Subaru has improved one of the world’s best engines for the new STI, enhancing performance, improving fuel efficiency, and reducing emissions.
Most notable, perhaps, is the addition of Subaru Intelligent Drive (SI-DRIVE), which effectively gives the STI three engines in one. SI-DRIVE regulates the engine control module and Electronic Throttle Control in three settings:
- Intelligent (I), for smooth, efficient performance driving
- Sport (S), for all-around performance driving
- Sport Sharp (S#), for maximum driving performance
(For more on SI-DRIVE, see the Summer 2007 issue of Subaru Drive magazine.)
Other engine highlights include:
- A larger intercooler with a new high-efficient, cross-flow design, which improves performance
- Dual Active Valve Control System (DAVCS), which varies both intake and exhaust camshaft timing for enhanced low-end torque and high-rpm horsepower, for improved fuel economy, and for reduced emissions
- Iridium-tipped spark plugs with square positive electrodes, for reduced emissions and better fuel economy
- Cast-iron exhaust manifolds and high-efficiency catalytic converter, which improve emissions as well as performance
- A high-flow fuel pump, which improves performance and fuel economy
- A high-flow, low-restriction exhaust system with stainless-steel quad tailpipe outlets
Improved features and new ones make the STI even more fun to drive.
Modifications to the 6-speed manual transmission improve shifting smoothness and durability. Changes include:
- Triple cone synchronizers and modified synchronizer chamfer for first and second gears
- Carbon cone synchronizers for fourth, fifth, and sixth gears
- Reduced shift friction in the synchronizer sleeve for improved shift feel
- Revised gear ratios to optimize the engine’s performance characteristics
The capabilities of the Driver Control Center Differential (DCCD) AWD system have been expanded. Along with six manual settings, it has three automatic modes:
- Automatic – center differential lock-up controlled by the Transmission Control Module
- Automatic Minus – less center differential lock-up for driving on high-traction surfaces
- Automatic Plus – more center differential lock-up for driving on low-traction surfaces
(Read more about the basic DCCD system in Drive Performance Version 1.2)
Other mechanical additions for 2008 include:
- Incline Start Assist – momentarily helps prevent the car from rolling backwards when starting on an uphill grade
- Vehicle Dynamics Control (VDC) system (stability control) with three modes:
(Read more about VDC in the Online Exclusive for “2008 Subaru Impreza” in Fall 2007 Drive.)
(Also, see an application chart here for tailoring SI-DRIVE, DCCD, and VDC to preferred driving habits and/or road conditions as part of the Online Exclusives for this article.)
Front and rear suspension systems feature lighter weight in the front and a completely different design in the rear.
Reduced unsprung weight in the front improves handling and stability.
- The front suspension’s aluminum L-arms are 2.2 pounds lighter than the previous design
- Box-section front crossmembers eliminate the need for a front subframe, reducing weight
- The rear suspension’s double wishbone design incorporates a subframe, which helps isolate road noise and shock from the passenger cabin for a quieter ride
- Rear bushings are tuned to reduce noise, vibration, and harshness
Both front and rear wheel track are wider than the WRX – 1.3 inches in front and 1.5 inches in back.
Eighteen-inch wheels and tires are standard – 5-spoke wheels with 245/40 R18 93W Dunlop SP Sport 600 summer tires.
Brake system features include:
- Brembo® brakes with black calipers carrying STI logos
- Ultra-low expansion brake hoses, which provide more positive brake feel and pedal strength
- Improved power brake booster
- Rear calipers increased in width from 35 mm to 45 mm to improve clamping force and overall performance