Anybody know what the difference is between 4WD and AWD. Is is just that 4WD vehicles are just temporarily in 4WD, whereas AWD are full time? Thanks

Correct. AWD is really full-time, automatic, no-need-to-unlock-the-hubs, four-wheel drive. It's gradually replaced 4WD systems on many SUV's and virtually all cars.

4WD - sends equal power to all four wheels<br>AWD - transfers power to the wheels with the best grip<p>I'm sure someone will have a more technical explanation. Also, I think there is a write-up in the FAQ section on the differences.<p>CraigB

<ul><li><a href="http://www.eskimo.com/~eliot/awd.html">4WD/AWD article</a></li></ul>

Though generally AWD = permanently engaged whereas 4WD could be partime automatically engaging, manually engaging, but could also be "Full-time 4WD"etc.<p>You would not be wrong to substitute one for the other, though each manufacturer has selected their "prefered" marketing lingo.

4WD, it's got a center differential that transfers power to the two axles. At that point, there is yet another differential, one on the front axle, and the second on the rear. Power is transferred equally at all times to all the wheels, with the differential making a difference only when one or more wheels go into a free spin, at which time, the standard differential does it's job of locking up the free spinning wheel. That's it.<p>AWD, (Quattro more specifically) uses a center differential to distrubute power and has both an additional front and rear differential as well like a 4WD, but unlike a 4WD, it has actuated worm gears, which are torque sensitive (right hand thumb rule) and the theory is this; the wheel with the least torque is the one that has the least grip, and so F=mA, as a cross product, the one with the most torque will be the one with the most grip and you set up the worm gears to react in reverse so the harder the wheel pushes (torque, sum moments) the harder the engines and gears push back. This creates then a situation where power can be distributed to the most torquey tire. I think Quattro can deliver up to 75% of it's power to just one wheel. <p>There are various AWD systems, Quattro being the best and most efficient, and Subaru getting an honorable mention from me. Other systems such as Volvo's use a solid state liquid silicone system. In isolated chambers, there is slotted disks in silicone fluid. Like corn starch, silicone locks up in solid state when torque is applied. So this becomes your transfer gear in essense. Pretty cool, not super efficient, but it gets the job done. <p>If you have more questions, go ahead and send me an email and I'll email you the Patents on Quattro, as well as the white papers if you want. Information overload I'm sure but they are available upon request if you need them.<p>Albert

<p>that 4WD systems (trucks, SUV's, et al) employ a center differential. Engine torque is distributed a fixed 50/50 to the front/rear via a transfer case. Each axle then distributes its torque to its wheels through some kind of differential (open, limited slip, etc.)<p>The quattro system (all generations) is AWD because it has a center differential which allows the front/rear torque split to change. The original quattro system used a standard open center differential which transfered torque to the wheels with the LEAST grip. That's why a dash mounted center/rear differential lock switch was needed. This switch allowed the driver to lock the center diff. so the torque split would be 50/50 front and rear. The rear diff. could also be locked 50/50 left and right.<p>In 1988, a Torsen center diff. replaced the open center diff; This worm gear design (as you described) transfers torque (up to about 78% front and rear) to the axle with the most grip, thus eliminating the need for the center diff. lock. A rear diff. lock switch was used to lock the rear diff. 50/50 left and right.<p>To be honest, I'm not quite sure on this but what I think Audi has done (on the latest quattro generation) is to make the rear diff. lock automatic (hence the EDL nomencalture). If this is not the case. please let me know. <br>

EDL is a traction control system using ABS, NO locking.<p>Several SUV's are true AWD and a few are hybrids. Jeep options include a switchable open/limited slip/locking center diff.