Subaru Symmetrical All-Wheel Drive – Four Decades of Evolution

Unlike other automakers, Subaru’s all-wheel-drive philosophy includes the engine choice, which is why the brand only sells AWD with boxer engines (with a few exceptions, like the Justy).

Another intriguing aspect of Subaru’s technology is the way the all-wheel-drive solutions are integrated into the vehicle in a different manner, as the manufacturer usually provides this system as standard for its vehicles.

Naturally, cars like the BRZ, some Kei cars, and a few small exceptions come with a single driven axle, but the rest of the range has all-wheel-drive as standard.

As in the case of BMW’s xDrive, the system is always on, and the user cannot turn it off. We also like the fact that the Japanese company integrated the center and front differential in the transfer case, a smart solution to keep weight down.

Subaru claims that its system is superior to its equivalents thanks to the symmetrical factor of the components it uses. In turn, the company does not adapt FWD or RWD configurations for AWD, but uses all-wheel-drive as a base for its vehicles, thus ensuring perfect integration.

The first ever Subaru with all-wheel-drive was the Leone Station Wagon, which came to market in 1972 with the system. Over the years, the Japanese carmaker has manufactured 15 million vehicles with the technology. Currently, it offers several types of all-wheel-drive, and we will detail them below.Viscous center differential AWD – Mechanical AWD for manual transmission models

Subaru installs this system on manual-transmission versions of the Legacy, Forester, and Impreza. It features a 50:50 torque distribution under normal conditions. This system uses a combination of technologies to achieve all-wheel-drive, including a bevel-gear-type center differential and a viscous limited-slip differential.

This is the heir of the original AWD system from the Japanese brand, and the locking center differential can send up to 80% of available torque to the axle with the best traction. The center diff achieves this through a viscous coupling that reacts to mechanical differences in wheel grip. No computers are required for the operation of this system.

Subaru’s reputation stands with this sturdy solution, and its principles are applied even on the top spec Impreza WRX. The origins of this system can be traced back to the 1980s, and these rarely cause issues to their owners, and it is found it rally cars which bear the Subaru logo.Active Torque Split AVD – Electronically controlled

This is an evolution of Subaru’s original Multi-Plate Transfer solution (launched in 1981) that regularly provides the front axle with 60% of available torque. However, the system has an electronically controlled set of clutches that adjusts torque distribution between the front and rear axles.

The Active Torque AWD comes as standard for the Legacy and Outback 2.5i, the Exiga 2.0i, the Lineartronic versions of the Impreza, and the automatic transmission Forester. The goal of this system is to provide stable and safe drivability in any situation, regardless of driver skill.

The first system that followed this philosophy came in 1987, on the Subaru XT. The car featured an electronic active torque split system, and it was biased to the front, where it provided 60% of torque in regular driving conditions. The system switched to 50:50 distribution when slippage was detected.Variable Torque Distribution AWD

The technology with this name is fitted to the Outback 3.6, Exiga 2.0GT, Legacy 2.5 GT with automatic transmission, and the WRX STI with an automatic transmission. Unlike the systems described above, this one comes with a 45% front, 55% rear torque distribution in normal driving. The system is electronically controlled to enhance turning performance in sporty driving.

To achieve this goal, Subaru implemented a planetary gear with an electronically controlled hydraulic multiple-disc clutch in the center differential, which comes with a limited-slip. Depending on road conditions, the system can provide a torque split of 50:50 front/rear, but it emphasizes the rear wheels in normal driving to deliver a dose of sportiness.

Instead of a purely mechanical solution, the VTD electronically controlled technology is proactive, and shifts torque between axles more quickly. A variation of this system is used on CVT-equipped Subaru models, which come with the same hardware, but with a front-biased torque split of 60%. This applies to the XV Crosstrek, the 2015 Impreza WRX with CVT, and with the 2014 Legacy, to name a few.Multi-Mode DCCD AWD

This system only comes on the Subaru WRX STI with a manual transmission, and it provides a performance-oriented driving experience. Subaru employs a Torsen differential with limited slip and an electronically controlled center limited-slip differential.

The torque split is 41/59 front/rear, chosen by Subaru for “optimum vehicle dynamics.” Depending on the generation of WRX STI involved, it uses a limited slip front and rear differential, or
a brake-based torque vectoring system to improve power transfer and tighten the turning radius of the car.

On top of it, there are two center differentials. One is electronically controlled while the other is a mechanical unit that is capable of reacting more quickly than its electronic counterpart, even though it's reactive.

Both differentials described above are at the center of the car and work harmoniously with a planetary gear. The mechanical differential reacts quicker than the electronic systems, while the latter allows the driver to choose how the system responds by selecting a predefined driving mode. Depending on the adopted setting, the car is biased to the mechanical or electronically controlled diff.

The system promises a balance between agility and stability, while managing a significant amount of torque, provided by the turbocharged boxer engine. The Auto [+] Mode keeps more power to the front wheels for increased traction, while the Auto [-] Mode provides more to the rear ones. The Manual Mode has six steps with predefined settings, while the Auto Mode selects the best solution for changing conditions.