/Wish List]
  [Privacy Statement]  [Contact Us] [About Us] [Shipping] [Careers]

Copyright © Pelican Parts Inc. -    DMCA Registered Agent Contact Page

Page last updated: Sat 12/3/2016 02:32:49 AM