This article also appears in
Subscribe now »

Adjustable torque bias and a torque-vectoring system help the Subaru WRX STI power through tight mountain curves.

List of enhancements is long for MY2015 Subaru WRX STI

Following the introduction of its WRX rally car replica at the Los Angeles Auto Show in November, Subaru let the other shoe drop Jan.14 at the North American International Auto Show in Detroit by unveiling the cherry-on-top STI variant.

This car reinforces the WRX’s performance capabilities with still more power and more race-oriented suspension tuning than the regular WRX. The STI’s turbocharged 2.5-L boxer four-cylinder produces 305 hp (227 kW) and 290 lb·ft (393 N·m) using 14.7 psi (1.0 bar) of boost, while achieving U.S. EPA federal Bin 5 Tier 2 emissions. EPA fuel economy is forecast to be 17 mpg city and 23 mpg highway.

Most of the STI’s changes from the regular WRX are tweaks, but in the case of the transmission an entirely different six-speed manual gearbox is used. It is an Aisin-supplied design, hand-assembled in-house at Subaru, according to Todd Hill, WRX Car Line Manager. The transmission features triple-cone synchronizers on first and second gears, with a double-cone synchronizer on third gear, and is improved for reduced noise and vibration with reduced shift effort compared to the previous model’s transmission.

The STI’s center differential lets drivers adjust the car’s torque bias to suit the situation. The system has three performance modes and six driver-selectable differential locking settings. This adjustable center differential combines with a helical limited-slip front differential and a JTEKT Torsen limited-slip differential to help driver put power to the ground in the ideal fashion.

Meanwhile, active torque vectoring applies the inside front brake when turning to help rotate the car around corners, using input from the steering angle sensor and yaw sensor.

The STI has stiffer springs and features inverted MacPherson struts with aluminum lower control arms in the front for minimized unsprung mass, which is critical for a car aimed at providing traction on rough off-road surfaces. Brembo brakes have four-piston front calipers and two-piston rears.

The standard STI rolls on 18-in cast aluminum wheels, but the limited edition, 1000-unit Launch Edition features the STI’s iconic gold-colored forged aluminum BBS wheels instead.

All of these suspension upgrades bolt to a new unibody which is substantially upgraded from the previous generation's, with greater use of high-strength steel. Setting the WRX and STI even further apart from the Impreza base car are thicker floorpan sheet metal, gussets at frame joints, and reinforcements on the strut towers, which contribute to a 40% improvement in torsional rigidity and a 10% improvement in bending rigidity compared to the previous model, according to Hill.

The WRX and STI wear almost entirely unique sheet metal compared to the Impreza, carrying over only the roof, decklid, and window glass. The STI substitutes an aluminum hood for the WRX’s steel one.

Subaru has added an engine sound delivery system, using a hose with a tuned diaphragm to deliver enhanced engine sound to the passenger cabin. This is a passive system that boosts preferred frequencies but that does not create any fictional sound, as an electronic system can do, emphasized Hill.

When engine sound isn’t the best entertainment option, the driver can switch on a 440-W Harman-Kardon nine-speaker sound system.

Continue reading »