Subaru AWD Systems: A Subaru Kind of Day

April 5, 2010
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If ever there was a car company whose success was born of mud and snow, it's Subaru. It first got going in the United States in the 1970s, with awkward-looking little front-wheel-drive cars - back when front-wheel drive was still a novelty - and soon achieved notoriety with its four-wheel-drive wagons. The combination of traction and toughness made Subarus particularly appealing in areas of the country that see lots of snow and mud. So it was fitting, then, that to demonstrate the two different all-wheel-drive systems in its Impreza Subaru carved a vaguely pear-shaped track in the spring-thawing snow and mud outside of Park City, Utah.
1003 Z 03+subaru Utah Adventure+front Three Quarter View
In a nod to the brand's more recent history - the WRX's competition in rallying - Subaru brought along Norwegian rally great John Hauglund to teach us how to use the Impreza's all-wheel-drive system to get around the track, rally-style. He diagrammed a corner to show the difference between a rally driver's line and the racing line - basically, a rally driver will apex much later to get onto the power sooner. Hauglund also advised stabbing the brakes with the left foot to weight the front wheels and help turn-in but to stay on the power during cornering to take advantage of the front wheels pulling in the direction you want to go.
Our first opportunity to put his teachings into practice was in the WRX. The WRX has a fairly traditional standard all-wheel-drive system, in which a viscous coupling apportions torque from front to rear based on available traction, with no input from the driver. The default torque split is 50:50. The track conditions were icy, with a generous helping of mud and rocks. The rally-style driving was definitely fun, and you can see how braking early, then using the throttle through the corners, helps make the most of the all-wheel drive.
Later we headed out in the STI. You may think of the STI as a WRX turned up a notch, but it's actually turned up quite a few notches. Higher boost makes for significantly more power: 305 hp to the WRX's 265 hp, and you could really feel it even on this very short track. The driver can calibrate the engine response (throttle mapping and the ECU) with a knob on the console to select Intelligent (the mellowest), Sport (quicker), or Sport Sharp (quicker still).
The all-wheel-drive system is also significantly different than in the WRX and offers extensive driver adjustability. Even left to its own devices, the STI system is more sophisticated, with a default rear bias (41:59) and two different types of limited-slip differentials at the front and rear to apportion torque from side to side. The center differential's automatic mode can be finessed in two directions: Auto (-) makes for less center differential lock, aiding steering feel and making for more rear bias. Auto (+) creates additional center diff lock for better AWD performance in slippery conditions. As if that weren't enough, there's also manual mode, with six levels of differential locking. The rally driver's favorite is manually selected full lock, which works best for loose surfaces and improves braking, although it makes for heavier steering. That's the one we went with, as the track was rapidly becoming more like a rally stage with the sun melting the ice and the mud and rocks taking over. The looser surface and the higher speeds (thanks to the more powerful engine) just about negated the efforts of the more sophisticated AWD system, however, as we powered around the circuit. Still, it was interesting to swap back and forth between varying levels of differential lock, giving the car a more or less lively rear end.
1003 Z 02+subaru Utah Adventure+front Three Quarter View
The event really pointed up the differences between the versions of the Impreza. The STI is a very sophisticated machine, with probably more driver adjustability than most owners will fully appreciate. The WRX is still sporty but easier to use - and may be a more compelling choice for the coming model year, as Subaru is giving it the STI's wide-body styling. The base Impreza, which we also briefly spent time in, is the plucky, growling, little Subaru that best embodies the spirit of the first Subarus, which earned their reputation in the mud and snow.

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2015 Subaru Impreza

2.0i PZEV AWD 4-Dr Sedan I4
starting at (MSRP)
$18,195
Engine
2.0L I4
Fuel Economy
25 City 34 Hwy
2015 Subaru Impreza