Which Type of AWD Your Car Has Matters More Than the AWD Itself
The difference between on-demand AWD and full-time AWD is bigger than you think.
In the modern era of cars, all-wheel-drive is mostly accepted as the ideal form of motoring traction. What began as a niche feature on Audis and Subarus has increasingly become the new-car norm as buyers want an edge over bad weather. After all, what could be better than having all four wheels being used to propel the car forward? But the truth is not all AWD systems are created equal, and I’m here to tell you why that matters. Like anything automotive, subtle differences can change the entire experience.
First and foremost: If you’re looking for a beginner’s guide to AWD, or need to know whether you should add it to your next midsize crossover purchase, look elsewhere. We’re nerding out here and talking primarily about performance car applications. And remember, when it comes to traction in bad weather, having the right tires is the most important thing you can do.
There are several different permutations of all-wheel traction out there. To start, we have full-time systems and part-time or on-demand systems. Part-time means some cars engage AWD when needed (like when driven wheels lose traction) and full-time systems have power going to all four wheels at all times. Most passenger cars automatically send power to the non-driven wheels when it’s required, and so most of the time the cars are actually front or rear-wheel driven.
Some examples of that part-time system are Volkswagen’s 4Motion, often simply known as “Haldex” after the Swedish company that manufactures these systems for the VW Group; BMW’s xDrive; and AWD systems used by Honda and Toyota. Subaru still has that slightly unique full-time AWD called Symmetrical AWD in part because all the drive axles are the same length side to side.
So you may ask, “What’s the issue with that? It’s all the same thing, right?”
No. No, it isn’t. The primary issue with part-time systems is that you get a delay in engagement. Whether it’s a mechanical or software limitation, a lot of these systems (looking at you, Haldex) take a perceptible amount of time to engage and cause a strange disconnected feel to the drivetrain and power delivery. Refer to the above video from Volkswagen to demonstrate how Haldex/4Motion works. This clutch-type system is common in the industry because it saves fuel when disengaged, compared to the constant drivetrain loss of a permanently engaged AWD system.
For example, you’re driving in inclement weather, winter, or dirt and want to get going from a stop. With a full-time AWD system, all wheels will spin immediately or just hook up and go, with no delay in power at a fixed front/rear torque split. With a part-time system, you can launch the same way and (in the case of a front-wheel-drive car) the front wheels will spin first. Then the car will take some time to detect the wheelspin, it will then feed power to the rear wheels over a few tenths of a second as it sees fit. It will then slowly, usually over a few tenths of a second to a few seconds, disengage the rear wheels as less traction is needed. Most of the time these systems can send up to 50 percent of power to the back, but the keywords are “up to,” meaning that it’s usually less power. So the car is still mostly front-wheel-drive, which sucks.
Why? Because the car decides when and where power will go, and the car won’t always know what to do and could leave you stranded in a tough spot or leave you unsatisfied during spirited driving.
Full-time systems don’t suffer from this uncertainty. Also, in performance driving situations that most of us won’t usually be in, the part-time systems make the car behave strange at or near the limit of handling, or even well below it. For example, when I drove a Golf R, it is slightly peculiar in the way it delivers power exiting a turn. It doesn’t send much power to the rear wheels and that power is delayed so it sort of see-saws from mild to moderate power understeer before settling down. The same GTI will have some understeer under power, or with the latest trick VAQ limited-slip differentials can tighten their line with power. But, most importantly, it will behave consistently, repeatably, compared to the on-demand all-wheel-drive.
There will always be that few tenths of a second where the system decides what to do, and that will constantly annoy me in a performance-driving situation. I’d rather just manage a front-wheel-drive traction situation rather than applying power, letting it be front-wheel-drive for a split-second and having it understeer under power before it sorts itself out. I can drive around physics, I can’t drive around a confused car. What’s even weirder is that some systems don’t engine brake with all four wheels, like Haldex, where it only engine brakes with two wheels.
The most beloved all-wheel-drive systems of them all are rally-derived, full-time systems. Stuff like Subaru’s Symmetrical AWD and Mitsubishi’s Super All-Wheel-Control (S-AWC) is legendary and both are truly excellent systems. When you give it throttle, there is no waiting, it simply does. Both of the aforementioned systems use mostly mechanical limited-slip differentials so there is a nice partitioning of power across all four wheels. These systems are permanently engaged, have a center differential to distribute power front-to-back, and usually have a fixed front-to-back power split, instead of the variable split of part-time stuff.
But even if you don’t own or want a sporty car, this still matters to anybody who regularly drives on rough roads or bad weather. Of course, winter tires for the folks in the cold climates are a must before you should even worry about AWD and off-road tires for people taking their small SUVs off-road. In either situation, a great AWD system can make the difference in the most critical moments, not necessarily in your everyday life. When you’re stuck in a rut or a snowbank, a good full-time AWD will give you much more of a fighting chance than a part-time system.
Back to the performance angle for a minute: There is an authority to Subaru’s AWD system that gives it a unique and charismatic feel from the moment you start letting the clutch out. It feels special and unique from the get-go, where many other part-time systems really don’t. I’ll give a pass to rear-wheel-drive-based systems like Nissan’s ATTESA that went into GT-Rs, where it is mostly rear-wheel-driven but pumps power to the front when needed, mostly because you can unplug a simple sensor and make it RWD on-demand. Somehow, it also manages to feel more natural and fun, but it still doesn’t have the greatness of a full-time system.
Alright nerds, if I hear another dude say that their Golf R is better because it’s all-wheel-drive, start taking bets on the first time that same owner is gonna start researching Haldex tunes because they realize that it is a compromised performance system—even in its newest iterations. It uses open front and rear differentials and behaves strangely in performance driving, and adds a lot of weight. For me, it’s having permanently driven wheels at all times. Whether it’s front, rear, or the whole lot of them, they should always be connected. And, if they aren’t, I should decide when they are connected, like a 4×4 truck.
But if it were up to me, I prefer two driven wheels for my fun cars. Do you think I’m wrong? Let’s hash it out in the comments.