DRIVES and DIFFS

I've been doing a lot of reading about what is 2WD, AWD, 4WD, 4x4, as well as open diffs, limited slip diffs, locking diffs. These are my conclusions and definitions as I understand it. It's confusing as people and makers regularly mis-state what a vehicle has, including my 323 GTX. Some makers make up their own terms, and the addition of electronics to the mix ads another variable.
Most vehicles are two wheel drive (2WD) with an open differential. That means, while both wheels have equal traction, they have equal power. However, as one wheel loses traction, the differential will feed it more power as it's the path of least resistance, and the wheel with traction gets less and less power (torque). This is why one wheel spins on ice and the other does nothing. However, it HAS to work this way normally, as when you turn, the outside wheel is going faster than the inside wheel and therefore they need to turn at different speeds.
All Wheel Drive (AWD) vehicles are all the rage now. They deliver power to all four wheels. I've never seen so many vehicles available with it. They generally add 100lbs to the weight of the vehicle. They claim to offer more control on poor road conditions. However, they can have the same problems as listed above: one wheel with no traction may cause the vehicle to have no other wheels with power.
Full 4WD is normally a bad thing, and retards control as it want all wheels to spin at the same speed, meaning it doesn't want to turn. Full 4WD may be useful on dirt, snow, ice, rocks, etc. A 4WD vehicle has all four wheels turning at the same speed regardless of traction.

A Differential controls how the power gets to the wheels. In normal conditions, any axle that gets power gives it to both wheels.
An Open Diff is the worst at keeping you moving forward when something weird happens traction-wise, but it creates the least problems when things are normal. With an open diff there is very little resistance to the wheels turning at different speeds. When you turn a corner, the open diff is happy to let the outside wheel turn faster than the inside wheel. The outside wheel is considered the one with less traction, so it spins faster. It does power both wheels, and makes sure all the power being given it goes somewhere.
A Limited Slip Diff (LSD) is a bit more picky. It allows the wheels to turn different speeds- to a point. After that point it will give more power to the wheel that has more traction. An LSD may work only when accelerating, only when decelerating, all the time, or even somewhere in-between. And the point that it engages may also vary. Finally, many LSD have an upper limit of engagement, after which it gives up trying to move torque to only one wheel.
A Locking diff locks both wheels together so they turn at the same speed. It's simple, but vehicles don't have their wheels turning the same speed when turning, so the vehicle won't like it when you try to turn and may buck as the inside wheel skips to keep up with the outside wheel. the less overall traction, the less it needs to buck as the wheels slide and skip to keep going the same speed.

With all wheel drive, and you have three differentials: Front & Rear- which drive the wheels, and Center- which puts power to the Front and Rear Diffs. The center diff, or Transfer Case, works just like the regular diff styles, so it may be open, LSD, or locking. It may also be disengaged entirely, sending power to only one axle. With all 3 diffs being open style, you can have 100% of engine power going to one wheel, the wheel with the least resistance. A center LSD proportions power front and rear, and may act as a 2wd drive vehicle until a loss of traction is detected. A center LSD may have an upper limit on how much power goes front or rear. A locking center diff sends 50% of the power front and rear. Once it gets there it's up to the other diffs where that power goes.

Finally, add in electronics. Until recently, all diffs worked in some mechanical fashion, through clutches, gears, or other spinning momentum for detecting loss of traction (slip). Today, electronic sensors can do it too. Once detected, the computer may engage an LSD (or 2 or 3), cut the throttle, activate the brakes, or any and all of the above. It may happen to drive wheels, turning wheels, or free wheels, to any wheel. In some vehicles some of this may be activated manually.


So, when my Mazda 323 GTX has a sticker on it that says All Time 4WD, it's lying. It has AWD, and a locking center diff. I assume the front and rear diffs are open. So, when I lock the center, I know that power is going front and rear, and that 2, 3, or 4 wheels will be rotating.