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Sunday, June 4, 2017

 1:27 PM         No comments
There is no published standard, but there are a few rules of thumb.

AWD practically always refers to a four wheel drive system that is primarily intended for use on paved roads.  These systems operate automatically with no intervention from the driver.  Some AWD systems include some level of control that optimizes function for road conditions, but no AWD system requires that the driver engages the control to enable four wheel drive.  All AWD systems have some form of mechanical, electronic, or electromechanical center differential that allow for speed differences between front and rear axles and use on dry pavement.


AWD systems are either full time (4 wheels driven) or part-time automatically engaging systems that send little or no power to one axle unless the other axle slips or the computer deems it necessary to transfer power.  Most performance AWD systems are full time systems.  Some systems, like the Audi TT's and Volvo AWD systems are part time but automatically engaging, sending a variable amount of torque to the second axle (generally the rear, though Porsche has a backwards setup on newer AWD 911 models that employ Porsche Traction Management).

4WD generally refers to systems that are intended for off road use.  This is a much less clearly defined term than "awd" as many soft roaders like the CR-V and Toyota Highlander are deemed 4WD when in fact their systems are ill suited for any off road use.  Many 4WD systems are part time systems that do not employ a center differential.  This is important to know because the center differential is a device that allows for rotational differences between the front and rear axle.  A part time 4WD system will never be referred to as AWD.

4WD trucks (they are almost always trucks except for 80's subarus) with part time systems cannot operate in 4WD for extended periods on dry pavement with excess wear on tires and possible damage to the drivetrain.  The advantage of part time 4wd is additional traction on loose surfaces and greater control- the driver can select true 2wd at any time to conserve fuel.  The disadvantage is the risk of damage to the system caused by driver error as well as an inability to react to sudden changes in traction. Most part time systems include a low range that allow for crawling slowly over obstacles.

Full time 4wd systems are like AWD systems, employing a center differential.  Many may also include a 2 speed transfer case and can operate on pavement with no driver intervention.  Some can be disengaged and put in 2wd mode, but they are still considered full time since they can be used at all times.
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