About applying traction to "wheels that grip." Yes, that makes a car or truck generally safer in general to moderate conditions. But, in the worst severe conditions, it usually works worse then a part-time 4WD setup. This is why I hate locking differentials. They tend to lock and send power to both drive wheels on an axle instead of just the one with the best traction. So, if both wheels break loose and spin, the vehicle will often veer sideways and go off the road. Not so bad on moderately flat roads. But, if you're trying to climb a hill and the road is crowned in the middle (like most are), that locking diff. can be a nightmare.
On the other hand, an open differential sends power to both wheels when traction is equal. But, if traction varies, the wheel with the worst traction gets the most power. This is why a soft suspension along with non-solid drive axles usually gives much better traction than a heavy springed vehicle with solid drive axles (e.g. many trucks and SUVs). I suspect this is whey older 4WD Subarus have always been so good. Soft springs, equal weight distribution, small tires, and NO solid axles. Traction stays pretty equal on all four wheels.
I don't want to make broad general statements since "4WD" and "AWD" don't really have any precise meaning. And today, with electronic braking controls, computers, etc. , many systems vary. If each wheel has an elentronic braking device and a slip-sensor, it's possible to always keep only the best wheels with power - by braking the one lesser traction.
Some AWD vehicle have ways of locking all four wheels in emergency situations, and some do not.
Generally speaking, AWD usually refers to a vehicle that can be driven on dry hard roads with all wheels engaged because it has a differential between the front and rear drive. 4WD does not and is intended for part-time use.
Now, specific to Subarus. Any true 4WD Subaru I've owned will do much better then any later AWD Subaru - in the worst conditions. No comparison. But, all is a trade-off. Nice thing about the later AWDs is you've always got assist going to all the wheels. It's not very strong if you get in a tough spot, but overall much nicer to use.
Many times when my steep hill is unplowed and covered with a foot of snow, my 4WD Subaru will go right up, and my AWD Subaru would get stuck. Note my 4WD trucks will also go up fine. But what about coming back down? All my trucks are death-traps in icy conditions, even with four studded snows. My Subarus usually can come down fine, even with summer tires.
Also note, there are subtle differences over the years.
True 4WD Subarus only came with stick-shift. Mine has a five speed trans plus a hi-low range transfercase like a pickup truck. Production ceased with the Loyales around 1993. Legacys are differenet and only came in AWD. Same with Imprezas which are 2WD or AWD - and never 4WD.
Older Subarus called "4WD", but with automatic transmissions - are not true 4WD. They have a pressure-activated slip-clutch inside the transmission that separates the front from the back drive. Under low throttle, it allows slippage and under full throttle it gets more pressure and locks up. That has never made any sense to me. It is also a wear item and most with high miles don't work at all anymore.
Later AWD Subarus that I've owned apply some pressure to all four wheels much of the time, but not all the time. Works good in moderate snowing conditions and can be useless in severe conditions. Maybe some new ones have a manual lockup but I'm not aware of any. My newest is a 2000, so I haven't kept up with the latest.
My wife's parents just bought a new Ford Excape AWD and that DOES have a button you can push for "lock up" in very bad conditions. It locks the front and rear drived together and works very well.
Like I said earlier, AWD has no specific meaning. I've got a Chevy Astrovan AWD that has a viscous-clutch between the front and rear drive. No way to lock it and not very good in real bad conditions. I also have a 1999 AWD Dodge Grand Caravan that has an even more complex system that also does not ever have any true lockup.
By the way, my old 1960s John Deere 440 log skidder has a pretty neat setup. Has hydraulically activated locking differential operated by a foot pedal. I'd love to have a car or truck with the same - all under operator's control.
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