Years ago, there was a clear difference between an all wheel drive vehicle and a four wheel drive vehicle. Now there is XDrive, 4 Motion, Quattro, 4 Matic, All 4, and it can be difficult to tell if something is all wheel drive or four wheel drive.
The definition of a four wheel drive vehicle is a vehicle that allows the front and the rear driveshafts to spin at the same speeds. The only time you will find this in a vehicle with a transfer case or a locking differential. In a vehicle with a transfer case, the driver can choose between two wheel drive hi range, four wheel drive hi range or four wheel drive low range. In either four wheel drive state, both the front and rear driveshafts get 100% of the power all the time. This type of four wheel drive system is found in pick up trucks or full SUVs with part time four wheel drive. In a full time four wheel drive vehicle, it operates most of the time as an all wheel drive vehicle except when you lock the center differential. What this does is ensure that both the front and rear driveshafts turn at the exact same speeds thus rendering it a proper four wheel drive vehicle. You might find this four wheel drive system in premium SUVs like Land Rovers and Porsche Cayennes.
A car like Subaru Impreza WRX however is all wheel drive. A WRX does not have a transfer case or a locking center differential, instead it has a viscous center differential which varies the torque split from front to rear through a system that is not locked. Because The front and rear wheels can not turn at the same speed, cars like this are all wheel drive. Another type of all wheel drive system that you will see is an on demand all wheel drive system. In this case, the car operates as a front or rear wheel drive car until the wheels lose traction. This engages the wheels not previously being driven creating an all wheel drive car. Companies like Mini and Volkswagen use this in their all wheel drive cars. In some cars the driver can decide the torque split from front to rear. In cars like the Ford Focus RS or the Subaru Impreza WRX STI, the driver can send in some cases up to 70% of the power to the rear wheels.
Hopefully that clears up some of the confusion in this world of creative marketing names. If you want to learn more about how different four wheel drive systems work watch a quick video here.
