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GTR vs Evo X vs STI: which has the best AWD system?

A few weeks ago, I made a post explaining mainstream AWD system types and how they compare, pros and cons, etc. including some simple diagrams to show where the power goes and how much. As promised, this post will focus on specific cars and what AWD systems they use, especially ones that that have more or less been defined by their AWD systems, and the best place to start may be with a bombshell; the Nissan GT-R.

Nissan GT-R (R35)

The GT-R has built a reputation around having monster traction and very approachable performance, thanks to its AWD system - Advanced Total Traction Engineering System for All-Terrain (ATTESA) - and what it can do for you. But the GT-R doesn't actually use the most mechanically sophisticated type of AWD systems discussed in the previous article, namely a "true" AWD with a centre differential. Instead, it uses a clutch pack to transfer power.

RWD-based clutch-type AWD schematic - Rams Eye The Track Guy ©

The R32, R33, and R34 Skyline GT-R's used a system that looked basically identical to the traditional RWD-based clutch type AWD system digram. This is the same type of AWD architecture you'll see in BMW X-drives, Cadillac RWD-based AWD cars like the ATS and the CTS, etc. Front engine > transmission > transfer case > power split, with a clutch pack in or at the transfer case controlling how much power you send to the front axle. The R35 GT-R is hugely similar to those older Godzilla's, with the main exception being a rear transaxle instead of a front transmission.

Instead of the transmission being bolted directly to the engine, there is a rear transaxle (transmission/differential as a single package) located at the back for better weight distribution. Power goes from the engine to the transaxle using a torque tube and then you have a drive shaft/prop shaft running back again to the front axle from the rear transaxle to send power up front. Like the earlier GTR's, a clutch pack varies lock to control how much power to send up front. 0% lock sends no power to the front, all power going to the back. 100% lock splits power evenly (50:50) between front and rear, with infinitely variable adjustment in between.

RWD-based clutch-type AWD schematic with rear transaxle - Rams Eye The Track Guy ©

So what makes the GT-R special and why not use a centre differential? Well, a clutch-type AWD system is actually very capable as mentioned in the previous post. It is very flexible because it can control exactly how much power to send to the secondary axle simply by varying clutch engagement. A "true" AWD system with a differential has a fixed torque split which, while can be biased to the rear axle for better handling, is still fixed. It can never act like a RWD car if you don't need the extra traction in the front. It's always splitting power between the front and the rear axles. The only time it modulates power split is if one axle (i.e. one or both wheel/tires on one axle) starts to spin AND you have a limited slip mechanism to send power to the axle with more traction.

What does this mean on track? It means that the GTR sends almost all of its power to the back and only sends power to the front as needed to utilize available traction. When it does, it only sends as much power to the front as needed to avoid overpowering the rears, no more. The exact same reason why those "part time" AWD systems got a bad reputation in FWD-based cars (i.e. always driving the fronts until fronts lose slip) is the same reason they are great for RWD-based vehicles; they're mostly RWD vehicles except when you need more traction. The downside to those systems is two things:

1. Unlike a true AWD system that always splits power, this system has to "act" (or react) to transfer power, meaning there is a time delay between the computer deciding it needs power up front and actually sending power up front.
2. It can overheat due to the constant slip of clutches in the clutch pack unless they are fully locked, which is rare.

R35 Nissan GTR at Atlantic Motorsport Park - Graham MacNeil ©

The GTR gets around both of those issues by using a high capacity, high pressure, fast acting hydraulic system that activates and modulates the clutches much faster than a traditional clutch-based AWD system like you'd find in a BMW or a Cadillac (or FWD-based AWD system like Haldex). As mentioned in the previous post, if the system is sized properly to handle the heat, it can be very powerful.

What makes the GT-R system even more powerful is that it uses an active rear differential that can vary lock, much like the electronic limited slip differentials described in my earlier post Limited Slip Differential Types Compared. Short of a torque vectoring differential, this is the most capable limited slip differential you can get. It's the same type Chevy uses on Camaro SS 1LE and ZL1's and Corvette Z cars, the same one used in BMW M cars, and the same one Porsche uses in the 911 GT3 RS and GT2 RS. All of those cars are RWD and they make such great use of available traction at the rear axle alone using just an electronic limited slip diff that they can beat just about anything on track. Imagine what you can do with the same type of system, coupled with the ability to send power up front as needed when you do exceed the rear end grip.

R35 Nissan GTR at Atlantic Motorsport Park - Kevin Doubleday ©

In short, the GTR maximizes use of every ounce of available traction at the back. When you do exceed the available grip in the back, the fast acting clutch pack carefully (but quickly) modulates power to the front axle as needed to avoid the rear axle losing traction. This means you avoid using the front axle as much as possible to leave it for steering and braking duties like a RWD car AND you maximize use of available traction at all four contact patches whenever you need. In other words, it's called Godzilla for a reason.

Mitsubishi Evolution (Evo) X

Unlike the GTR, the Evo X and previous Evo's use a "true" AWD system. Born and bread in the world of rallying, the Evo needs a more robust and consistent system. Rallying is like offroading for speed freaks. As mentioned in the previous AWD post, most hardcore offroaders like traditional 4x4 systems because they are dependable and consistent. You put the car in 4x4 (or 4 high/4HI) and power is always split. No computer guessing or variable power distribution. Using a centre differential instead of a clutch pack means consistent and fixed power split front to rear.

Mitsubishi Evo X AWD Schematic - Rams Eye The Track Guy ©

With that said, as mentioned in Why Open Differentials Don't Work on Track, an open differential alone can only send as much power as the low traction side can handle. On an axle, that means the wheel/tire with the least traction dictates how much power the other wheel gets gets. In an AWD system centre differential, that means the low traction axle determines how much the other axle gets. To avoid this problem, the Evo X's All Wheel Control (AWC) system uses an electronic limited slip centre differential that can variably lock the front and rear axles together to limit slip at either axle as needed. Earlier Evo's were mechanical but the late model Evo's use an electronic limited slip differential, using an electronically controlled clutch pack to lock the two axles together if one axle starts to spin.

The schematic I drew is not an exact representation of the physical arrangement of components inside the Evo's transaxle. I found the exact arrangement sometimes difficult to use to illustrate the flow of power. In the schematic, red is the centre diff housing and it gets engine power. It splits power between front (grey) and rear (black) axles. An electronic multi-plate clutch pack (green) can variably lock front and rear axles if either starts to spin. In essence, the Evo takes the rear differential type of the GT-R and puts it in the middle. What that does is distribute power between the front and rear axles as consistently and effectively as possible.

Mitsubishi Evo X at Atlantic Motorsport Park - Kevin Doubleday ©

After that, the Evo uses a gear-type mechanical (like Torsen and Quaife) front limited slip differential to maximize available traction up front. In the back, it uses a true torque vectoring differential to control how much power to send to either side. That allows maximizing use of available traction at the back like the GTR, but goes one step beyond that by inducing steering moment/yaw at the back if wanted by sending torque to the outside wheel. Mitsubishi calls it Active Yaw Control (AYC) and it's one of the earliest applications of torque vectoring differentials in main stream cars.

What does this mean on track? It means that whenever you put your foot down, the Evo X is always splitting power between the front and the rear axles and does its best to maximize use of traction at every contact patch. It uses limited slip diffs front and back (torque vectoring in the back). With a transverse engine layout ahead of the front axle and a front transaxle, the Evo X has a balance that approaches a FWD car instead of a RWD car like the GTR, with most of the powertrain and transmission weight ahead of the axle and a roll centre ahead of the driver. The Evo takes advantage of that by allowing weight transfer to the front to maximize front grip but, at the same time, has all the tools in place to send power away to the rear axle. as needed to avoid overpowering the fronts.

Subaru WRX STI

The STI is designed with a very similar mentality to the Evo X. The main difference is that it has a longitudinal flat-four boxer engine as typical for Subaru and a longitudinal transmission like a RWD car. But like the Evo, the STI uses a centre differential to consistently split power between the front and the rear axles. Up to 2017, the STI used a mechanical centre differential - a Torsen, like an Audi Quattro system - plus an electronic clutch pack just like the Evo to provide additional lock as needed, up to 100% locking if need be (a Torsen cannot provide 100% lock). As of 2018 and beyond, the STI makes do with only an electronically controlled limited slip differential.

WRX STI AWD schematic - Rams Eye The Track Guy ©

Up front, the STI is once again similar to the Evo and uses a gear type Torsen limited slip differential in the front. In the back, however, the STI uses another Torsen limited slip differential instead of an electronic differential. Subaru's SI-Drive controller manages torque split front to back and side to site to determine where to send power between all four wheels.

What does it mean on track? Like the Evo and unlike the GTR, the STI is always splitting power between the front and rear axles. In power distribution, however, it is stuck somewhere between the two. The STI has a front to  rear power split of 41% to 59% (absent lock up from the centre clutch pack). The electronic clutch pack can variably lock the front and rear axles to manage slip at either axle and send power to the axle with more traction. The front and rear limited slip diffs manage slip side to side on either axle and send power to whichever wheel has the most traction. The SI-Drive controller then supplements all the differentials using variable brake lock to slow down any individual excessive wheel spinning.

You can read more about using the brakes to limit slip in the earlier post Limited Slip Differential Types Compared. It's the same technology used in a very wide array of specially marketed cars from off roaders (Jeeps) to hot hatches (Focus ST, GTI without performance pack or PP) to even supercars (McLaren).

4th gen WRX STI at Atlantic Motorsport Park - Kevin Doubleday ©

Driving the STI, it does not seem to be designed to allow weight transfer to the front for added grip and agility, not as much as the Evo anyway. You can read more about how the two compare in my Focus RS vs STI vs Evo X comparison. Long story short, the lack of a torque vectoring rear differential and the handling balance (i.e. further from neutral) indicates how the STI was balanced, which is not to encourage rotation, but simply to promote stability. The rear power bias lets it take better advantage of stability (i.e. more grip at the rear axle) that Subaru bakes into the suspension tuning, but never at the expense of traction. If you want to slide or rotate the STI, you have to grab it by the scruff of the neck, chuck it into a corner, and give it a boot full of power.

Which one is best?

The burning question is which one is the best AWD system. You can't know for sure what the designers of each system were thinking, but you can kind of guess based on background and application. Nissan comes from a background of sports cars and road racing. It is thinking in terms of a sports car - front engine, RWD. Its solution to increased traction is obvious. Build a RWD-ish sports car with best type of limited slip differential (short of torque vectoring) to maximize use of available traction at the rear axle, then add an electronically controlled clutch pack to send power to the front as needed to improve traction.

R32 Nissan Skyline GTR AWD at Atlantic Motorsport Park - Kevin Doubleday ©

On the other hand, the Evo and the STI come from a background of rallying where speeds are high on low traction/low grip surfaces and you always need to be splitting power. Their AWD systems reflect that. If you're always splitting power, the best system is a true AWD with a centre differential. Then they employ limited slip mechanisms/technologies at the front, centre, and rear differentials so that power can be distributed side to side and front to back to make best use of every little bit of traction you can find. I wouldn't be surprised if the STI does away with its centre diff (and the Evo if Mitsubishi ever brings it back) in favour of electronic clutch packs as controls and heat handling capacity of those systems get better, but for now, that's the way to go for this application.

All three cars - if stock/factory - will understeer at the limit. But they behave differently up to that point. The GTR feels like a very safe and approachable RWD car with mountains of traction. A RWD car that never wants to bite. It understeers the most at the limit, likely tuned that way because it has the most rear power bias, but that also gives you the most flexibility in using power to rotate the car with the most rear power bias. The Evo X feels like a playful hot hatch that never runs of front end grip and claws its way around a corner like a far lighter and more nimble performance car. The STI is somewhere in between. It doesn't feel as tossable and playful as the Evo, feeling more like a RWD car. The upside of that is that you can kick the back end out if you have power biased to the rear using the C.Diff controller.

4th gen WRX STI at Atlantic Motorsport Park - Kevin Doubleday ©

On the road in poor driving conditions (ice/snow), the GTR system is, theoretically, the least capable. It has no limited slip mechanisms up front and sends most of the power to the back until there is slip. While it is a very fast acting AWD system, I can't be convinced that it is as good as consistent power split. The Evo and the STI are better, in my opinion. This is not surprising since the Evo and the STI came from driving (very fast) in "poor" driving conditions like rally stages. They both are always splitting power. The Evo system is more agile but the STI system feels more secure and it is the one I'd prefer driving in Canada winters.

On track, however, I'd rather drive the Evo over the STI because it's sharper and more fun. With that said, for a pure track toy, I have a soft spot for the GTR system over the other two because it's the one that most closely approaches a RWD car. But I would want to do something about the weight, since it's the heaviest by about 400 lb. or nearly 200 kg...

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