Those familiar with a boxer engine will recognize the faint, loping vibration that makes its way through the flywheel and down the center of the car at idle. It's comforting to anyone who's owned an original Beetle (or 911). The other thing about boxers, compared to some inline four-cylinder engines, is an abundance of acceleration-producing torque at low- to mid-range engine rpm. Here the Impreza 2.5i has it all over most of its competitors, generating an impressive 166 pound-feet. And there's an added benefit to this good low-end response. In the past we've found the Impreza better suited than many four-cylinder cars to an automatic transmission. Acceleration is brisk, and thanks to the new variable valve operation, the 2006 Impreza 2.5i is more flexible than its predecessor. The engine breathes a little better than before at high rpm, and it keeps pulling strong further up into the rev range.

The 2006 Impreza WRX gets even more extensive improvements than the 2.5i, and a new engine entirely. Until now, the WRX was equipped with a smaller, 2.0-liter version of the boxer, powered up with a high-boost turbocharger. Now it gets the 2.5, still with a turbo and intercooler. Horsepower has increased by 13 to 230 hp; at least as significantly, torque increases by 18 to 235 pound-feet.

Along with the engine, the WRX gets aluminum suspension arms front and rear (as do some very expensive cars). The steering ratio is even quicker than before. The brakes, already large for this class, now have four-piston calipers in front. It's serious stuff, and should dissuade anyone who might still think of Impreza as an economy car.

The best news: the WRX may be more fun than ever. Some may lament the passing of the higher strung 2.0-liter engine, but the 2.5 is more mature. It feels more substantial at all speeds. It still makes revs quickly, to the point that under hard acceleration you might bump the rev limiter before you realize it's time to shift up. But the real, meaty grunt comes lower in the engine's power band. The power comes more evenly, and in greater quantity, and the ratios in the manual transmission are spaced nicely for the engine's power curve. The shifter is accurate, with even less free movement when it slots into a gear, and reasonably quick.

How quick is the WRX? This one will go 0-60 quicker than just about any compact around. Quicker than a Honda Civic Si, an Acura RSX Type S, a BMW 325i or an Audi A3 2.0 or A4 3.2. But more than the sheer speed, it's the feeling in the gut as the turbo spins, and the fun of working the engine. The pedals in the WRX are perfectly placed for our taste, and the controls operate with a light touch. More than ever, this little sedan feels like it's more than the sum of its parts.

The steering is light, too, maybe a touch lighter than we'd wish, but the front tires respond quickly and the WRX turns into a corner crisply and accurately. During quick, repeated direction changes, its weight transfers smoothly, never herky-jerky, from side to side. Near the limits of tire grip, the WRX understeers, or plows just a little. You can just give it a little more gas, because the all-wheel-drive will tuck the front end in nicely and tighten the WRX's trajectory and guide it through the curve almost as if a cable was pulling it underneath. Even with a pronounced bit of body lean through fast curves, the WRX just sticks. That lean isn't a bad thing at all, once your sensibilities adjust. It means the WRX has good compliance, or wheel movement, and it keeps the tires pressed to the road surface even on very bumpy roads. Hammering down the Midwest's worst roads, the WRX presses on, unflustered, and grips as good as some cars do on smooth pavement. The ride is quite comfortable, because the suspension does a great job of keeping those road shocks from finding their way through the floorboard and seat bottoms. A steady battery of potholes hardly produces a vibration through the dash or steering column or roof pillars.

No doubt that Ring Frame Reinforced body helps here. With the structural improvements, the 2006 WRX also gets more sound deadening material in the headliner, under the carpet and in the shift boot. For the first time, it's equipped with an underbody tray previously reserved for the WRX STi, This device effectively smoothes airflow under the car by removing obstruction that would force the air to change direction.

It works. At any speed you're likely to get away with on the highway, there's virtually no wind noise. Get the car going fast enough, and you might see the hood ruffle just a hint, but you won't hear it or feel it. The only thing you can really put your finger on is that faint loping vibration from the boxer engine, and it's not unpleasant. It's this new level of smoothness, and the sophisticated balance of ride quality and handling, that moves the 2006 Impreza above its predecessors to a new level of sophistication.

The WRX brakes work great, too, but it's more than impressive stopping power. The ABS is tuned in outstanding fashion. Try a panic stop on those battle-scared roads and the WRX stops like a champ (and itt is, in the World Rally Championship), without losing composure, even if you are turning at the same time.