Pelican Technical Article: Limited Slip Differential (LSD) Installation Wayne R. Dempsey
Time: 8 hrs
Tab: $2500
Talent:
Tools:
Torx set, slide hammer, gear puller
Applicable Models:
986 Boxster (1997-04) 987 Boxster (2005-08)
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
This article is one in a series that have been released in conjunction with Wayne's new book, 101 Performance Projects for Your Porsche Boxster. The book contains 312 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 950+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Boxster owner's collection. The book is currently available and in stock now. See The Official Book Website for more details.
Check out some other sample projects from the book:
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: Wheel Bearing Replacement) and your CV joints (Pelican Technical Article: Replacing CV Joints and Boots / Axle Replacement) 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. In 1999, there was a change from two 46mm inner diameter differential bearings to one 46mm and one 50mm. When you install your 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 outwards against the clutch plates, which in turn forces them outwards 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%) 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%) 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%).
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 towards 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, like 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 Boxster and 996 engines have a somewhat high-RPM power band (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 power band, and maximize the power output to the wheels. The six-speed Boxster 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 1st gear for actual track use, which effectively creates a true 5-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 power band is where the drivetrain would shine, delivering peak power in a power band closely matched to the transmission and the type of race track. 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.
Another option may be the installation of the Boxster S six-speed transmission into a Boxster that normally had a five-speed. The proper gear ratios can make a world of difference - taller gears in the five-speed tend to make the car feel slower, even if the same engine is installed. If I compare the five-speed Boxster with the 3.4 engine, to my six-speed 996 with the same 3.4 engine, the Boxster seems slower, because the gear ratios are taller. If you wish to install the Boxster six speed transmission into your five-speed Boxster, the swap is pretty straightforward, but time consuming and expensive to acquire all the parts. You need the six-speed transmission, a six-speed clutch slave cylinder, the 3.2 clutch package and flywheel, the six-speed shifter and shift cables, a set of 3.2 axles, the updated transmission mounts and brackets, and a handful of other small odds and ends.
Figure 1
The first step is to remove the halfshafts from the transmission. Begin by removing the center bolt that fastens the halfshaft to the transmission. To pull out the halfshaft, I used a slide hammer, combined with an old CV joint, as shown. Place the end of the slide hammer shaft against the halfshaft flange and then fasten it down with two CV bolts. Tap the hammer along the shaft and the halfshaft should slide out of the transmission. Another method you can use involves placing two bolts into the halfshaft flange and then using them to wedge the halfshaft out of the transmission (inset photo). This is the method documented in the Porsche factory manuals.
Here's a photo of the halfshaft after it has been pulled out of the transmission. Although it's more difficult in the tight space, you can remove the halfshaft 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 halfshafts 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 halfshafts removed, you can then pull off the differential cover. Use a Torx socket tool to 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 transmission has a large o-ring on the differential cover that I recommend replacing when you reseal it.
With the cover removed, you should be able to simply 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. The curved gear on the right (yellow arrow) is attached to the pinon shaft and mates with the ring gear that is attached to the differential. There is a magnet in the case that attracts debris and metallic parts that have worn in the transmission. 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. 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. 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. 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 Boxster with stiffer suspension, and higher power engines.
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 (see text for details).
Here's a side shot of the GT LSD installed back into the transmission. The inset photo shows how the halfshafts sit inside the differential (shown without the differential cover installed).
When reinstalling your halfshafts, be sure to use a new circlip on the end as mentioned in the Porsche factory manuals. The part number for all Boxsters (1997-08) is 012-409-413.
