Subaru's constant all-wheel-drive technology - as used in the Impreza, Liberty and SVX - is designed to offer maximum traction on stable and slippery surfaces. Unlike an on-demand off-road style all-wheel-drive, the Subaru system features a limited-slip centre viscous coupling. This allows the car's front and rear tyres to follow their natural turning radius through a corner without causing driveline "binding". In addition, the viscous coupling (VC) apportions engine torque to the front and rear axles depending on the amount of tyre slippage at either end. This is known as the "torque split", where the majority of drive is automatically passed to the axle with the most traction.
Up until now, Australian Subaru AWD owners have had to make-do with the front-to-rear torque split characteristics of the factory viscous coupling. However, thanks to the recent availability of STi VCs, you can now enjoy a changed distribution of torque between the front and rear axles. We talked with Sydney's Middleton Rally Team to scoop all the inside info...
The Subaru Viscous Coupling
The Subaru's centre VC is hidden in the rear extension on the gearbox - the transfer case. This casing contains the viscous coupling unit as well as rear transfer gears (which are employed to reduce tailshaft rpm). Notice that - once removed - the VC assembly looks very similar to a conventional differential, except it contains a viscous limited slip centre. Here, the function of limiting front-to-rear drive slip is performed with a series of drive plates whose movement is opposed by a high viscosity silicone fluid.
Testing
The first step in testing your existing viscous coupling is to remove it from the vehicle. Note that there's no need to drop the entire gearbox - the transfer case simply unbolts once the tailshaft and gearbox cross member are removed. The transfer casing is then split in two and the VC is pulled out.
There are major differences between those VCs found in vehicles built before and after the MY99/Version 5-6 STi. Earlier models have a circlip on the coupling, which - when removed - allows the housing to be split and the VC cartridge to come out and to be disassembled. Post MY99s - with their stronger, 6-bolt gearbox housing - have a sealed VC assembly that's impossible to open and repack. The viscous liquid in these couplings, incidentally, is said to be very tacky and smelly.
Once the viscous coupling is extracted from the rear transfer case, MRT can perform their unique lathe test. Torque is passed from the lathe's chuck through a makeshift "pinion shaft" and into the test VC cartridge. Measurement of the torque that passes through the VC cartridge is derived from a torque wrench that's connected at a right angles to cartridge and its output shaft. Notice that testing post-MY99 VCs (which cannot be opened) requires the use of various different lathe components. Spinning the whole assembly at 100 rpm, a typical Liberty RS/Impreza WRX VC should see around 10kg/m indicated on the torque wrench. If it comes up much less, the VC is considered worn out - something that's often caused by running unequally sized tyres on the car.
Replacement VCs
There are two different viscous couplings available to suit all MY Impreza/Liberty/SVXs. Subaru Technica International (STi) produces both a 12 and 20kg/m VC to replace the standard 10kg/m VC. Notice that a slot-in cartridge is available for pre-'99s, and an entire VC assembly is available for more recent MYs. You cannot modify the original couplings. The 12kg/m item would make a subtle improvement for any mildly modified WRX, but the 20kg/m version - which is intended for competition use - certainly gives the more pronounced difference. Notice that lathe testing the 20kg/m unit is very difficult, because the extra torque transmitted through it tries to rip the torque wrench out of the operator's hands. In other words, it's bloody stiff!
And the cost? Depending on exchange rates, a 12 or 20kg/m viscous coupling - for any MY Impreza/Liberty/SVX - will cost approximately $1500. If you're in Sydney, you can choose to pay a further $250-300 in fitting labour (plus gaskets).
Effects
According to MRT rally ace Brett Middleton, the 20kg/m VC makes a major difference to the WRX's on-road performance. However, note that moving to the 20kg/m VC without any other swaybar or suspension mods will cause the car to "understeer like a pig". More typically, however, vehicles requiring a STi VC already have adjustable swaybars and suspension - which can be tailored to suit the characteristics of the new coupling.
Once correctly set-up, it's claimed that your car's turn-in response should not be jeopardised, mid-corner stability will be slightly improved and - most noticeably - you'll be able to power out from the apex much more efficiently. Instead of spinning the inside front wheel as soon as you get on the power, the car will send torque to all four wheels and accelerate away. That also means that more throttle can be applied earlier in the corner.
We're told that WRX drag racing enthusiasts will also find improvements with a STi VC. Any full-bore launch with the 20kg/m coupling will give much reduced chance of wheelspin. Conversely, braking with the heavy-duty VC provides a more equal spread of braking force to each wheel. A by-product of this is that the Subaru ABS system will kick in much later, as there will be reduced tendency to lock an individual tyre. Brett says that handbrake turns become virtually impossible with the 20kg/m coupling - even with a hydraulic handbrake. He also suggests that the biggest disadvantage of the 20kg/m coupling - on a streetcar - is its low speed manoeuvring characteristics. With increased driveline lock-up, there is noticeable fighting between the front and rear wheels.
This aside, these STi viscous couplings look like they might become the next "must have" item for the modified WRX brigade. They're certainly worth looking into if you've already spent some loot modifying your Rex and you're a keen driver...