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Upgrading the differential in your BMW is one of the best ways to improve handling, acceleration, and all-around performance. Whether you're looking to alter the final drive gear ratio, or add limited-slip functionality to get all that power to the ground in the turns, we've got the knowledge to guide you in the right direction.


Differential Tech Info

There are two types of differentials found in BMWs. In the majority of non M cars, the differential that came in your vehicle from the factory are called an open differential. In most M cars, your vehicle came from the factory with a limited slip differential. See descriptions on each type below.

Open Differential: An open differential is the simplest type of differential. When a car is driving in a straight line, both drive wheels are spinning at the same speed. The pinion gear turns the ring gear and carrier and none of the pinions within the carrier are rotating—both side gears are effectively locked to the carrier. An open differential will always apply the same amount of torque to each wheel. There are two factors that determine how much torque can be applied to the wheels: equipment and traction. In dry conditions, when there is plenty of traction, the amount of torque applied to the wheels is limited by the engine and gearing; in a low traction situation, such as when driving on ice, the amount of torque is limited to the greatest amount that will not cause a wheel to slip under those conditions. So, even though a car may be able to produce more torque, there needs to be enough traction to transmit that torque to the ground. If you give the car more gas after the wheels start to slip, the wheels will just spin faster. Now what happens if one of the drive wheels has good traction, and the other one is on ice? This is where the problem with open differentials comes in. Remember that the open differential always applies the same torque to both wheels, and the maximum amount of torque is limited to the greatest amount that will not make the wheels slip. It doesn't take much torque to make a tire slip on ice. And when the wheel with good traction is only getting the very small amount of torque that can be applied to the wheel with less traction, your car isn't going to move very much. The solution to these problems is the limited slip differential.

Limited Slip Differentials

Clutch Type: The clutch-type LSD is probably the most common version of the limited slip differential. This type of LSD has all of the same components as an open differential, but it adds a spring pack and a set of clutches. Some of these have a cone clutch that is just like the synchronizers in a manual transmission. The spring pack pushes the side gears against the clutches, which are attached to the carrier. Both side gears spin with the carrier when both wheels are moving at the same speed, and the clutches aren't really needed -- the only time the clutches step in is when something happens to make one wheel spin faster than the other, as in a turn. The clutches fight this behavior, wanting both wheels to go the same speed. If one wheel wants to spin faster than the other, it must first overpower the clutch. The stiffness of the springs combined with the friction of the clutch determine how much torque it takes to overpower it.

Getting back to the situation in which one drive wheel is on the ice and the other one has good traction: With this limited slip differential, even though the wheel on the ice is not able to transmit much torque to the ground, the other wheel will still get the torque it needs to move. The torque supplied to the wheel not on the ice is equal to the amount of torque it takes to overpower the clutches. The result is that you can move forward, although still not with the full power of your car.

Viscous Type Differential (M Variable): Th­e viscous coupling is often found in all-wheel-drive vehicles. It is commonly used to link the back wheels to the front wheels so that when one set of wheels starts to slip, torque will be transferred to the other set.

The viscous coupling has two sets of plates inside a sealed housing that is filled with a thick fluid. One set of plates is connected to each output shaft. Under normal conditions, both sets of plates and the viscous fluid spin at the same speed. When one set of wheels tries to spin faster, perhaps because it is slipping, the set of plates corresponding to those wheels spins faster than the other. The viscous fluid, stuck between the plates, tries to catch up with the faster disks, dragging the slower disks along. This transfers more torque to the slower moving wheels -- the wheels that are not slipping. When a car is turning, the difference in speed between the wheels is not as large as when one wheel is slipping. The faster the plates are spinning relative to each other, the more torque the viscous coupling transfers. The coupling does not interfere with turns because the amount of torque transferred during a turn is so small. However, this also highlights a disadvantage of the viscous coupling: No torque transfer will occur until a wheel actually starts slipping.

A simple experiment with an egg will help explain the behavior of the viscous coupling. If you set an egg on the kitchen table, the shell and the yolk are both stationary. If you suddenly spin the egg, the shell will be moving at a faster speed than the yolk for a second, but the yolk will quickly catch up. To prove that the yolk is spinning, once you have the egg spinning quickly stop it and then let go -- the egg will start to spin again (unless it is hard boiled). In this experiment, we used the friction between the shell and the yolk to apply force to the yolk, speeding it up. When we stopped the shell, that friction -- between the still-moving yolk and the shell -- applied force to the shell, causing it to speed up. In a viscous coupling, the force is applied between the fluid and the sets of plates in the same way as between the yolk and the shell.

Helical Type Differential: The helical type differential is a purely mechanical device; it has no electronics, clutches or viscous fluids. A helical differential works as an open differential when the amount of torque going to each wheel is equal. As soon as one wheel starts to lose traction, the difference in torque causes the gears in the differential to bind together. The design of the gears in the differential determines the torque bias ratio. For instance, if a particular helical differential is designed with a 5:1 bias ratio, it is capable of applying up to five times more torque to the wheel that has good traction. These devices are often used in high-performance all-wheel-drive vehicles. Like the viscous coupling, they are often used to transfer power between the front and rear wheels. In this application, the helical type is superior to the viscous coupling because it transfers torque to the stable wheels before the actual slipping occurs. However, if one set of wheels loses traction completely, the helical type differential will be unable to supply any torque to the other set of wheels. The bias ratio determines how much torque can be transferred, and five times zero is zero.


What happens when you change to a shorter gear ratio—numerically higher—is that you increase the acceleration of the vehicle by increasing the mechanical advantage through torque multiplication, which makes it easier for the vehicle to apply its power to the wheels. The engine will rev higher in any gear, but it will have less RPM drop when upshifting, enabling the vehicle to always stay in the power band.

By adding limited-slip functionality to a BMW differential, you enable the vehicle to put the power to both wheels instead of just one. You can also alter the behavior of the vehicle during corner entry, improve mid-corner stability, and optimize corner exit speed by changing the ramp angles and the preload of the differential.

Diffsonline 2-clutch option: This is a BMW limited slip carrier which uses the traditional 2-clutch setup. The unit is set to 40% static lock unless a lower amount is requested by the customer, and it utilizes the stock ramp angles of 45/45 (acceleration/deceleration). This limited slip differential is much more durable than the factory M Variable factory unit and is well-suited for street and track use. We recommend using Redline 75W110 gear oil for this application.

Diffsonline 3-clutch option: This is a BMW limited slip carrier with a third clutch pack. The unit is set to 40% static lock unless a different amount is requested by the customer, and it utilizes the stock ramp angles of 45/45 (acceleration/deceleration). This limited slip differential is much more durable than the factory M Variable unit and is well-suited for street and heavy track use. We recommend using Redline 75W110 gear oil for this application.

Diffsonline 3-clutch option with 30/90 ramps: This is a BMW limited slip carrier with 3 clutch packs and ramp angles which are modified from stock. The unit is set to 40% static lock unless a different amount is requested by the customer, and it uses ramps which are modified to 30/90 (acceleration/deceleration). This limited slip differential is much more durable than the factory M Variable unit and is well-suited for street and race use. Other ramp angles are available by request. We recommend using Redline 75W110 gear oil for this application.

Diffsonline 3-clutch option with 35/60 ramps: This is a BMW limited slip carrier with 3 clutch packs and ramp angles which are modified from stock. The unit is set to 40% static lock unless a different amount is requested by the customer, and it uses ramps which are modified to 35/60 (acceleration/deceleration). This limited slip differential is much more durable than the factory M Variable unit and is well-suited for street and race use. Other ramp angles are available by request. We recommend using Redline 75W110 gear oil for this application.

Diffsonline 4-clutch option with 35/60 ramps: This is a BMW limited slip carrier with 4 clutch packs and ramp angles which are modified from stock. The unit is set to 40% static lock unless a different amount is requested by the customer, and it uses ramps which are modified to 35/60 (acceleration/deceleration). This limited slip differential is much more durable than the factory M Variable unit and is well-suited for street and race use. Other ramp angles are available by request. We recommend using Redline 75W110 gear oil for this application.

Diffsonline Compact (medium) Race Carrier: This unit weighs in at just 14 pounds while other options weigh 22 pounds or more; the lower reciprocating mass helps to give you a competitive edge over the competition. This unit uses 6 clutches and single-driven ramps; making it the best working and most tunable limited slip on the market. The preload is controlled by coil springs so that preload is not lost as the unit wears. Since this is a Diffsonline exclusive product, we offer custom tuning assistance for your race team, and we can also cut any ramp and set preload anywhere you wish. We have a great baseline setting which covers 95% of cars running normal suspension setups. We recommend Redline 75w140, Motul Comp FF, or Castrol SAF XJ fluid for this differential. NOTE: Please contact us for your specific requirements on this unit.

This unit is a race carrier and is recommended for race/track applications only, and this is the go-to unit for BMW's running ST in Grand Am (E82, E90 or F30), and in any serious race environment. Many podium finishes have also been achieved in World Challenge by cars utilizing this setup.

Diffsonline's 210mm (large) Race Carrier: This unit weighs in at just 21 pounds while other options weigh 28lbs or more; the lower reciprocating mass helps to give you a competitive edge over the competition. The unit uses 12 clutches and single-driven ramps; making it the best working, most tunable limited slip on the market. The preload is controlled by coil springs so that preload is not lost as the unit wears. Since this is a Diffsonline exclusive product, we offer custom tuning assistance for your race team, and we can also cut any ramp and set preload anywhere you wish. We have a great baseline setting which covers 95% of cars running normal suspension setups. We recommend Redline 75w140, Motul Comp FF, or the Castrol SAF XJ fluid for this differential. NOTE: Please contact us for your specific requirements on this unit.

This unit is a race carrier and is recommended for race/track applications only, and this is the go-to unit for the BMW M3 in any serious race environment.

Wavetrac: Wavetrac's helical limited slip carrier. The beauty of the Wavetrac is that it is ideally suited to high performance road cars that see track use. It is very strong, yet transparent to both driver and traction control electronics during regular driving. On the track or in spirited road use, it provides two wheel drive without the handling issues that traditional clutch type diffs create (there is no tendency to push/understeer with a Wavetrac.) The Wavetrac outperforms other gear type diffs because unlike them, it won't lost drive when one axle is unloaded (which happens more often than you'd think. Lastly, one of the most underrated benefits of the Wavetrac is its smooth delivery of the power near the limit. It's patented design makes for a very predictable, smooth addition of power when you need it most- when things are at their limits: on the edge of traction mid corner and you're using throttle to keep it there; or coming into a corner a little hot, but quickly back on it to maintain exit speed…These situations are often subjective to the driver, but the benefit can be felt. The Wavetrac makes you faster because it lets you drive with confidence.

QUAIFE ATB: This differential is QUAIFE's Automatic Torque Biasing unit. QUAIFE does not specify bias ratio details and generally, does not use the term. The QUAIFE ATB differential is an all helical gear design. The angle and tooth form of the helix used, the number of pinions used, and many other factors (including the particular car fitted) all contribute to how the differential behaves and biases drive torque. Each QUAIFE differential application is designed to offer the best compromise between performance and durability. We recommend Redline 75w90 fluid for this application.

OS Giken: This limited slip carrier utilizes up to 28 clutch plates—depending on application—to enable the unit to completely lock while never fully disengaging. As supplied by Diffsonline, this unit comes set to the OS Giken factory setting. This differential requires exclusive use of OS Giken's own 85w250 differential oil for proper and consistent operation. This unit is guaranteed to arrive in working order. OS Giken parts come with no warranty.

Core charge: This enables us to pull a differential from our own stock and rebuild it for you ahead of time. This reduces the downtime for your vehicle and can be used to purchase additional differentials for your car if you need different ratios or limited slip setups for varying conditions or tracks. To receive your core deposit back, all cores must be returned to Diffsonline LLC within 15 days of receipt unless other arrangements are made in advance.

Differential break-in instructions

Fill your differential with the recommended fluid as described above. We recommend Red Line, Motul, and factory Castrol SAF XJ fluids for most differentials, but the OS Giken and Kaaz limited slip units require their own thicker fluid for proper operation. If you are experiencing chattering in your differential while cornering, you can add a limited slip additive from Red Line or GM. If you have a question about your specific application, email us, and we will get back to you with a recommendation.

All differentials purchased through Diffsonline LLC require a 1,200-mile break-in period. During this period, you should keep constant vehicle speeds under 80 mph and drive the vehicle moderately. This will allow the gears, bearings and limited slip parts to break in properly and without excessive heat. After 1,200 miles the fluid should be changed in your new differential with the same recommended fluid. After the initial flush, we recommend changing the fluid every 24K miles in a street driven car, or every 12K miles in a car that sees autocross and track day use.

For racing applications, one day of track use is recommended for break-in. Change the differential fluid after the one-day break-in procedure using the same recommended fluid. We recommend changing the differential fluid every other time you change the engine oil, or whenever the differential has gotten extremely warm.

Note: The differential fluid lubricates the ring and pinion gears, which transfer power from the driveshaft to the axles. The limited-slip unit requires fluid to keep all of the internal parts healthy and moving properly. Differential fluid changes areas imperative as changing the oil in the engine, and for the exact same reasons. Metal-to-metal contact wears down surfaces and creates heat from friction, which weakens parts and can lead to failure.

Normal operations

During initial break-in, it is not uncommon for the rear end to have some chatter in the form of tire scrubbing or internal LSD noise depending on the vehicles limited slip choice. It is normal for some freshly rebuilt differentials to have a tight spot while turning by hand before break-in, as differentials are assembled and shipped dry except for some oil on the clutches and bearings during assembly. These conditions remedy themselves upon fluid fill and break-in process. Differentials with high levels of preload have difficulties unlocking depending on driving surfaces, such as rain, snow, gravel, etc.


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