Motor Mouth: The pros and cons of AWD vs. 4WD

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AWD is the new black

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Fifteen short years ago, Jaguar’s X-Type was roundly condemned, in part because it offered North American luxury intenders no powertrain choice other than all-wheel-drive. Fast forward to 2015 and the Detroit Free Press predicts that, this year, more than 30% of all the vehicles sold in North America will have some form or another of four-wheel-drive. And in the luxury segment where the X-Type was once so loudly castigated, virtually every top-of-the-line sedan — think BMW 7 Series, Mercedes-Benz S-Class and Audi A8 — is only available with quatre roues motrices . Indeed, irony of ironies, despite boasting exotic — and fuel conserving — all-aluminum construction and the finest interior in the entire luxury sedan segment, Jaguar’s range-topping XJ only became a hit after the company unveiled its Instinctive All-Wheel-Drive system in 2013.

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Motor Mouth: The pros and cons of AWD vs. 4WD Back to video

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Why AWD/4WD?

Despite sometimes grandiose advertising to the contrary, the main advantage of all-wheel-drive is acceleration, especially at low speeds and, of particular interest to those of us north of the 49th parallel, in slippy-slidey winter driving conditions. It is also, in its more traditional 4WD guise, absolutely essential for serious off-road driving and, in the case of some high-end sports coupes and sedans equipped with something called torque vectoring, a boon to high-speed handling.

Also read: Do you think AWD makes you invincible in the snow? Think again

It must be emphasized, however, especially in light of a recent spate of storms up and down the Eastern Seaboard, that driving all four wheels is not a substitute for snow tires . 4WD does not allow Jeeps to defy gravity, quattro does not repeal the laws governing traction and a summer-tired 4Matic Mercedes is no more likely to claw its way up a snowy incline than a FWD Toyota with a set of grippy Hakkapeliittas.

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AWD versus 4WD

The simplest way to differentiate between all-wheel- and true four-wheel-drive systems is that the latter, engineered for maximum off-road traction, always has a two-speed central transfer case with a locking differential (and, often, locking front and rear diffs as well) while the former uses some form of (non-locking) front, rear and centre open differential. As to why 4WD is not used universally — as it offers superior traction — besides the fuel economy penalty caused by the added friction and weight, there is the little matter of all four wheels of your vehicle not travelling at the same speed (for instance, in a turn the inside tires travel a shorter distance than the outside wheels). That’s the cause of the “binding” we’ve all experienced in 4WD trucks/SUVs. In tight corners, the differential’s rigidly locked internal gears are trying to turn at different speeds because the tires, with their firm grip on dry pavement, can’t. It’s also why every 4WD truck has levers, buttons or even, in the case of Land Rovers, a rotary dial to disable the system when they are driven on regular roads.

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Part-time AWD

For simplicity’s sake, all-wheel drive, then, is 4WD civilized for the masses. Like 4WD, the objective is to drive all four wheels to maximize traction. How they drive all four wheels, however, is why there is so much confusion among those shopping AWD.

AWD systems can be divided into part-time or full-time, manufacturers typically labeling the part-time systems “on demand.” On-demand systems are essentially two-wheel-drive vehicles — typically front-drivers, especially at lower price points — to which some form of rudimentary all-wheel-drive system has been adapted. They typically only send power to the rear wheels when slippage — due to wet or icy roads — is felt at the fronts. The majority of the time, most AWD sport-cutes — Honda CR-V, Hyundai Santa Fe, Toyota RAV4 et al — operate as an ordinary front-wheel-drive vehicle.

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Because they direct power to the second axle so infrequently, the main advantage to part-time systems is to minimize the fuel economy penalty normally associated with AWD. The downside is that most such systems, again despite manufacturers’ advertising to the contrary, are purely reactive, only powering all four wheels after slippage has occurred. That is why, even on AWD-equipped sport-cutes, you can actually feel some initial wheelspin from the front tires when taking off from a stoplight.

Full-time AWD

And then there are full-time AWD systems — typified by Subaru’s Symmetrical All-Wheel-Drive and Audi’s Quattro — that always transfer power to all four wheels. Depending on the system, the torque distribution can be rear-biased, front-biased or, as in the case of Subaru, a 50/50 split. Even so, how they distribute power varies from manufacturer to manufacturer. Audi, for instance, uses a Torsen system in its centre differential, while Subaru uses either a mechanical viscous clutch (for manual transmission models) or a more sophisticated electronic version on CVT-equipped Foresters and Outbacks. Either way, what separates full-time AWD systems from their lesser part-time alternatives is that they always send a significant portion of the engine’s power to all four tires.

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The $64 question: What to buy

With so many different manufacturers making so many divergent claims, choosing between AWD systems can be just plain confusing. Nonetheless, the first step is simply determining what your needs really are.

Also read: When is buying an AWD or 4WD car right for you?

Do you live in Vancouver with its once-a-year snowfalls? Then a part-time AWD system will almost assuredly suffice, providing enough traction while maximizing fuel economy. However, ensure, even here, that you are realistic in your expectations. Honda’s Real Time system is most definitely a light-duty unit while the Haldex system used in Volvos, Volkswagens and even Land Rover LR2s is a far more robust example of on-demand AWD.

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Those in northern hinterlands will be much better served by an “always-on” full-time system, the increased fuel consumption (usually about five to 10% compared with the 2WD alternative) a small price to pay for enhanced traction.

As to exactly which marque you should buy, with on-demand AWD the key differentiator is whether the system is predictive or reactive. The former is able to start powering all four wheels before things turn nasty, the latter has to wait until wheels start slipping to get down to business.

With full-time AWD, one of the primary decisions is choosing between torque biases. Jaguar’s Instinctive AWD, for instance, defaults to a 25/75 front-to-rear torque split, meaning its XJ behaves much like a rear-wheel-drive car. Its German competition typically pumps more torque to the front wheels, the question then becoming whether you want more benign, safety-oriented understeer (front torque bias) or sporty, tail-wagging oversteer (rear-drive bias).

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And, if you want simply the heaviest-duty AWD this side of a Range Rover, you can’t go wrong with a Subaru. CVT transmission Foresters offer an X-mode which can, through the miracle of computer controls, lock up the clutch to emulate the locking centre differential of a true 4WD system. Indeed, Subaru claims its AWD is all but on par with the butchest of off-roaders in the traction department. There’s a reason why that the more mountainous and snowier the terrain the more often you see Subarus.

AWD makes supercars more manageable for us mere mortals

Of lesser import in wintry months, but no less telling re: the benefits of full-time AWD is how many sports and supercars now transmit their power to all four wheels. Purists — hoary, old and, to be truthful, at least a little delusional — decry the “whimpification” of their steroidal steeds. The truth is that any 500+ horsepower car without either all-wheel-drive or electronic stability control (or preferably both) is all but undriveable to we mere mortals. So, while a 887-horsepower AWD’ed Porsche 918 is more than manageable, no car has struck more fear into Yours Truly than the 725-hp rear-wheel-drive Ferrari F12 Berlinetta. Even Lamborghini, once reputed the most wayward of supercars, has tamed its errant behaviour through the magic of Haldex: A 700-hp Aventador is not appreciably harder — albeit much faster — to drive at its limit than a Ford Fiesta ST. And the easiest supercar of all to drive on a racetrack is a Porsche 911 Turbo, its all-wheel-drive system making a doddle of 520 horsepower.