The 2008 Impreza WRX STI is far more than a modified version of the WRX. The two models share a basic architecture, some interior trim and very little more. Nearly every major mechanical system in the WRX STI is unique to this model: engine, transmission, DCCD All-Wheel Drive system, Multi-Mode Vehicle Dynamics Control stability control, special suspension, brakes, plus interior appointments.
MAJOR CHANGES TO POWERTRAIN AND DRIVETRAIN FOR 2008
Subaru optimized the entire powertrain and drivetrain in the 2008 Impreza WRX STI to provide better performance and a greater degree of driver-selectable control. The heart of the Impreza WRX STI is a revised version of the 2.5-litre intercooled turbocharged Subaru Boxer engine used previously. The 2008 model is further enhanced for high performance with the addition of Subaru's Dual Active Valve Control System (Dual AVCS) variable valve timing technology and the Subaru Intelligent Drive (SI-DRIVE) system.
Dual AVCS controls valve timing on both the intake and exhaust valves, adjusting camshaft timing to employ the ideal intake and exhaust valve overlap under all engine operating conditions. The system helps eliminate the traditional compromise between low-end torque and high-rpm horsepower and also helps to enhance fuel economy and reduce emissions.
Subaru Intelligent Drive (SI-DRIVE) technology adds a new dimension of driver engagement. The system allows the driver to custom-tailor the car's driving characteristics by choosing from among three selectable modes - Intelligent, Sport and Sport Sharp - using a rotary dial on the centre console. By doing so, the driver is optimizing engine performance, control and efficiency.
TURBO BOXER ENGINE PROVEN ON ROAD AND TRACK
The STI's 2.5-litre horizontally opposed 4-cylinder engine is based on a specially reinforced semi-closed deck engine block with high-strength cast pistons. A semi-closed deck design provides the same cooling efficiency as an open-deck cylinder block but with the higher strength of a full closed-deck. Special reinforcing ribs in the block provide additional strength, and the crankshaft is nitride-hardened. Compression ratio remains 8.2:1. The turbocharger in the WRX STI engine produces 14.7 PSI of boost, compared to 11.9 PSI in the standard WRX.
Further enhancing quick throttle response in the WRX STI is a drive-by-wire Electronic Throttle Control (ETC). This system eliminates the physical linkage between the throttle pedal and the engine. The ETC system provides more precise control and more flexible response than a conventional throttle and also helps to reduce fuel consumption and emissions.
Iridium-tipped sparkplugs provide a more precise spark than platinum-tipped sparkplugs. A precious, silver-white metal, Iridium is one of the densest materials found on Earth. An Iridium-tipped sparkplug requires less voltage to spark, burns fuel more efficiently and sparks at leaner air/fuel mixtures to ensure optimal performance. The WRX STI exhaust system features a more efficient high-flow rear catalytic converter, low backpressure muffler and two sets of dual-outlet chrome tailpipes.
HOW SI-DRIVE WORKS
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. In Intelligent mode, the system adjusts the horsepower for maximum efficiency, and switches to a more relaxed throttle response curve. By controlling throttle angle and response of the Electronic Throttle Control system, SI-DRIVE restrains peak power somewhat. One effect is smoother response while commuting in traffic.
In addition, Intelligent mode's more relaxed responses can be used to complement the low-traction-surface driving safety provided by the All-Wheel Drive System. (Subaru recommends that WRX STI drivers install proper winter tires for snow conditions.)
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 and put down the power sooner. As its name might suggest, Sport Sharp mode ramps up the performance feel of the car and is ideal for tackling twisty roads.
EXCLUSIVE 6-SPEED MANUAL TRANSMISSION
As before, the Impreza WRX STI comes equipped exclusively with a 6-speed manual transmission. Gear ratios remain the same as in the 2007 model. This unit is designed to handle high-torque capacity while providing short, smooth shifts. All gears are helical-type with diameters and thickness chosen for high-strength. Triple cone synchronizers on 1st (new) and 2nd gears, a double-cone synchronizer on 3rd gear and single-cone carbon synchronizers on all other forward gears help ensure durability. A parallel link gear selector shortens shift throws, while sliding bearings on the shift fork rods reduce friction.
REVISED DCCD ALL-WHEEL DRIVE WITH VDC
The 2008 WRX STI features an enhanced version of the Driver Controlled Centre Differential (DCCD) All-Wheel Drive System. The DCCD system is one of the most advanced and 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 centre differential augments the electronically controlled differential to enhance torque transfer performance. A steering sensor input enhances torque transfer response by accurately relaying the car's cornering situation.
The newly enhanced DCCD AWD System has four modes in the 2008 WRX STI: Auto, Auto (-), Auto(+) and Manual. Auto is the same as in the previous model. The new Auto (-) Active Sport setting shifts the torque bias to the rear and also opens the centre 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. There are also six manual differential locking settings.
Set in Auto mode, the electronically managed continuously variable transfer clutch varies the torque distribution ratio as needed to suit driving and road-surface conditions. Auto mode provides the best all-around performance for most drivers and circumstances, varying the front/rear torque distribution automatically via the limited-slip differential, depending on driving conditions and driver input. The system determines the ideal power distribution in response to vehicle acceleration, deceleration, steering angle, cornering force and wheel slippage.
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 selects manual mode with a console-mounted button and then adjusts the rocker switch on the centre console to select from among six levels of centre differential locking. Increasing the lock factor keeps more power at the front wheels, which the driver might want in certain driving conditions, on particular roads 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.
The WRX STI driver can also custom-tailor the level of dynamic intervention offered by the Vehicle Dynamics Control (VDC) system. There are three settings: VDC Normal, Traction (less ABS engagement) and Off. The Normal setting provides the highest level of system engagement, utilizing traction control (TCS) and VDC. The Traction setting is more performance-oriented, with less-restrictive VDC and the engine torque-reduction control disengaged. The Off setting allows the driver to disengage the VDC and the traction control functions.
BREMBO BRAKES WITH SUPER SPORT ABS
Subaru is again equipping the Impreza WRX STI with the advanced Brembo performance brake system, which employs ventilated 4-wheel disc brakes to help ensure stopping power and consistency commensurate with the vehicle's high-performance capability. Rotors are slightly larger than on the 2007 model: the front discs measure 33 cm (13 in.) in diameter (vs. 32.5 cm [12.77 inches] on the previous model) and utilize 4-piston, fixed-position calipers. The rear discs are 32 cm (12.6 inches) in diameter (vs. 31.25 [12.3 inches]) and use 2-piston, fixed-position calipers. The rear caliper pistons are larger in size to provide increased braking force.
Electronic Brake-force Distribution (EBD) helps to reduce stopping distances, and the Super Sport Anti-lock Braking System (ABS) helps to reduce understeer compared to conventional ABS systems. EBD provides precise control of braking pressure between the front and rear wheels based on the vehicle load to improve stability and stopping distances. Brake Assist analyzes the force and stroke on the brake pedal and increases hydraulic boost to provide stronger emergency braking, also activating the ABS system early if needed.
Super Sport ABS, using input from a lateral g-sensor, a steering angle sensor, a yaw sensor and a brake pressure sensor, can control the braking forces at each rear wheel independently, optimizing braking ability during hard cornering to help reduce understeer. In contrast, conventional ABS systems control the rear wheels as a pair, reducing the braking force to both rear wheels equally.
ABOUT SUBARU CANADA, INC.
Subaru Canada, Inc. is a wholly owned subsidiary of Fuji Heavy Industries Ltd. of Japan. Headquartered in Mississauga, Ontario, the company markets and distributes Subaru vehicles, parts and accessories through a network of over 90 authorized dealers across Canada.
CONTACT: Elaine Griffin (905)568-4959
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