FONTANA, CA — The Subaru WRX STi is a car with multiple personalities.

As an everyday driver, it has great road manners. Its horizontally opposed 2.5L turbocharged 4-cyl. produces a staggering 300 hp (helped by Subaru's first-ever use of variable valve timing for its signature “boxer” engine layout). But that power is not at all unruly, thanks to continuously variable power distribution from Subaru's excellent all-wheel-drive system.

But when the testosterone starts a-flowin' and the need for speed overwhelms, the WRX STi lays it down like the best 1960s muscle car.

Actually, it's better — because the same AWD system that makes the STi a nice casual driver takes this modified Impreza to rally-car heaven, then parks it at the American Graffiti drive-in. Four wheels pulling hard are better than two. Duh.

For the launch, Subaru took journalists to the California Speedway here to thrash about on a handling course and autocross track. Here, the AWD system shines. The STi gets its name from Subaru Technica International, the auto maker's motorsports arm.

The WRX STi doesn't have electronic stability control. And Subaru contends it doesn't need it — engineers say a well-executed AWD system can perform many of the same functions. Typical stability control systems use a yaw sensor to detect when a front or rear slide is imminent, then strategically brake a single wheel to prevent it.

The advanced AWD system in the STi instead achieves stability by apportioning extra torque to individual wheels.

There are two components to this AWD marvel. First, the Driver Controlled Center Differential generally channels 65% of the power to the rear wheels and 35% to the front when in automatic mode.

During hard cornering, DCCD begins to lock the differential, bringing the power split closer to 50/50. Subaru says the system allows aggressive drivers to maximize handling in corners while maintaining good control. For extra handling “tunability,” there's a unique, center-console thumbwheel that allows the driver to manually choose how much torque is split between the front and rear by selecting from among six levels of center-differential locking. Want to get jiggy with the back end? Dial the thumbwheel all the way back, and the tail swings like it belongs to Jennifer Lopez.

Second, STi has Subaru's excellent new Suretrac limited-slip front differential, a faster-acting gearless unit that responds to torque differences between the front wheels to increase traction and reduce understeer.

All that sounds like a long-winded way to get stability control. But DCCD, Suretrac and the new Super Sport ABS work in tandem to give driving enthusiasts a performance edge that Subaru says isn't available from brake-activated stability control systems.

The STi also has an intercooler, with 33 tubes packed into its compact 18.5-in. (47-cm) width. It can be sprayed with distilled water to initiate extra cooling of the intake charge, theoretically boosting power.

The intercooled 4-cyl. propels the STi to 60 mph (97 km/h) in 4.8 seconds. That's faster than a normally aspirated Porsche 911.

The STi manages decent fuel economy: 18 mpg (13L/100 km) in the city, 24 mpg (9.8L/100 km) on the highway. Subaru looked everywhere to cut weight. The car has no floor mats and no audio system, as well as lightweight rear glass. Use of forged aluminum saved 4 lbs. (1.8 kg) per wheel.

Brembo brakes and Bridgestone Potenza 225/45ZR17 tires reinforce the purpose of the STi. There's barely any tread on the tires, so driving on snow is not an option. An automatic transmission is not available.

The STi hit U.S. streets in late May, priced at $30,995. Its older brother, the Subaru WRX, with a smaller turbocharged horizontally opposed 4-cyl. that makes 227 hp, sells for about $25,000.