[s-cars] Re: WRX (was NAC, but more turbo content)

TM t44tq at mindspring.com
Wed Jan 15 10:24:27 EST 2003

Excerpt from Subaru.com re: AWD systems:

As you'll see, it's a 45/55 F/R split, continuously variable.
Actually more sophisticated than the Audi Torsen system.



The Subaru All-Wheel Driving System - Four Types
All models equipped with the five-speed manual transmission feature a
viscous coupling center differential to split the power 50/50 between
front and rear wheels. The viscous coupling responds to wheel slip and
transfers power to the front or rear wheels with the best traction.
Featured on most Subaru models equipped with the 4-speed electronic
automatic transmission (4EAT) is Active All-Wheel Drive. This system
uses an electronically-managed continuously-variable transfer clutch.
This All-Wheel Drive system can anticipate wheel slippage and transfer
power to the front or rear wheels even before slippage occurs.
Impreza WRX models with the optional four-speed electronic automatic
transmission (4EAT) also feature Variable Torque distribution (VTD)
All-Wheel Drive.
he Subaru Outback H6-3.0 VDC combines VTD with a new stability system
called Vehicle Dynamics Control (VDC). This is the most advanced
All-Wheel Drive that Subaru has ever offered in the United States. -
[link to VDC Piece]
Variable Torque Distribution (VTD) All-Wheel Drive uses an
electronically controlled hydraulic transfer clutch that works with a
planetary gear type center differential to control power distribution
between the front and rear wheels. Under normal driving conditions, the
VTD system splits power 45% front and 55% rear to deliver more of a
"performance"driving feel. The VTD All-Wheel Drive system responds to
wheel slippage by sending the power to the wheel or wheels with the best

The Subaru All-Wheel Driving System keeps you safe and sound by
adjusting to changing road conditions in a split second, providing
outstanding traction on virtually any wet or dry surface in any weather.

Continuous All-Wheel Drive: Simple, Effective, and Reliable
In vehicles with the 5-speed manual transmission, the All-Wheel Drive
uses a viscous coupling in a center differential inside the transaxle
case. The viscous coupling contains a series of opposing discs attached
to the front and rear output shafts, surrounded by a silicone fluid. In
normal operation, power is distributed equally between the front and
rear wheels (50/50 power split). Slippage at the front or rear wheels
causes a rotational difference between the front and rear discs in the
viscous unit, which then shears the fluid.

The shearing action heats the fluid, causing it to thicken. As the fluid
thickens, power transfers from the slipping wheels to the wheels with
the best traction. When the slippage ceases, all the discs turn at the
same speed, restoring the 50/50 power split. The process is quick and
imperceptible to the driver and passengers.

The continuous All-Wheel Driving System is simple, compact, and
virtually invisible during operation. Its traction adds a significant
margin of safety on slippery or unpaved roads, and enhances dry-road

Active All-Wheel Drive
Subaru models equipped with the 4-speed electronic automatic
transmission (4EAT) employ Active All-Wheel Drive. Active All-Wheel
Drive optimizes power distribution before slippage occurs.

Instead of a viscous coupling center differential, 4EAT-equipped Subaru
vehicles feature an electronically managed variable transfer clutch in
the transaxle tailshaft. Power transfer is governed by slippage in the
clutch plates, which use a special friction material that easily handles
the loads generated during power transfer.

The electronic Transmission Control Module (TCM) controls the All-Wheel
Drive multi-plate clutch. Active AWD can adjust the power split in an
instant, depending on many input factors. If the front wheels begin to
slip, the TCM increases hydraulic pressure on the clutch, sending power
to the rear wheels. As the front wheels regain traction, the TCM reduces
pressure on the clutch, increasing slippage of the plates and
transferring power to the front wheels.

Intelligent Control
With Active All-Wheel Drive, the TCM monitors input from speed sensors
on the front and rear output shafts and also takes input from the
throttle position and transmission. All of these factors cause the TCM
to choose how aggressively it adjusts the power split.

Subaru Active All-Wheel Drive varies the power split according to how
you drive the vehicle. Under acceleration, weight transfers to the rear,
and the system responds by transferring more power to the rear wheels
(in a front-wheel drive vehicle, this weight transfer on acceleration
can cause wheel spin, compromising traction). When braking, weight
transfers to the front of the vehicle, and the All-Wheel Driving system
transfers power to the front wheels to help reduce stopping distance.

Compared with standard front- or rear-wheel drive, All-Wheel Drive also
enhances cornering performance. When you enter a turn, lifting off the
gas and applying the brake transfers power to the front wheels for
greater steering control. As you exit the turn and accelerate, power
transfers to the rear wheels for added traction under acceleration.
Power transfer occurs quickly and imperceptibly, just as it does with
the continuous AWD in a manual-transmission.

Limited-Slip Rear Differential
Many models in the Subaru lineup feature a viscous limited-slip rear
differential for even greater traction capability. If one rear wheel
loses traction, the differential automatically diverts power to the
other wheel. A viscous limited-slip differential not only helps traction
in slippery on- or off-road conditions, but also serves as a cornering
aid. As you enter a curve and weight transfers to the outside wheel, the
inside wheel can lose traction. The viscous limited-slip differential
transfers power to the outside wheel, which has greater grip.

VTD All-Wheel Drive
Impreza WRX models with the optional four-speed electronic automatic
transmission (4EAT) features Variable Torque distribution (VTD)
All-Wheel Drive. The new VTD system uses an electronically controlled
hydraulic transfer clutch that works with a planetary gear-type center
differential to control power distribution between the front and rear
wheels. VTD All-Wheel Drive splits the power 45 percent front/55 percent
rear, with the slight rear-wheel bias enhancing the performance driving
feel. VTD All-wheel Drive constantly monitors throttle input to account
for weight transfer and responds to driving conditions to continually
optimize power distribution on all road surfaces.

VDC: A New Level of All-Wheel Driving Performance and Control
The Outback H6-3.0 VDC gets its name from Vehicle Dynamics Control, an
advanced stability system designed by Subaru to help enhance control
under poor driving conditions. VDC represents the next level of active
driving safety and performance.

The Subaru Vehicle Dynamics Control (VDC) system monitors vehicle
stability and automatically reacts to help the driver maintain
directional stability at all speeds and on all surfaces, more precisely
than even the most skilled driver can do. If VDC senses a loss of
control due to understeer, oversteer, drift conditions or wheel spin, it
quickly takes corrective action. The vehicle seems to respond almost
intuitively to the needs of the driver, no matter how demanding the

- VDC and Variable Torque Distribution
The Outback H6-3.0 VDC features Variable Torque Distribution (VTD)
All-Wheel Drive, the most advanced All-Wheel Drive system that Subaru
has ever offered. VTD works in conjunction with the Subaru VDC system to
provide unequalled traction and stability. Under normal conditions, VTD
splits power 45% to the front wheels and 55% to the rear wheels to
deliver more of a performance driving feel. VDC adds another layer of
driving control, an all-wheel, all-speed Traction Control System (TCS).
Should any wheel begin to spin, VDC applies braking force to that wheel
to keep spin under control. If the driver is applying more throttle than
available traction will allow, the VDC System signals the Engine Control
Module (ECM) to momentarily reduce engine output by turning off one or
more fuel injectors.

- VDC Control Module and Sensors
At the heart of the VDC system, the VDC Control Module coordinates data
from sensors throughout the vehicle to determine if the vehicle is
responding accurately to the driver's intentions. If there is the
slightest difference between what the driver is asking the vehicle to do
and what the vehicle is doing, the system issues a precise, extremely
rapid series of directions to control the All-Wheel Drive components,
the engine output, and braking force to individual wheels. For example,
to correct understeer (front-wheel drift), VDC applies split-second
brake pressure to the inside rear wheel to help pull the vehicle back on
course. To correct oversteer (rear-wheel drift), VDC applies
split-second brake pressure to the outside front wheel to bring the rear
of the vehicle back in line.

VDC coordinates signals from its on-board sensors to help ensure that
the driver's intentions are reflected in the vehicle's behavior. One of
these sensors, the Steering Angle Sensor, measures the amount and
direction that the driver is turning the steering wheel. The Brake
Pressure Sensor monitors the level of braking force applied by the
driver to help determine any corrective measures needed to restore
vehicle control. Wheel Speed Sensors measure the rotational speed of
each wheel, helping to determine cornering rates and whether there is
any wheel spin. The g-Force Sensor measures lateral acceleration, or how
hard the vehicle is cornering, while the Yaw Rate Sensor senses the
rotation of the vehicle around its center axis, to help determine
vehicle stability and traction.

- VDC and the Transmission Control Module
In addition to its transmission shift control duties, the Transmission
Control Module (TCM) aids wheel traction. When a loss of traction is
detected, the VDC system signals the Transmission Control Module (TCM)
to adjust power distribution between the front and rear wheels, helping
to ensure that the wheels with the best traction receive the most power.

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