Actually, a lot of things can beat a Subaru in 4WD. Let's break it down to shifting power from front to rear, and from left to right. We'll see in a bit why the left-right shift is extremely important.
Transferring power between left and right wheels Why is this important??
Most AWD systems have open differentials at the front and rear axles. This transmits power to through the path of least resistance - in other words, to the wheel that is
already spinning.
In a worst case scenario where you're stuck, the additional axle with power won't be able to save you when there is a wheel on each axle with limited/no grip (on ice/snow/in the air).
Better systems will use the brakes to generate resistance in order to shift power to the other wheel. This reduces the torque available to actually move the car forward. In some scenarios, this means that you won't have enough torque left to get unstuck.
One thing that I've noticed is that a lot of older systems (-2000) offered locking front/rear differentials, and a lot of car based systems (2000-2010) with open differentials do not have brake based L/R transfer, as this relied on stability control.
Example: Left wheels in both front and rear axles have no grip.
https://www.youtube.com/watch?v=FJZxVefta68 Implementations Acura: :bowdown:
Differential based system on BOTH axles. Able to transfer 100% of power to either left/right wheels.
Audi (TRANSVERSE Haldex based systems in A4+ models): Brake based system. :fuckthatshit:
Audi (Longitudinal Torsen based systems in S/RS models): Electronic locking differentials [either open or 50/50 L/R]/LSD ("Sport Differential") available. :notbad:
BMW xDrive: Brake based system. No LSDs except for in M models. :fuckthatshit:
Mercedes: Brake based system. :fuckthatshit:
Subaru: Open differentials front and back. Brake based system :fuckthatshit:
Subaru WRX/STI: LSD on rear axles. :notbad:
Transferring power between front and rear axles Subaru (manual transmission): Viscous coupling where different front/rear axle speeds generates pressure and locks up.
Audi A3s, Volkswagens, and Volvo also use this system.
PROBLEM: Since this depends on rotational speed, tire sizes must be kept constant, making repair costs for punctures/vandalism quite high.
If the difference in pressure is too great, there would be damage. Also can't lockup.
Subaru (auto/CVT transmission):
Electronically controlled
clutch engages to transfer power to the rear wheels. No center diff.
Shifts power between 60:40 and 50:50.
BMW xDrive is similar.
PROBLEM: In some systems (most notably Honda's implementation), the clutch cannot sustain prolonged or significant amounts of torque.
https://www.youtube.com/watch?v=jkiv-bWbLIo Acura SH-AWD There are a couple different versions, but let's look at the simplest (1st gen RDX/2nd gen MDXs, and last gen TL). The RL adds extra overdrive capability for more of a RWD feel.
2 differentials with planetary gearsets AND clutches in both front AND rear axles - the planetary gearsets reduce load on the clutches. 2015+ MDXs only have clutches in the front/rear diffs. Can shift up to 70% power to the rear axle.
PROBLEM: Limited to 50% power to the rear in some scenarios due to diff design. More expensive maintenance.