The 2008 Subaru Impreza WRX STI has a heritage of power and control. Previous models have been the foundations for countless racing victories and championships. The new WRX STI promises the same with its 305-horsepower, turbocharged, intercooled Boxer engine and six-speed manual transmission (only!).
But there’s more. Power and control incorporate enhanced technology. As suggested by new switchgear on the dashboard and center console and by markings within the instrument cluster’s center-mounted tachometer, a driver has some things to learn before wringing out the most from the car.
Today’s electronics now allow the driver to tinker with engine response characteristics, the manner in which the All-Wheel-Drive system fights for traction, and the degree to which braking and engine management help maintain vehicle stability. These capabilities are made possible by standard Vehicle Dynamics Control (VDC), Driver Controlled Center Differential (DCCD), and Subaru Intelligent Drive (SI-DRIVE).
Beginning with basic Anti-lock Braking System (ABS) technology, electronic control units have evolved from those that helped to prevent lockup during braking so the driver maintained steering control to more sophisticated systems. Today, ABS controls take information from a host of sensors, and that information enables advanced braking, traction, and stability-control functions for the 2008 WRX STI.
Using input from a lateral g-sensor, a steering angle sensor (new for 2008), a yaw sensor, and a brake pressure sensor, Super Sport ABS can control the braking forces at each wheel independently. The system enhances straight-line braking and optimizes braking ability during hard cornering by helping to reduce understeer. In contrast, conventional ABS systems control the rear wheels as a pair, reducing the braking force to both rear wheels equally instead of independently.
Super Sport ABS works in conjunction with VDC and DCCD, and it is complemented by Electronic Brake-force Distribution (EBD) and Brake Assist.
EBD helps to reduce stopping distances and to improve stability by precisely controlling braking pressure between the front and rear wheels based on vehicle load.
Brake Assist analyzes the force and stroke on the brake pedal. If the control unit senses an emergency situation, the system increases hydraulic boost to provide enhanced braking, also activating the ABS system early if needed.
The WRX STI driver also can custom-tailor the level of dynamic intervention offered by VDC. Changes are made by pushing the OFF switch on the dashboard to the left of the steering wheel column.
VDC has three settings:
|SETTING||DRIVING CONDITION||MODE SWITCH||WARNING LIGHT|
|Normal||Daily driving conditions||Default – no action needed; to return from Traction or Off settings, press once||None|
|Traction||Performance-oriented driving conditions||Hold in for more than two seconds||Green|
|Off||Emergency – mud or snow||Press once||Yellow|
The 2008 WRX STI features an enhanced version of the DCCD All-Wheel-Drive system. DCCD is one of the most advanced and most performance-directed AWD systems on the market, regardless of vehicle segment. The system employs planetary-type differential gears with a nominal 41:59 torque split. A mechanical limited-slip type center differential augments the electronically controlled differential to enhance torque transfer performance. Steering sensor input improves torque transfer response by accurately relaying the car’s cornering situation. Lockup is controlled by the Transmission Control Module, with setting selected by switchgear on the center console.
Set in Auto mode, the electronically managed continuously variable transfer clutch can vary the torque distribution ratio in response to driving and road-surface conditions. Auto mode provides the best all-around performance for most drivers and circumstances. The system determines the ideal power distribution in response to vehicle acceleration, deceleration, steering angle, cornering force, and wheel slippage.
The three Auto modes are:
The six manual differential locking settings remain the same as in the previous model. In manual mode, DCCD allows the driver to vary the front-to-rear torque distribution (up to 50:50 maximum) to optimize AWD performance for varying driving conditions. The driver first selects manual mode with a console-mounted button and then pushes or pulls the rocker switch on the center console to select from six levels of center differential locking. Increasing the lock factor sends more power to the front wheels, which the driver might want in certain driving conditions or to suit an individual driving style. A helical-type limited-slip front differential and TORSEN* torque-sensing limited-slip rear differential optimize power distribution side-to-side, helping to prevent the inside wheels from slipping during cornering.
|SETTING||DRIVING CONDITION||MODE SWITCH||MODE DISPLAY|
|Auto||All-around performance for most drivers and circumstances||Push AUTO/MAN to select between Automatic and Manual modes||AUTO|
|Auto [-]||For dry roads – quick response from the steering wheel and smooth driving||Pull the control switch rearward when in the AUTO mode||AUTO [-]|
|Auto [+]||For slippery roads – emphasized traction control||Push the control switch forward within the AUTO mode||AUTO [+]|
|Manual||Driver’s choice of initial limited-slip-differential torque – from minimal to almost completely locked (50% front:50% rear)||Push AUTO/MAN to select between Automatic and Manual modes, then push the control switch forward to increase the initial limited-slip-differential torque||[six bars that indicate the level of limited-slip-differential torque desired]|
Click here to read more about DCCD in Version 1.2 of Subaru Drive Performance magazine.
The SI-DRIVE system is integrated into the Engine Control Module (ECM). SI-DRIVE enables three distinctively different modes of vehicle performance characteristics by regulating the ECM and by fine-tuning the Electronic Throttle Control system. The three modes are:
With Intelligent mode selected, the system reduces engine torque and maximum power and switches to a more relaxed throttle response curve. Intelligent mode can provide smoother response while commuting in traffic. In addition, Intelligent mode complements the low-traction-surface driving safety provided by the All-Wheel-Drive system.
Selecting Sport mode provides quick throttle responses and powerful, linear acceleration, making it ideal for everyday sporty driving.
In Sport Sharp mode, SI-DRIVE modifies the engine’s electronic throttle mapping to deliver lightning-quick throttle response. As its name might suggest, Sport Sharp mode enhances the performance feel of the car and is ideal for tackling twisty roads and for track driving situations.
Click here to read more about SI-DRIVE in the Summer 2007 issue of Subaru Drive magazine.
What’s the lay of the land? What kind of weather are you having? What’s traffic like out there? (Sounds like the morning weather and traffic report.)
Confused as to which of the WRX STI SI-DRIVE, DCCD, and VDC settings to use? We thought that might be a problem at first – at least until you’ve had some experience with the car. After that, you’ll have your own preferences.
For starters, the chart below has some suggestions for different types of weather and/or performance driving.
|Snow/Dirt||S or I||Auto +||Normal|