Electromechanical clutch LSD
An emergent technology, electromechanical LSD systems have become increasingly prevalent in AWD and performance applications over the last decade. What once was available at high-dollar in Rally and circuit racing is now commonplace in streetcar applications especially in AWD systems. Electromechanical LSD systems commonly use a planetary or bevel gearset with electronically-activated continually variable transfer clutches.

This type of LSD system incorporates the benefit of being partially torque-sensitive and partially speed-sensitive. Like torque-sensing LSDs, Electromechanical clutch arrangements are predictive and do not require wheel slip to provide limiting torque, relying on various sensors around the vehicle to "map" the limiting torque. Conversely, unlike torque-sensing LSDs, these systems do not fully lose traction when a wheel is allowed to rotate freely. Since the limiting torque is controlled fully by a computer, various chassis sensors are referenced to vary limiting torque. While this is considered the most complete system, it is also the most expensive and complex. Like other clutch-based systems, Electromechanical clutch LSDs will wear the clutches over time.

Uses: Porsche 959, Subaru DCCD system, BMW X-Drive,

Alternatively, Electrohydraulic clutch systems use a gerorator pump to produce hydraulic pressure, which can be diverted to a hydraulic locking mechanism. This hydraulic clutch can be computer controlled to very the split of power to each axle. Like the Electromechanical systems, the hydraulic systems CAN use a clutch, or uses a friction fluid like what is found in the typically torque converter of an Automatic.

Uses: Haldex AWD systems, on demand AWD, MazdaSpeed6, and Subaru 4EAT/5EAT/CVT

AS OF WRC2011, Active Differentials such as the electromechanical variety have been banned from the WRC top-tier competition.

Viscous LSD
Viscous LSDs or vLSDs are a speed-sensing LSD utilizing the mechanical force of a viscous fluid to accomplish torque transfer between output shafts. Most commonly vLSDs implement a silicone fluid within a housing containing stacked “disks” between the output shafts.

As one output shaft begins to spin faster than the other, the disks begin spinning within the silicone medium. Taking advantage of the properties of adhesion and cohesion, the silicone fluid transfers energy from one output shaft to the other in an attempt equalize the speed difference. This type of LSD has obvious advantages. Firstly, the vLSD is low-cost compared to geared LSDs. Second, vLSDs are very low maintenance and perform surprisingly well. Like all speed-sensing LSDs, the Viscous LSD must experience wheelslip before the device can transfer torque between output shafts. As such, the vLSD is considered a “reactive system” and is less effective in performance applications. At the limit, this type of differential can cause sudden loss in traction before regaining traction ultimately resulting in a loss of control. Furthermore, repeated operation of this type of differential can heat the silicone fluid resulting in a permanent loss of the binding properties of the fluid. Luckily vLSDs fail as open differentials.
vLSDs have been used within center and axle differentials for street and rally cars in AWD applications over the years. A once common LSD in the 1980s and 1990s, vLSDs have been systematically replaced by electromechanical systems of similar cost. This is the type of LSD utilized by Subaru in its “Symmetrical AWD” implementation on manual transmissions and within the rear differential in USDM WRXs between 2002 and 2007. Many iconic Rally-homologated roadgoing cars in the early 1990s utilized vLSDs such as the Celica GT-Four, Lancia Delta Integrale, Subaru Liberty WRC, Mitsubishi Gallant VR4, Ford Escort RS Cosworth and many more. Later vehicles such as the DSM variant of the Eclipse and 3000GT used vLSDs.

eLSD
eLSDs or Virtual LSDs are becoming increasingly more common thanks to advances in computer control and software. With an eLSD, the differential is physically an open type differential. Instead of applying friction within the differential housing to facilitate torque transfer, the eLSD system uses onboard speed sensors to monitor each wheel and modulate the brakes to shift the speed bias. For example, during hard cornering, the computer will modulate the brakes on the inside wheels shift the speed bias between the inside and outside wheels thus transferring torque. The advantages of such a system is the exceptional low cost and the low maintenance required. eLSDs perform a very similar job to other speed-sensing LSDs (especially the clutch-type) at a fraction of the cost. However, this system also has significant disadvantages compared to a traditional LSD. Firstly, since the eLSD uses the braking system, during track events a car equipped with an eLSD may experience brake fade quicker than a car without an eLSD. Perhaps even more unappealing is the unpredictability of the system compared to a traditional LSD. To investigate this second point, one must consider the more complex mechanics of the system. With a traditional LSD, the equalizing friction is contained within the differential housing implying that the suspension and wheel hubs do not see a moment of force as the LSD shifts the bias within the housing. By applying friction at the brakes, as the eLSD shifts the bias from inner to outer wheel the half-shafts and suspension will experience a moment of force that can alter the handling characteristics of the vehicle. Some eLSD system can anticipate the change in handling utilizing onboard yaw and roll sensors, but this increases the cost and complexity of the overall system. In general, a traditional LSD is considered superior in conditions when speed and torque differences between inner and outer wheels are common. As an added effect, traditional LSDs separate the duty of equalizing output shaft speed without compromising the braking system.
Cars that use Brake Vectoring: 2015+ Subaru WRX, Ford Focus ST, Volkswagen Golf R and GTi, BMW F-Body among many others

NEXT UP: 2-Way, 1.5-way, and 1-way L

 

Nice writeup! Here's a great old-timey video explaining the basics of a simple differential:

 

Although not specifically related, I've seen some questions on here about how the viscous coupled center differential in the WRX works. Here's another excellent video explaining viscous couplings:

 

Nice write-up!

and nice Videos Isaac

 

Common AWD Configurations in Performance Applications

ATTESA-ETS in the 2009+ Nissan GT-R
The GT-R’s ATTESA-ETS is primarily rear-biased in normal driving. The engine sends power to a rear-mounted transaxle, which contains the main gearbox and the rear final drive. A separate multi-plate clutch pack in the transaxle can send torque forward through a secondary driveshaft to the front final drive. In steady-state cruising, very little torque is sent to the front, typically near 0%, with the ability to send up to about 50% forward when rear slip is detected. The rear differential is electronically controlled with torque vectoring capability, the center coupling is an electronically controlled clutch pack, and the front differential is open.

S-AWC in the Lancer Evolution IX (JDM) and Lancer Evolution X (all markets)
S-AWC uses an active center differential (ACD) with a hydraulic multi-plate clutch to vary lockup, which changes the effective torque split between front and rear. It is not fixed at 50/50 in use. The rear differential incorporates Active Yaw Control (AYC), which uses a planetary gearset and clutch packs to overdrive the outside rear wheel in a turn, helping the car rotate. The front differential is a helical LSD on most performance trims. Torque distribution is managed by combining center diff lock control, rear torque vectoring, and brake-based stability control.

Audi Quattro in the B7 S4/RS4
Most longitudinal-engine Audis from this era used a Torsen type C center differential. The base torque split in the B7 RS4 was 40% front and 60% rear, with the ability to bias from roughly 70% front to 85% rear depending on traction. The B7 RS4 had an optional active rear sport differential with two clutch packs to vector torque side-to-side. Both the B7 S4 and RS4 used open front differentials, and the S4 had an open rear unless equipped with the sport diff option. Brake-based stability control could assist in maintaining traction when a wheel was unloaded.

Audi Quattro in the B8 S4/RS4
The B8 platform introduced a crown gear center differential. This is a purely mechanical torque-biasing unit that can send anywhere from about 15% to 70% of torque to the front axle depending on conditions. The base split is still 40% front and 60% rear. The RS4 carried over the active rear sport differential, and the S4 had it as an option. The front differential remained open in both.

Symmetrical AWD with DCCD in the 2008+ STI
The STI’s top-level AWD system combines a planetary gear center differential with a nominal 41% front and 59% rear split (35/65 for 2004-2006 USDM STI), paired with an electronically controlled multi-plate clutch pack. The Driver Controlled Center Differential (DCCD) varies lockup from nearly open to fully locked, changing how torque is shared when slip occurs. It does not directly change the base torque split. The rear differential is a mechanical Torsen-type LSD (2007 STI and later), and the front differential is a helical LSD. From 2015 onward, the STI also uses Active Torque Vectoring on the front axle, which applies the inside front brake to help the helical diff send more torque to the outside front tire. For 2018, Subaru removed the mechanical center differential, making the DCCD a fully electronically controlled clutch pack for faster response and lower cost.

2004-2006 and 2018-2021 USDM STI do not incorporate a mechanical center LSD

Symmetrical AWD in Subaru Manual Transmissions (non-DCCD)
Most manual-transmission Subarus without DCCD use a viscous center coupling with a fixed base torque split. Commonly this is 50/50, though some JDM performance models used a 35/65 base split with the viscous coupling as a slip limiter. A standard viscous coupler progressively increases lockup as the speed difference between front and rear grows. Breakaway torque ratings are sometimes quoted in kilogram-force meters (kgf-m), which is a torque value, not a literal force in kilograms. Rear differentials varied by model and trim. Many performance models like the WRX, Legacy GT, and Forester XT used a viscous rear LSD, while base models had open rears.

Symmetrical AWD in Subaru Automatic Transmissions (E4AT, E5AT, CVT)
Older 4-speed automatics without VTD used an electronically controlled center clutch that normally sent about 90% of torque to the front, with the ability to increase rear torque transfer to about 50% when slip occurred. Performance models with the 4EAT used Variable Torque Distribution (VTD), which combined a planetary center gearset with a multi-plate clutch. VTD has a base 45/55 split and can bias torque in either direction as needed. The 5-speed automatic and high-torque CVTs also use VTD. Front and rear differentials are open on most trims, with viscous rear LSDs on some performance-oriented models.

 

Audi Quattro in the B8 S4/RS4:

Quick correction on this section -- I've recently read that the Crown differential was only used in the B8 Audi RS5 and the S4/S5 ONLY with the S-Tronic automatic transmission option. The manual version of the S4/S5 used the previous Torsen Type-C.

 

Expanded the FAQ to include some common AWD configurations!
If you have any questions, want clarification, or have suggestions on another AWD system, feel free to PM me and I’ll make updates.

To clarify a common misconception:
A limited slip differential does not change the nominal torque split between axles. The base split is set mechanically, usually by the design of the center differential’s gearset, and stays the same given equal traction at all four tires.
What an LSD can do is bias torque toward the axle (or wheel) with more grip when there is a traction difference. Clutch-type, viscous, and helical/Torsen units all do this in different ways. In a torque-biasing diff like a helical, this can happen without obvious wheelspin, but there still has to be some difference in load between outputs for torque to shift.
So when you read something like “This AWD system varies torque split from 100% rear to 50/50,” that is not quite right for a purely mechanical center LSD. The base split does not move. The biasing just changes the effective torque distribution temporarily when traction differs. Electronic torque vectoring systems can actively adjust torque distribution beyond the limits of a mechanical LSD, but those are a different category altogether

Interesting addendum:
With the 2015 STI, Subaru closed much of the handling gap to the Evo X, reducing the safe but understeer-heavy balance common in earlier models. What is interesting is that Subaru and Mitsubishi took almost opposite paths to get there. Mitsubishi’s S-AWC adds an active rear differential (AYC) that can overdrive the outside rear wheel in a corner. By giving that wheel slightly more rotational speed, it helps rotate the car and counteract understeer. Subaru added Active Torque Vectoring to the front axle. ATV lightly brakes the inside front wheel, letting the front helical LSD send more torque to the outside wheel that is carrying more load.

How that works in the STI:
In a corner, lateral load transfer gives the outside front tire more traction than the inside. The STI’s front helical LSD biases torque toward that outside tire up to its design limit, called the torque bias ratio (TBR). Subaru has not published an exact figure, but it is roughly in the 2 to 3:1 range, meaning the outside tire can get about 2 to 3 times the torque of the inside if the inside still has some grip.
At the extreme limit, if the inside tire is completely unloaded and free-spinning, the diff cannot send useful torque to the outside. Any multiplier on zero is still zero. That is where ATV comes in, applying brake force to the inside wheel to create resistance. That resistance gives the diff something to work against, so more of the engine’s torque flows to the outside tire that can actually use it.

Comparing philosophies:
In both systems, the goal is to create a yaw moment by generating a torque difference between inside and outside wheels, helping the car rotate into the corner. Mitsubishi achieves it by adding torque to the outside rear, while Subaru does it by creating resistance on the inside front.
Mitsubishi’s approach is more energy-efficient and keeps the brakes dedicated to braking, avoiding the extra heat load that brake-based vectoring adds. Subaru’s approach is simpler and lighter, but in prolonged hard driving, the brake work from ATV can contribute to fade. Both systems have tradeoffs. AYC adds cost, weight, and complexity. ATV is cheaper and lighter but less thermally efficient.

 

Crown LSD:

 

Torsen LSD:

 

Active Torque Vectoring: Clearing Up the “Dragging the Wheel” Misconception

This post is aimed at providing clarity to a common misconception that Subaru’s Active Torque Vectoring (ATV) improves turn-in and cornering simply by “dragging” the front inside wheel. While there is some truth to that phrasing, it often leads people to picture it like a person walking straight, then lazily dragging their left foot so they drift left. A better way to think of it is this: instead of dragging one foot, the person puts more effort into pushing off with the other foot. You still turn, but the cause and effect are different.

Quick refresher on the diff side of things
Torsen is a contraction of “Torque” and “Sensing.” A Torsen or a helical LSD can bias torque toward the wheel with more grip. The amount it can bias is called the Torque Bias Ratio (TBR) — for example, a 2.5:1 TBR means the wheel with grip can get up to two-and-a-half times the torque of the slipping wheel.

The STI’s front differential is a helical LSD that works on the same principle as a Torsen. Subaru has never published an official TBR for it, but it is generally in the range of 2 to 3:1.

What happens in a corner
When cornering, lateral weight transfer loads the outside front tire and unloads the inside front tire. The helical diff senses this load difference and biases torque toward the outside wheel up to its TBR limit. This helps “pull” the car through the turn and reduces under-power understeer.

However, if the inside front tire is completely unloaded and has no usable grip, the helical can no longer send meaningful torque to the outside wheel — multiplying zero still gives zero.

Where ATV comes in
ATV applies light braking to the inside front wheel, not primarily to slow the car, but to give the differential resistance to work against. That brake torque allows the diff to send more engine torque to the outside wheel that has grip.

For example, if the brake adds resistance equal to 10 lb-ft at the inside wheel, and the diff’s TBR is 2.5:1, the outside wheel could receive about 25 lb-ft of engine torque instead of nothing. Yes, the inside tire is being “dragged” slightly, but since it is unloaded, most of that brake force is not acting to slow the car — it is working through the diff to feed torque to the loaded wheel.

What about the WRX?
The WRX also has ATV, but its front differential is open. An open diff always sends equal torque to both outputs, so to give the outside wheel 25 lb-ft, you must apply about 25 lb-ft of brake torque to the inside wheel — much more than the STI’s helical needs. This extra brake work increases heat and can contribute to fade sooner under hard driving.

The takeaway
ATV is not just “dragging” a wheel. It is using brake force as a tool to feed torque to the wheel with grip. In the STI, the helical diff makes that process more efficient. In the WRX, it still works, but it is less efficient and more brake-intensive.

 

Added to help clarify a misconception!

 

Reserved for Expansion4

 

great read! thank you zax! love the new stuff

 

I do recommend that anyone who had read through this FAQ, read through it one more time. I found a lot of errors in my original text. I'm sure there are still more technical errors to be corrected.

 

Yes, I though the 04-06 STIs came with a Clutch type rear LSD and not a Torsen.

Otherwise, great article!

 

Question for you Zach... What difference would it make to have a LSD up front as well? Am I wrong in thinking I have an open front diff? I'm having a hard time making the connection. I saw a post a while back about it and it popped into my head while doing hw, so I thought I would read up on your LSD sticky. I realize I have a rear VCU LSD and a viscous center coupler(03 wrx wagon). What's the deal on the front? Is there a way to upgrade all of this? I'm aware of doing the DCCD swap and rear sti diff, haven't heard much about the front.

 

Just FYI, your rear LSD is probably an open diff by this point. They will wear out and "fail" to functioning like an open differential.

 

Sorry, just saw this.

Whether or not you choose to install a front LSD really depends on the way in which you intend to use the car. There is a reason that most high-performance AWD road cars opt for an open front LSD e.g. GT-R, Lamborghini Huracan, Audi R8, etc. At the limit, a front LSD can add a bit of push understeer on-throttle. This is mitigated by utilizing LSDs with low TBR and clever tricks like Active Torque Vectoring on the front axle. Granted, the aforementioned high performance cars have a natural torque split heavily biased to the rear.

The use of a front LSD on the STI and Evo is a relic of a bygone era. The front LSD helps to keep traction on loose surfaces like you would find on a gravel or snow rally stage, particularly when executing pendulum turns and the like. Of course the real rally cars have much more aggressive diffs tuned to particular stages which leaves the road going STI and Evo a bit of an oddity.

The rear LSD is a far more useful (and prevalent) tool in Motorsport as it will allow the car to maintain grip under "squat" or heavy acceleration. There is an interesting opposition to everything I just mentioned. If you are keen to the news, you'll notice that the 2018 Focus RS will ship with a front LSD. In addition, the Golf R is offered with that front "XDiff" which is really just brake modulation. The reason these cars benefit from a front LSD is that they are primarily FWD-biased and will channel a lot of torque through the front wheels on power-on. To gain maximum grip on Accel the front LSD is needed just like a FWD car.

TL;DR you don't see a lot of front LSD installs because most people around here and NASIOC build their cars for street use and road racing. If you venture over to a rally message board, that will definitely change.

Also, Isaac is right. At this point, your rear LSD is effectively an open diff as the silicone fluid in the LSD breaks down over use. The same is not true for your center diff unless it constantly sees a lot of traction loss.

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Oh, BTW...

The "DCCD swap" suggests that an STI transmission is used and most STI transmissions come with a front LSD. Ergo, the "DCCD swap" would also mean front diff swap to LSD.

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If it's anything like the lsd swaps put in jeeps it makes low speed turning in the snow super wonky too. Not as bad as a locked diff or a lunchbox locker but my experience the tires wanted to hop and it really cut down on turning radius.

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My question is; What type my ‘16 wrx cvt is equipped with?


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Symmetrical AWD in Subaru Automatic Transmissions (E4AT, E5AT, CVT)
Older 4-speed automatics without VTD used an electronically controlled center clutch that normally sent about 90% of torque to the front, with the ability to increase rear torque transfer to about 50% when slip occurred. Performance models with the 4EAT used Variable Torque Distribution (VTD), which combined a planetary center gearset with a multi-plate clutch. VTD has a base 45/55 split and can bias torque in either direction as needed. The 5-speed automatic and high-torque CVTs also use VTD. Front and rear differentials are open on most trims, with viscous rear LSDs on some performance-oriented models.

 

Thanks Zax, I knew it was probably somewhere in there but wasnt totally sure. Is says that both front and rear axles are open, except wrx, gt, etc So does it mean my ‘16 wrx cvt has limited slip? Just unsure because it says there is an exception but doesnt specify what type it is. Thanks! @zax ;


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Thanks Zax, I knew it was probably somewhere in there but wasnt totally sure. Is says that both front and rear axles are open, except wrx, gt, etc So does it mean my ‘16 wrx cvt has limited slip? Just unsure because it says there is an exception but doesnt specify what type it is. Thanks! @zax ;

No, the LSD in the rear axle was dropped from the WRX in 2008.