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Tokyo marked the global launch of the hot-rod rally-champ WRX STI -- or at least the one the rest of the world will be getting. The North American model breaks cover next month at the Los Angeles International Auto Show. What are the differences? The biggie is under the hood, where the global model stays true to the World Rally Championship homologation regs with a 2.0-liter boxer engine boosted to 14.7 psi by a twin-scroll Mitsubishi turbocharger to produce 295 SAE horsepower and 299 pound-feet of torque (Wiki editors note: The 308 PS/304 bhp/311 pound-foot power figures widely quoted everywhere were generated using Japanese Industry Standard testing procedures, which differ from our SAE test).
We'll continue to get the 2.5-liter engine boosted by a single-scroll Mitsubishi turbo that's said to produce slightly more horsepower (a shade over 300 SAE net, we hear) and slightly less torque. Yes, it's counter-intuitive for the bigger engine to produce less twist, and we won't believe it until Subaru engineers spill the beans next month, but the 2.5 is said to have a broader torque curve and to feel torquier, and it won't have to work quite as frenetically to make a few more ponies. Gearing is the same, so ours should outperform the global STI. We'll get Dunlop tires instead of the Bridgestone Potenza RE050As everybody else gets, but we're promised the rest of the suspension design and tuning is identical.
Let's recap the basics for the less-devoted WRX STI fans before digging into the newly divulged details. All WRX STIs will be five-doors -- the shorter overhangs and longer wheelbase (relative to last year's sedan) improve high-speed maneuverability in Finnish forests. Relative to the WRX hatch, the grille, fascias, fenders, rear quarters, and side sills are unique, designed to accommodate the fat 245/40R18 tires. The front fascia and rear wing balance front and rear lift at zero (an aftermarket front lip spoiler is said to provide a tiny bit of downforce). Inside the gauges, wheel, center console, and seat trim are unique (but to these eyes, not sufficiently upgraded to suit the expected price tag of around $35,000).
More functional upgrades include SI-Drive (Subaru Intelligent Drive, which alters the throttle map for relaxed efficiency, sport, or sport-sharp modes, all accessed by a console-mounted rotary knob); multimode vehicle dynamics control (VDC), allowing on, off, and sport modes (the latter disables engine-torque management); and a four-position Driver Controlled Center Differential. The high degree to which the personality of the WRX STI's dynamic behavior can be tailored is cited as the reason Subaru doesn't need to offer different models of its top rally performer to match the many flavors of EVO.
The DCCD system is the coolest, most innovative part of the new WRX STI. It was designed and is built by Subaru, and is claimed to be unique in the world as the only hybrid mechanical/electronic limited-slip differential. The unit, which is the same size and weight as last year's purely electronically locking LSD, is geared for an inherent 41/59 front/rear torque split. The mechanical differential can vary this ratio between 55/45 and 25/75 front/rear, after which the electronics can take the ratio to 75/25 or 5/95 front/rear (see How DCCD Works for more nitty gritty details).
This hybrid system delivers the twin benefits of automatic, no-delay locking within the mechanical range of operation (which is inherently quicker and more effective than purely electronic diffs), and a range of lock-up capability that's wide enough to keep the car on track without stability-control intervention. And the electronics allow tailoring of the differential's behavior among three automatic modes, and six degrees of manual lockup ranging from zero electronic torque routing to the front, to full-lockup, routing up to 75 percent of the twist frontward. The automatic settings tailor the torque bias based on input from sensors monitoring throttle and brake inputs, engine- and vehicle-speeds, lateral g, and yaw rates. In addition to full auto, there are settings biased to provide slightly more or less front torque (the former is best for slippery-road driving, the latter for sporty dynamics in the dry). There are mechanical torque-sensing limited-slip differentials on the front and rear axles as well.
Those are the major product highlights that can be gleaned from tire-kicking and engineer-interviews. I sampled the car today on Japan's famous 2.8-mile Fuji Speedway circuit, but I had to promise not to tell you about how it feels until November 9th. Click back then for the rest of the story.
How DCCD Works:
Wheel spin at one axle or the other causes a pair of cams with angled teeth on them to pull a series of clutch plates together tightly enough to shift the static 41/59 front/rear torque distribution 15 percent in either direction. If the computer thinks more lock is necessary, an electromagnet energizes to pull a second set of clutch plates into contact, causing a secondary ball-ramp to engage, adding considerable additional pressure to the primary differential-locking clutch pack and allowing an additional 20-percent torque shift to either axle. Elegant, simple, light, compact -- and exclusively Subaru.
2008 Subaru Impreza WRX STI - First Look - Motor Trend
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