Can someone red pill me on the various nuances of All Wheel Drive systems between manufacturers?

All you need to know is that Audi quattro is the best system.

What are the Japs and Americans running? Haldex?

Haldex Gen 4+ can pre-lock the clutch based on throttle input

Not an unusual system. Porsche uses something comparable. However if traction is everything you want a center differential to apportion power instead. Systems like ATTESA will work for snow but you're not going to go off road with it.

Why won't it work offroad?

That's because it is.

Want to do skidz? Flip the switch and make it stay in RWD.

Want to put 1000hp down to all 4 wheels with the stock diffs and driveshafts?
Go for it

Subaru has true AWD in the sense that (most of) their longitudinal systems are 50/50 power split in terms of front to rear, and can shuffle that around a bit. Downside is that their boxer is in front of the front axle, which causes understeer.

BMW X-Drive is a similar system in most cases, although it's usually more rear-biased. Very little downsides, except weight, and it uses a seperete center diff iirc.

Volkwagen Haldex systems (branded 4Motion) are considered shit, because they're FWD until you slip, and even then cannot send more than 50% of the power to the rear wheels. This is completely opposite to Audi's true TorSen systems (branded Quattro in most cases, although they do have some Haldex stuff branded Quattro as well). This system allows for permanent AWD, with a variable power split, up to 25/75 and 75/25. Downside to the Audi system is that they usualyl design it with the engine's CoG in front of the front axle, again, causing understeer.

Read this: awdwiki.com/en/home/

Pretty sure mazda runs haldex for their awd models. Dunno about toyota and the others tho.

ATTESA is the best AWD system, hands down.

It works like this, grab a nice straight 6, now put a manual gearbox behind it. Now you have a center diff behind the gearbox with essentially a straight connection for the rear driveshaft which goes to an LSD in the back and drives the rear wheels.
I6, manual, RWD with an LSD. The car stays and behaves like this most of the time, pottering around town or WOT on a freeway for example.

But now it's raining and you're trying to put power down out of a corner, what do you do?
Well, the center diff sends power sideways a few inches, pic related, and powers another driveshaft up to the front of the car.
Because of the way the center diff works, it can send up to 50% of the power to the front wheels, making it a true AWD system. You can disable this manually on R32s by pulling a fuse or wiring in a switch, R33/34 are more complex but they all essentially work the same way, RWD until slip detected, start throwing power at the front until slip no longer detected or 50% reached.

Now there's a problem, though. You have a driveshaft coming up to the front of the car on 1 side, but you need to power both wheels or else it's a 3 wheel drive.
So this means you need to raise the engine to get the driveshaft under it, right?

Nope. What you do is you cut a fucking hole straight through the sump and jam the driveshaft through that, letting the engine still sit fairly low.


Now, you might be thinking to yourself 'hold on, this dickhead just described the same thing a patrol or landcruiser uses, it's just 4x4'.
And you'd be right. ATTESA AWD is essentially a 4x4 system but with automatic torque split meaning it can do variable torque front/back unlike a 4x4 system which locks everything at the same speed.

Daihatsu is Haldex-style iirc.
Honda and Toyota use Haldex-style AWD, as well as FWD ICE's combined with electric RWD to make AWD setups. The NSX is of course electric FWD+ICE RWD.
Mistubishi uses full-time RWD biased AWD on some offroaders, FWD-based Haldex-style on their shitboxes, and the Evo used RWD-biased, FWD-based AWD.
Nissan uses ATTESSA style RWD-biased AWD, and FWD-biased stuff on their crossovers.
Subaru has been discussed ITT before. Mainly longitudinal 50/50 AWD.