The Limited Slip Differential
written by Gregory Brown
Originally published in Velocity, February 2001
official magazine of thePorsche Owners Club (POC)

In previous Velocity articles, I've concentrated on how suspension" and tires can affect our Porsches. Another item which has significant impact on car handling is the limited slip differential. This item, because it is located inside the gearbox, is frequently mistaken for a part that helps the drivetrain function. It actually isn't needed for the drivetrain to function, but should be considered an optional piece that improves handling. The limited slip differential could almost be considered more a part of the suspension than the transmission, because of the huge impact it has on handling.

One of the more desirable options that a Porsche can have for track use is and always has been a limited slip differential. Initial analysis says that a limited slip differential is designed to limit wheel spin while exiting from a comer. This allows a smoother exit from a corner, since the inside unloaded tire is forced to rotate at about the same rate as the outside loaded tire. All of this is true, but as cars have progressed, so has the role that the limited slip plays. No longer is the equation quite so simple. In this issue of Velocity, I'm going to take a closer look at limited slips and their close relatives, and see how these components affect the race car on track.

Porsches have basically four different types of differentials. The first style is a plain "open" differential. This is pretty much what all street Porsches are supplied with from the factory, unless the "limited slip" option was chosen. This differential has a set of gears that allows the outer tire in a comer (which follows a larger arc) to travel that extra distance without any hindrance or help from the inside tire. If the inside tire needs to travel twenty feet and the outside tire needs to travel thirty feet, this is not a problem, and occurs without much extra effort inside the gearbox. Minimal heat and friction are incurred as a result of this effort. Unfortunately, this effortless, minimal friction has a downside. If you were to jack up one rear corner of a vehicle that had this "open" differential, start the engine, and attempt to move this vehicle, the tire off the ground would simply spin, and there would be no force transferred to the other rear tire still on the ground. This same effect would occur on track. The unloaded inner rear tire would have a severe tendency to spin, and the power would not be used to move the car down the track. This obviously slows the vehicle down, resulting in slower lap times.

The optional differential that Porsche offers from the factory is the limited slip differential. This is the style of differential that I am going to concentrate on in this article, along with discussions of the variants of this design. This differential still has the gears that allow the tires to move different distances through a comer, but these gears are kept from spinning freely by a series of friction plates which are held under pressure by a diaphragm spring. Now, as the car goes through the corner, the inside tire no longer moves without being influenced by the distance that the outside tire travels. The friction action attempts to keep these tires moving the same distance until the preset friction force is overcome. This style differential, because it depends upon friction to function, creates heat inside the gearbox. Depending upon the vehicle and the heat generated, this can become a serious issue, and may require that the gearbox oil be cooled. One also needs to remember that any friction that occurs before the power reaches the ground is a loss of acceleration. Simply stated, friction causes heat and takes power to do so. Porsche has made the limited slip differential almost forever, varying the quantity of friction plates and the spring pressures from model to model.

The third style differential that has been used in Porsches (hopefully in very limited applications) is a true locking differential. These are usually, in the Porsche world, stock open differentials that have had the gears that allow the wheels to move different distances welded together, which forces the tires to always move the same distance, no matter what. These differentials should not be used in cars that need to turn comers, and should be limited to only straight line drag race vehicles. Our rules do not disallow the use of this style differential, although they clearly should. One of the major hazards that this style differential has is that if one axle should fail, all power would be transmitted to the opposite tire. This would make the car instantly snap turn either right or left, depending on which axle failed. If you have one of these ill conceived "locker" units in your car," please consider removing it before you hurt yourself or someone else.

The last differential that we need to consider is the aftermarket "torque biasing" style. This style differential relies on multiple helix cut gears that are forced outward onto the housing of the differential when tire spin begins to occur. These differentials function almost totally as an open differential while off throttle, which makes them vastly inferior to a limited slip differential, as you will see. I have always considered this style differential to have very limited usefulness on a race course, so I will not spend time discussing them.

In order to understand how the Porsche limited slips function and how they have changed over the years, we need to try and understand the pieces inside the differential. I have borrowed a picture from the Porsche Workshop Manuals (Figure 1) in order to help explain the function of the limited slip differential.

Starting in the very middle of the differential, there are four small gears (#10) located on two shafts (#11). These gears are all in the same plane when assembled, which can occur because the shafts are each cut halfway through in the center. This allows the shafts to fit together and form a perfect "X." The small gears (called pinion or spider gears) have holes in them, just slightly bigger than the shafts, which allows them to fit over the open ends of the shafts and spin freely on the shafts. Two larger gears (#8), also called pinion or spider gears, mesh into the four smaller pinion gears. Note that these larger gears are splined on the inside. This is where the stub axles protrude out of the gearbox (where the axles are driven from). Outboard of all of these gears are two large metal blocks (#7), called thrust rings. These thrust rings have notches cut in them where the shafts for the small pinion gears are located and supported. These notches were originally—in all early Porsche limited slips—cut symmetrically. As these differentials progressed, Porsche found that by varying the angles of these notches, they could achieve different amounts of lock-up under certain circumstances. This is going to be a very important feature as we progress in this limited slip analysis. Referring back to our picture, one can see that the thrust rings fit over and surround all of the pinion gears. The large pinion gears protrude through the thrust rings and have splines cut on the outer portion, where the friction discs (#6) are located and driven. Note that the thrust rings each have four protruding square shaped blocks on the very outer diameter. These are held captive inside of the actual differential housing. Referring again to the picture, the friction discs are sandwiched between the thrust ring and an outer plate (#5). These friction discs have a coating on them which is designed to "grip" the surfaces that they touch. It is here that all of the work of the limited slip differential occurs and thus where all of the friction and resulting heat originate. Outboard of the outer plate, is a diaphragm spring (#4). This spring provides the "crush" that keeps all the parts together and provides the "preload" that determines how much friction the differential can generate.

It is important to note that the illustration which I've used to help explain the inner parts of the limited slip differential (Figure 1) has only a single friction disc and outer plate. Multiples of these items are to be found inside many differentials (especially the ones that we are interested in), which allow more available friction surfaces.

After the 356s and prior to about 1984, limited slips were pretty much cut and dry as supplied from the factory in a Porsche. These units were either called 40% or 80% limited slips, depending upon how the friction discs and outer plates were arranged. These limited slips had two sets of friction discs on each side of the thrust rings (for a total of four discs). The 40% limited slips had two of the friction discs against each other, without an outer plate between them. Because the two friction discs touch each other, without an outer plate to separate them, this effectively provides two friction surfaces. Since this occurs on each side of the differential, you
end up with a total of four friction surfaces in the entire differential. The 80% limited slips separated the two friction discs with an outer plate. This provided four friction surfaces on each side of the differential, which makes the 80% limited slip have a total of eight friction surfaces. Although certainly the number friction discs that actually contact the outer plates affect the amount of friction that the differential can generate, the amount of pressure that the diaphragm springs have also influences the amount of friction that the differential can produce. The more friction discs and the higher the preload from the diaphragm springs, the more torque it takes for limited slip to actually slip. The amount of preload can be varied since Porsche offers the outer plates in different thicknesses, which can be used to change the releasing torque.

This is where some of the fun starts. Early 40% limited slips (before 1984, in the 911 series vehicles) were designed to be set up so that it takes between 29 to 58 foot pounds of torque to make the differential slip. The 80% limited slips had higher settings. Beginning with the Carrera models (and on through the C-2 models), limited slips only had room inside for one friction disc and one outer plate on each side (like our illustration has). These limited slips are also called 40% limited slips, but have a setup that only allows 7 to 25 foot pounds of torque (and 25 foot pounds is almost impossible to get) before the unit begins to slip. How can one 40% limited slip have less pressure than another 40% limited slip and still be considered to have the same slip percentage? I have no idea, but this can't be good. We therefore call these single friction disc units "snow only" limited slips. They have limited benefits on the race course, because they release at very low torque numbers. If you have one, lose it for one of the better later limited slips that I'm going to discuss and your lap times will improve instantly!

A quick comment before I move onto the later, more significant limited slips that Porsche has created. The early multiple friction disc limited slips are excellent parts, as long as you keep the torque releasing numbers under control. Obviously, it would be wise to separate the friction discs that are found together on the 40% units in order to get more friction surfaces in contact with outer plates. This will increase the amount of life that the unit offers before needing to be serviced. However, we have found that creating excessive pressures (high release torque numbers) inside of these limited slips can cause extreme handling problems. If the limited slip releases at too high of a pressure, the long sweeping corners can become very difficult to drive, because the limited slips will release at different places in the same comer each consecutive lap. This can force a car to understeer one lap and oversteer the next lap, all in the same corner! Note that this will only occur in the long sweeping style corners. Any tight comer will force the unit to release and this will not be an issue. If you have a 40% or 80% limited slip that your favorite transmission guy has "leaned" on, and your car feels different each and every time you go through turn eight at Willow Springs or any other long sweeping comer, you might note this and discuss it with someone who is familiar with this phenomena.

Now we can move on and discuss the really fun/fast limited slips that Porsche has developed. The first significant change in limited slips was found in the C-2 body style Turbos—both in the 3.3 liter and the 3.6 single turbo versions. They also found their way into the European RS versions of the C-2s. The limited slips offered for these cars were termed 20/100 limited slips. The 20 refers to the amount of locking that these units had under acceleration, and the 100 number refers to the amount of locking that they had under deceleration. Yes, that is correct, the units became 100% locked under braking! Porsche achieved this by changing the symmetrical ramps on the thrust rings which support the small pinion gears. By cutting a shallower angle on one side of these thrust rings, the shafts were able to move and push the thrust rings outward towards the diaphragm springs. This can be clearly seen in Figure 2. The pressure is so high when this occurs, that the friction discs are unable to slip at all. This had very significant effects on these cars and defined what all future limited slips would be like. When the rear tires are locked under deceleration, they are forced to rotate at exactly the same rate. This makes the car push the front end. It was now possible, especially with ABS brakes, to get really deep in a corner, brake when the car was turning and not have the rear of the car instantly spin. Try this in an early car and you will immediately see that it is very difficult, if not impossible, to keep the car from spinning. As soon as you begin accelerating, the shafts rotate back and the limited slip became a 20% limited slip. These units worked fairly well in the Turbo style cars, but didn't work perfectly in the RS style cars. They would allow the inside tire to spin on acceleration, in some instances. This was primarily experienced in the higher torque naturally aspirated models and reared its ugly head when the turbo cars had the engines tweaked. The 20% locking percentage was simply too low to keep the tires from spinning, especially when the shafts slammed back on the steep angles from the locked position. In spite of these problems, Porsche knew that they had created something really significant in the differential department, especially when they combined this effect with ABS braking.

The 993 style street cars with the limited slip option almost all came with a 25/65 limited slip. This provided a 25% locking function under acceleration and a 65% locking function under braking. These limited slips also featured significant increases in the sizes of the internal parts, which made them less prone to wearing out the friction discs (except for a few of the very early versions, which were prone to really awful wear characteristics). These limited slips, along with the next generation ABS systems, really allowed the driver to get into the corner deep and brake hard while the car was turning.

The standard Motorsports version of this limited slip (for the 993 Cup Cars and the 993 RSRs) was designated a 45/65 limited slip. Motorsports also offered differentials and components that allowed one to modify these percentages a fair amount. One could have a 50/80 limited slip, for instance. These 45/65 limited slips and the next generation ABS systems were perhaps the single most significant changes that were made between the C-2 factory race cars and the 993 factory race cars. Sure, the rear suspension changed and this certainly helped make these cars faster, but only when the suspension was correctly adjusted, which happened very infrequently in this country.

This is not the final chapter in this tale. These wonderful 45/65 Motorsport limited slips had a few problems. We (and others) began to see cracks in some of the differential cases at the base of the ring gear. This problem could—and did—lead to some catastrophic failures. Although these failures were certainly caused by material and machining problems, I believe that the sturdiness of the internal components had much to do with the failures. These limited slips were very slow to wear out. Combine this with the effectiveness of the 45% locking function on acceleration and the 65% locking function on deceleration and the case simply was not able to cope with the higher loads generated. The Porsche Factory made billet cases out of chromoly for the GT-1 cars and recommended that all the GT3R vehicles switch to this case. (As far as I can tell, all of the year 2000 GT3Rs came with the cast cases.) These were relatively expensive, but seemed to solve the problems. One of the aftermarket suppliers (Paul Guard at Guard Transmissions) offers these differentials in several versions, all with billet chromoly cases. (Refer to Figure 3). Guard can also provide the racer with thrust rings that have notches cut for different locking ratios. There is enough room on the thrust rings to machine another set of angles that can allow the user to try either a 45/65 locking ratio or a 50/80 locking ratio, which Paul has done on the latest versions. This can be clearly seen in Figure 2. He can also provide the customer with a set of thrust rings that are cut for both a 50/80 locking ratio and an 80/80 locking ratio. The drivers in the GT3R race cars have been using these new higher releasing ratios with outstanding results. The higher 50/80 ratios and 80/80 ratios have shallower ramps on the thrust rings, which provides smoother transition from braking to accelerating than the original 45/60 style differential.

What does this all mean to you, the owner/driver of a club car? Well, if you can get one of these whammy zammy limited slips into a gearbox that you own, you will be totally amazed. Your lap times will improve the very instant that you make the change. Lap time improvements are not insignificant. We have been able to find literally seconds per lap with differential changes and so will you. Although there aren't many guarantees in life, this is certainly one of them.