Only a few weeks before the WRX marks its 20th anniversary in Australia, the new fourth-generation performance STI sedan has kept true to its predecessors by offering a raw performance car designed for everyday and motorsport use.
Subaru admits the updated STI was more an evolutionary change than anything else, with the Japanese company focusing primarily on fine tuning its ride, handling, steering response and rear tyre grip while leaving many components unchanged, including the engine and gearbox.
Engine and gearbox:
Compared with the previous model, the new Subaru WRX STI utilises the same EJ series engine and six-speed manual gearbox. Subaru has all but left the engine alone, allowing it to produce 227kW of power and 393Nm of torque, but it has improved the gearbox with the shift mechanism improved for better tactile feedback (in engineering speak, a detente has been added to the shifter arm shaft and a change on shifter rods made).
Subaru will offer the new STI in sedan-only body type and there's no word of an automatic transmission as of yet.
The new Subaru STI claims to have the same steering response (delay between steering input and lateral acceleration) as the new Porsche 911 at 10ms. That compares to 27ms for the previous model, 14ms for BRZ, 15ms for Mitsubishi Lancer Evolution X and 20ms for Volkswagen Golf GTI.
Subaru engineers have also modified the car’s turn-in ability at high G, which is said to be superior than that of the rear-wheel drive BRZ.
The steering ratio has changed from 15:1 to 13:1, the torsion bars have been stiffened by 125 percent and even the gearbox mount bushings are now 400 percent stiffer.
Rear Tyre Grip:
Subaru has been adamant that a stable rear end with more tyre on the ground is the key to the STI’s apparent physics-defying abilities, so it focused considerably in improving the fourth-generation vehicle in this area.
Lateral acceleration limit (in Gs) has been improved from 0.92 to 0.98 (Porsche 911: 1.0, Evolution X: 0.94). This has been a result of subtle suspension setup changes that allow for more tyre grip.
Subaru’s active torque vectoring - which applies brake to inner front wheel and distributes torque to outer front wheel - remains a key component for power delivery in the new STI.
Body roll is the nemesis of sports cars, yet there’s a fine line between an unbearable racecar-hard suspension and one which can be tolerated in an everyday vehicle. The new STI aims to find the right balance.
On a certain cornering test conducted by Subaru, the roll angle of the previous STI was found to be 1.75 degrees, which has now been improved to 1.5 degrees.
Subaru also claims the STI is not only better than the last one under an emergency handling test, which involves a double lane change at high speed, but is indeed better than several higher-priced competitors.
The new STI remains stable at up to 61km/h during the test, according to the Japanese brand, while the 911 Carrera S gives up at 59.5km/h, BMW 135i at 58.5km/h, Nissan 370Z and Porsche Boxster at 58km/h, while its traditional and desperately ageing rival, the Evolution X tops out at 56km/h.
For these changes to be possible, Subaru has changed the front and rear suspension. For the front, it has stiffened the cross-members, changed the rigid lower arm bushings, changed the stabiliser bar diameter from 21 to 24mm, upped the spring rates by 22 percent, lowered the lower arm rear bushings by 10mm, added stiffening plates to support front mounts and increased the front’s lateral stiffness by 14 percent.
The rear sees the subframe and trailing link bushings stiffened. The rear lateral outer bushings have changed to racing-type pillow ball, stabiliser bar diameter is up from 19 to 20mm, spring stiffness is up six percent, front link mount for new subframe is lowered by 2.3mm and 3mm of initial toe-in has been added.
There are also rear subframe supports, while the rear lateral arms have been moved 10mm outward. In total, lateral stiffness for the rear end is up 38 percent, which means an overall roll stiffness front and rear is up 24 percent.
With all the suspension changes taking place, the STI’s actual chassis construction has also changed to use more high-strength steels. Where the current model uses only 270MPa, 400-440MPa and 590MPa strength steel, the new model adds higher strength 980MPa and hot stamp steel and improves the overall mixture of the existing metal’s quality
Torsional and bending rigidity has increased for body stiffness and durability. The new car’s chassis is up 40 percent in torsional rigidity and 30 percent in bending rigidity.
Pricing is yet to be announced for the Australian market but we suspect it will remain around the $60,000 mark and be available in two specifications. Subaru sells roughly 200 WRX STIs per year.
2015 Subaru WRX STI Specifications
- Body: 4-Door wide-body design, 5-passenger
- Engine: Horizontally opposed (BOXER), turbocharged/intercooled; SI-DRIVE Engine Performance Management system
- Block: Specially reinforced die-cast aluminum with integral cast iron cylinder liners, five main bearings, semi-closed deck cylinder design
- Cylinders: 4
- Displacement: 2,457 cc
- Bore x Stroke: 99.5 x 79 mm
- Heads/Valvetrain: Aluminum alloy cylinder heads with double overhead belt-driven camshafts (DOHC), four valves per cylinder, Subaru Dual Active Valve Control System (Dual AVCS) variable valve timing (operates on intake and exhaust valves)
- Compression ratio: 8.2:1
- Fuel/Induction: Aluminum alloy intake manifold, sequential multi-port fuel injection (MFI), electronic throttle control (ETC) system, turbocharger, cross-flow intercooler
- Max. boost: 14.7 PSI
- Power: 227kW @ 6,000 rpm
- Torque: 393Nm. @ 4,000 rpm
- Ignition: Direct with iridium-tipped spark plugs
- Emission control: Bin 5 Tier 2-Federal (EPA) / LEV2-California
- Fuel requirement: 91 AKI minimum
- Symmetrical All-Wheel Drive with Driver Control Center Differential (DCCD) featuring three performance modes and six driver-selectable differential locking settings; mechanical and electronically controlled limited-slip center differential; helical limited-slip front differential and TORSEN® limited-slip rear differential, steering angle input sensor
- Stability system: Multi-mode Vehicle Dynamics Control (VDC) stability and traction control system with yaw sensor; Active Torque Vectoring
- Transmission: 6-speed manual transmission (WRX STI Launch Edition features STI short-throw shifter)
- Chassis: Unitized body construction with Ring-shaped Frame Reinforcement structure; high-tensile steel reinforcements at key structural and suspension-mounting locations
- Suspension: 4-wheel independent, high-performance STI sport-tuned
- Front: Inverted struts with forged aluminum-alloy lower A-arms with pillow ball joint mount coil springs, 24mm stabilizer bar
- Rear: Double-wishbone type with coil springs and damper units, stiffener bar, 20mm stabilizer bar
- Brakes: Brembo Performance Brake System, power assisted 4-wheel disc with 4-channel, 4-sensor Super-Sport anti-lock brake system (ABS) with g-sensor; Electronic Brake-force Distribution (EBD), Brake Assist and Brake Override
- Front disc: 13.0 in. ventilated with four-piston fixed position calipers
- Rear disc: 12.4 in. ventilated with dual-piston fixed position calipers
- Steering: Quick-ratio hydraulic power assisted rack-and-pinion
- Ratio: 13.0:1
- Turns, lock-to-lock: 2.5
- Turning circle: 36.0
- Wheels: 18 x 8.5J cast alloy (BBS® forged alloy wheels featured on WRX STI Launch Edition and WRX STI Limited models)
- Tyres: 245/40R18 97W
- Wheelbase: 104.3 in.
- Length: 180.9 in.
- Height: 58.1 in.
- Width 70.7 in
- Track: (front/rear) 60.2 in. / 60.6 in.
- Road clearance: 4.9 in.
- Headroom Front: 39.8 in. With moonroof: 37.2 in.
- Rear: 37.1 in.
- Legroom Front: 43.3 in. Rear: 35.4 in.
- Hip room Front: 52.1 in. Rear: 53.1 in.
- Shoulder room Front: 55.6 in. Rear: 54.2 in.