subaru

All all wheel drive Impreza, Legacy, Outback, Forester, Tribeca, and XV vehicles

With manual transmission (both 5- and 6-speed): (Edit)

Full-time all wheel drive with 50/50 torque split front-to-rear under normal conditions. Viscous coupling locking differential in the center inside the transaxle case, activates when wheels start to slip. Torque apportion from 80/20 tro 20/80 (source, because the coupling can't be locked completely?).

Figure: subaru impreza all wheel drive scheme

Figure: Subaru's manual transmission, center differential (10) and viscous coupling (11)

Figure: subaru manual transmission visco

Figure: Subaru MT viscous coupling locking center differential

With TZ1/ACT-4 (Active Torque Split) 4-speed automatic transmission: (Edit)

This system has no "normal" torque split as it is adjusted dynamically many times per second, but due to customer confusion on the topic, layman's terms emerged as torque split numbers as a way to describe the system as "always on" to the customer and sales force that did not posses the necessary background knowledge to understand it. These numbers have no real meaning other than to describe the system as active and always on (not part time).

It is a proactive automatic all wheel drive with 90/10 torque split front-to-rear under normal conditions (80/20 from 1996; 60/40 on Subaru Forester from 2009 and on - Source). Electronically controlled hydraulic multi-plate transfer clutch installed instead of the center differential in the transaxle's tailshaft. Sensors monitor parameters such as wheel slippage, throttle position and braking, to help determine needed torque distribution to the rear axle.

The hydraulic multi-plate transfer clutch ECU performs the following functions:

• a) adjusts the hydraulic pressure according to the accelerator pedal position and the vehicle speed;
• b) increases the hydraulic pressure when the gear shift lever at the position "1";
• c) increases the hydraulic pressure when the front wheels start to slip (this function deactivates when the speed exceeds 60 km/h or when the accelerator pedal is released);
• d) decreases the hydraulic pressure when the vehicle is turning;
• e) cuts off the hydraulic pressure when ABS is activated.

Figure: Subaru's 4-speed automatic transmission (type MPT) and hydraulic multi-plate transfer clutch (23)

With CVT transmission (2010 Legacy, 2012 XV): (Edit)

Proactive automatic all wheel drive with 60/40 torque split front-to-rear under normal conditions. Electronically controlled hydraulic multi-plate transfer clutch installed instead of the center differential in the transaxle's tailshaft. Sensors monitor parameters such as wheel slippage, throttle position and braking, to help determine needed torque distribution to the rear axle before the wheels start to slip.

Electronic traction control system on all four wheels.

Figure: Subaru XV CVT and multiplate clutch

Figure: Subaru XV transmission layout

Video: How Subaru XV all wheel drive and traction control system work

With TV1/VTD 4- and 5-speed automatic transmission: (Edit)

Full-time all wheel drive with 45/55 torque split front-to-rear under normal conditions. This torque split is static and is different than MPT only system in that the mechanical planetary center differential gear provides this ratio as calculated by the number of gear teeth. Electronically controlled hydraulic multi-plate clutch installed in conjunction with the center planetary-type differential. Sensors monitor parameters such as wheel slippage, throttle position and braking to help determine the amount of clutch lock. Optional viscous-type limited-slip rear differential.

Vehicles with 4-speed VTD: 2002 Impreza 2.5WRX (North American market), Forester 2.5XT (North American market), SVX (36/64 torque split), 2003 Legacy GT 4EAT+VTD, 1998-2003 6-cylinder Legacy Outback 4EAT+VTD, (anything else?)

Vehicles with 5-speed VTD: 6-cylinder Legacy, Legacy Outback, Tribeca (else?)

In North America, if a vehicle is equipped with VDC, most likely it has a VTD type all wheel drive.

The hydraulic multi-plate clutch ECU performs the following functions:

• a) adjusts the hydraulic pressure based on the produced torque and the road conditions;
• b) adjusts the hydraulic pressure when the vehicle starts moving, based on the accelerator pedal position;
• c) decreases the hydraulic pressure when the vehicle is turning (calculated by comparing the front and the rear's driveshafts speeds);
• d) increases the hydraulic pressure when the front or the rear wheels start to slip;
• e) cuts off the hydraulic pressure when ABS is activated;
• f) decreases the hydraulic pressure when the brake pedal is pressed and when the accelerator pedal is released;
• g) increases the hydraulic pressure when the gear shift lever at the position "1".

Figure: Subaru's VTD all wheel drive, planetary-type center differential (23) and hydraulic multi-plate clutch (24)

Figure: Subaru VTD all wheel drive system's planetary gear center differential

Figure: subaru vtd differential

Some models are equipped with viscous limited-slip rear differential. The models include: Impreza WRX, Legacy GT, Legacy Spec.B, Outback 2.5i, Outback XT.

Some vehicles are equipped with Vehicle Dynamics Control (VDC). VDC operates like most electronic stability control (ESC) systems. It can adjust engine output and adjust individual brake pressure. Any vehicle, with MT, 4AT, or 5AT transmission can have VDC. On vehicles with automatic transmissions, to retain/regain vehicle stability VDC tries to adjust the front/rear torque split to balance the available traction, rather than applying brake force. If the total traction is still not enough for stability and wheels continue to spin, the system will then use braking and reduced engine power to reduce momentum and help regain control.

See how VDC works in our Videos section.

Full-time all wheel drive with 50/50(?) torque split front-to-rear under normal conditions. (???)

Figure: subaru impreza wrc second generation

Figure: subaru impreza wrx sti transsmision third generation

Full-time all wheel drive with 35/65(?) torque split front-to-rear under normal conditions. Driver-controllable Center Differential System (DCCD).

"Helical-type" front differential (2004-... WRX STI), varies the torque delivered to the left and right axle shafts, depending on traction and engine load. Instead of locking the output shafts so that they rotate at the same speed, this differential sends more torque to the wheel with more grip. In addition, it makes a gradual adjustment for a more fluid response.

Read more about DCCD:

https://www.ringriders.ch/dccd.html

https://www.ringriders.ch/dccd_animated.html

https://www.driveperformance.subaru.com/version1_2/blueprint.asp

https://www.driveperformance.subaru.com/version5_1/blueprint.asp

https://forums.nasioc.com/forums/showthread.php?t=932701&page=7

Full-time all wheel drive with 41/59 torque split front-to-rear under normal conditions. Driver-controllable Center Differential System (DCCD). A limited-slip, planetary gear-type center differential, augmented by an electronically controlled center limited-slip differential, provides a performance-oriented 41:59 torque split. The mechanical limited slip differential has a quicker response and activates just prior to the electronic limited-slip differential. In any of the three automatic modes, the electronically managed continuously variable transfer clutch can vary the distribution ratio through the center differential as needed to suit driving and road-surface conditions. The DCCD AWD System has three automatic modes in the 2009 WRX STI: “Auto” is the same as in the previous-generation model. The new “Auto (-) Active Sport setting shifts the torque bias to the rear and also opens the center limited-slip differential (no locking factor), which improves steering feel. For driving on slippery surfaces, such as gravel or snow, the Auto (+) setting tightens the LSD. In manual mode, which offers six driver-selectable settings, DCCD allows the driver to vary the front-to-rear torque distribution to optimize All-Wheel Drive performance to suit specific driving conditions.

• Auto mode is the default setting on engine start-up. This mode is an all-round control mode appropriate for the majority of driving conditions. It automatically adjusts front/rear torque split to accommodate variations in traction, individual wheel speed, and yaw.
• Auto (+) mode biases front/rear torque toward a 50/50 setting but does fluctuate slightly for flexibility on clear patches. This near-neutral setting is ideal for rough roads and slippery conditions or situations when safety through traction is of the utmost concern.
• Auto (-) mode biases front/rear torque distribution toward 41/59, increasing steering response for quick and agile driving due to the near-consistent rear-wheel bias.
• Manual mode offers drivers a precise choice in torque split to exactly suit driving conditions and their individual driving preferences by pressing the Auto/Manual button and toggling forward (+) or backward (−) to adjust front/rear torque split. Only the real experts will get the most from this feature as it puts total decision-making in the hands of the driver.

2008 WRX STi is equipped with multi-mode VDC. Normally, the VDC button simply turned the stability control system on and off. Multi-mode VDC offers drivers three different combinations of stability control, traction control and ABS performance to suit any driving condition or driving style.

• Normal mode delivers VDC’s full capabilities for maximum safety in everyday conditions.
• Off mode allows some wheel spin to enable throttle steering in corners, and wheel spin on ice or in deep snow.
• Traction mode minimizes the intrusiveness of VDC (traction and stability control).

All versions of the SVX sold were equipped with automatic transmissions. Depending on the country, Subaru had two versions of their All-Wheel-Drive system for the automatic transmission, called ACT-4 or VTD. The first system, called ACT-4 (Active Torque Split) by Subaru, was the same setup commonly found on other Subaru models of the period, and used a variable clutch pack center differential using a 90% / 10% power split front to rear, which could transfer up to a 50% / 50% power split for maximum traction if the front wheels started to slip, allowing better fuel economy. This AWD system was offered throughout the entire production run, and was used in vehicles manufactured for sale in the USA, Canada, Germany, France and Switzerland. A sportier continuous traction delivery system, called VTD (Variable Torque Distribution) by Subaru, was used in vehicles for sale in Japan, England, the Benelux region of Northern Europe, Australia, Spain, Austria and Brazil. The VTD AWD system is a permanent AWD due to its 36% / 64% split. (Source: https://www.wikipedia.org)

Figure: subaru svx gear levers

Shift on the Fly: this refers to an less refined and effective, older Subaru 4x4 system that allowed the driver to engage the four wheel drive as the car was moving. Last used on the 1994 Loyale, but also used on older GL, DL, Subaru, Brat etc. Not currently used on any Subaru.

Part-time all wheel drive Vacuum-actuated front axle.

Figure: subaru libero

Figure: subaru libero

1985-1987 turbo and 1988-1989 non-turbo XT had part-time all wheel drive selectable by a push button atop the shifter. Automatic(?) all wheel drive was used on XT and XT6 with automatic transmission.

Figure: subaru alcyone xt

Figure: subaru xt logo

Subaru technology in detail (internal link)

Subaru Offers Four Versions of Symmetrical All-Wheel Drive for 2009 (.doc 73KB)

There is a lot of confusion about the differences between the various Subaru AWD systems (.doc 39KB)

Subaru AWD (.pdf 90KB)