Chapter 02 / Mechanics

The Five Kinds of AWD, From Subaru to Acura

"AWD" covers five distinct mechanical philosophies. The differences matter in snow, in cornering, and in fuel economy. Knowing which type your car uses changes how you should drive it and how much you should trust it.

Type 1: Full-Time Symmetrical AWD

Used by: Subaru (every model except the BRZ). Key characteristic: Always driving all four wheels. Never disengages. No driver input required.

Subaru's boxer engine is mounted longitudinally, which allows equal-length front and rear driveshafts. This is the "symmetrical" part of the name, and it gives the system an inherent balance advantage over front-wheel-drive-biased platforms. On manual-transmission models, the center differential is a simple planetary type with a 50/50 torque split and a viscous LSD that tightens under wheel slip. On CVT models, the Variable Torque Distribution (VTD) system uses a hydraulic clutch to run 45/55 front/rear and adjusts electronically.

The WRX STi uses Subaru's DCCD (Driver-Controlled Center Differential), which allows the driver to manually adjust the center diff bias from full open to full locked. The Outback Wilderness and Forester Wilderness models add X-Mode, an off-road stability system that uses the AWD hardware plus hill descent, traction control, and brake-based torque vectoring to crawl through mud and light trails.

Type 2: Full-Time Torsen AWD

Used by: Audi quattro (longitudinal A4, A5, A6, A7, A8 platforms), older Porsche Cayenne. Key characteristic: Gear-based center differential, instant mechanical response, no electronics.

The Torsen (torque-sensing) differential uses worm gears to react mechanically to torque differences between the front and rear axles. Default split is approximately 40/60 front/rear, biased rearward for a neutral handling balance. When one axle experiences slip, the Torsen gears redirect torque to the other axle in milliseconds, faster than any electronic system. Audi RS models add a rear sport differential that can vector torque left-to-right within the rear axle. The weakness: Torsen cannot completely prevent wheel spin if all four wheels lose traction simultaneously, which is why the Audi RS system also applies individual wheel braking via the stability control.

Type 3: On-Demand AWD with Multi-Plate Clutch

Used by: BMW xDrive, Mercedes 4MATIC (most models), Audi A3 and TT Haldex, Honda CR-V and Pilot Real Time AWD, Ford Escape Intelligent AWD, most Volvo XC models, Kia and Hyundai AWD systems. Key characteristic: Front-wheel-drive or rear-wheel-drive biased, engages the other axle via an electronically controlled clutch when needed.

BMW xDrive defaults to approximately 40/60 front/rear but can shift to 100 percent rear in Sport mode or when Sport is engaged. On M xDrive models (M3, M5, X5 M), the system can also send all power to the rear for controlled drifting. Mercedes 4MATIC on rear-wheel-drive platforms (C, E, S Class) is rear-biased and engages the front via a clutch pack. On transverse front-wheel-drive platforms (A, B, CLA, GLA Class), it is front-biased and engages the rear. Response time for these systems is 100 to 200 milliseconds, which is generally fast enough that you won't notice the delay during normal snow driving.

Type 4: Torque-Vectoring AWD

Used by: Honda i-VTM4 (Pilot, Passport, Ridgeline), Acura SH-AWD (MDX, RDX), BMW M xDrive (M3, M5, X5 M, X6 M), Ford Focus RS (discontinued). Key characteristic: Two independent rear clutch packs that can send torque to left or right rear wheel independently.

Honda's i-VTM4 can send up to 70 percent of total engine torque to a single rear wheel. The system reads steering angle, vehicle speed, throttle, and lateral G-forces and distributes torque proactively before understeer develops. In a right-hand corner, the system loads up the left rear wheel, which effectively yaws the car into the corner and eliminates the push (understeer) that most front-heavy vehicles exhibit.

BMW M xDrive combines the standard xDrive clutch center section with an electronically controlled rear differential that can also vector torque between the rear wheels. In Track mode, M xDrive goes to 100 percent rear-wheel drive, bypassing the front axle entirely.

Type 5: Reactive AWD

Used by: Toyota Dynamic Torque Control (RAV4, Highlander non-hybrid), Mazda i-Activ AWD, Kia and Hyundai Dynamax (base trims), Nissan Intelligent AWD (Rogue, Murano). Key characteristic: Front-wheel drive by default. Small rear coupling engages only when front wheels are already slipping.

Reactive AWD is the most fuel-efficient type because the rear driveshaft is often completely disconnected during normal driving. Toyota's Dynamic Torque Vectoring (DTV, on the RAV4 Adventure and TRD Off-Road) takes this further by adding left-right torque vectoring at the rear in addition to the front-to-rear split, and can completely disconnect the rear driveshaft for maximum fuel economy on the highway. The weakness: response time is 300 to 500 milliseconds, which means a full wheel-spin event at the front can occur before the rear engages. In steady deep snow this is noticeable as initial hesitation.

AWD System Comparison Table

Response times are approximate. Mechanical Torsen systems react in microseconds. Electronic systems vary by generation and calibration. Data verified April 2026.

Frequently Asked Questions

Data verified April 2026. Specifications vary by model year, trim, and configuration. Verify with manufacturer before purchase.