Explanation of Continuous, Active and VTD AWD systems - an honest attempt

There is a lot of confusion about the differences between the various Subaru AWD systems:

• Continuous (for 5MT)
  
• Active (for 4EAT)
  
• VTD (for 5EAT)
  
• availability or non-availability of Rear LSD

So heres an attempt to explain the differences....

1. Continuous AWD System:

The manual transmission’s all-wheel drive is referred to as a continuous all-wheel drive system. It uses a center differential located inside the transmission case that is controlled by a viscous coupling device. In effect, the center differential is a limited-slip differential.

In normal operation, power is distributed equally to the front and rear wheels. Plates are alternately attached to the front and rear output shafts inside the viscous coupling. When a rotational difference occurs between the front and back wheels, the plates inside the viscous housing shear inside the contained fluid (a type of silicone) heating it and causing the fluid to thicken. The thickened fluid causes the plates to transfer torque from those that rotate faster (the slipping wheels) to the plates that rotate slower (the wheels with the best traction).

This no-maintenance system is simple, compact and virtually invisible in its operation. The system can distribute torque from a 50:50 torque split for maximum traction to mostly front or rear wheel drive.

(Source: http://www.autoworld.com/news/Subaru/Subaru_All-Wheel.htm)

A. Continuous AWD with Rear LSD: Available with 5MT on WRX, Legacy GT, Outback 2.5i, Outback XT and with 6MT on Spec.B

B. Continuous AWD without rear LSD: Available with 5MT on Impreza 2.5i, Outback Sport and Legacy 2.5i

2. Active AWD System:

Active all-wheel drive is a term coined by Subaru to differentiate the all-wheel drive system in the automatic transmission (4EAT) from other "reactive" all-wheel drive systems on the market today. What makes this all-wheel drive system so special is its ability to anticipate traction needs and act before a wheel slips.

The mechanism that transfers torque fore and aft is contained within the transmission’s tailshaft. To the casual observer it looks just like a typical hydraulic clutch found in any automatic. The key difference in this clutch pack is its operation. It’s designed to slip according to how much all-wheel drive is needed. When an automatic’s clutch slips, it is due to a malfunction and will eventually burn up. But the multi-plate transfer (MPT) clutch uses a special friction material that easily withstands the friction loads generated during torque transfer. (Also referred to as VTC = Variable Transfer Clutch)

The MPT’s operation is controlled by the Transmission Control Unit (or TCU) and constantly changes dependent on how the vehicle is being driven. To get more all-wheel drive, the TCU increases the hydraulic pressure to the clutch for less slippage. Less all-wheel drive calls for more slip and the TCU reduces the hydraulic pressure to the clutch.

Under normal, dry pavement operation torque split is about 90% front and 10% rear. This distribution helps to compensate for the car’s weight distribution and resultant smaller effective rolling diameter of the front tires. As weight transfers to the rear of the vehicle, (i.e., under acceleration), the TCU shifts the torque split more toward the rear wheels. Under hard braking, torque is directed forward. Torque distribution is changed based upon how the vehicle is being driven. Throttle position, gearshift lever position, current gear and other factors combine to influence the TCU and it, in turn, selects a software map that determines how aggressively torque split will be adjusted.

Two speed sensors are used by the TCU to detect wheel slippage. One sensor monitors the front axle set, the other the rear axle set. Pre-programmed variables help the TCU differentiate between slipping wheels and normal wheel speed differentials as what occurs when cornering. A speed differential (front-to-rear) of up to 20% signals the TCU that the vehicle is cornering and torque is distributed to the front wheels to help increase traction during the turn. Anything above 20%, however, indicates to the TCU that wheel slippage is occurring and torque is then distributed to the rear wheels.

Another feature of the all-wheel drive system is its interaction with the anti-lock brake system. When ABS is engaged, the transmission selects third gear, reducing the unpredictability of engine braking and, thus, reducing the possibility of wheel lock-up. But all four wheels are still connected to the engine through the AWD system and are brought back up to overall vehicle speed quicker and can, therefore, be controlled again sooner. In a two-wheel drive system if the locking wheel isn’t a drive wheel, it can only be brought back up to overall wheel speed by whatever traction exists between it and the road. The quicker a wheel is controlled the better the stopping performance

(Source: http://www.autoworld.com/news/Subaru/Subaru_All-Wheel.htm)

A. Active AWD with Rear LSD: Available with Outback 2.5i

B. Active AWD without Rear LSD: Available with 4EAT on Impreza 2.5i, Legacy 2.5i, Legacy 2.5i Ltd

3. Variable Torque Distribution (VTD):

It has the MPT (multi plate transfer) clutch aka VTC as found in the Active AWD system but it also has a planetary-type center differential and a Rear LSD. The center differential provides the ability to have a default torque split of 45/55 front/rear (as against the 90/10 split in Active AWD). In every other aspect it is similar to the Active AWD in that it anticipates wheel slippage instead of reacting to it as in the case of a MT. We can say the the VTD is an advanced Active AWD system

Available with 4EAT on Impreza WRX

Available with 5EAT on Outback XT, Outback 3.0R, Legacy GT

4. Driver Controlled Center Differential (DCCD):

Uses an electronically managed multiplate transfer clutch and a mechanical limited-slip differential in conjunction with a planetary gear-type center differential to control power distribution between the front and rear wheels. Normally, DCCD splits power 41% front and 59% rear. Sensors monitor parameters such as wheel slippage, steering angle, throttle position and braking to help determine torque distribution and direct it to the wheels with optimum traction. DCCD also features a helical-type limited-slip front differential and a Torsen® limited-slip rear differential.

Available with 6MT on Impreza WRX Sti

Does my Subaru have a Rear LSD?

Another point of confusion is which Subarus have Rear Limited Slip Differentials (Rear LSD) and which ones do not. For this you can refer to the list of Subaru AWDs above where I have also listed the Subaru models which have that type of AWD. But to make things even clearer heres a list of Subarus with and without Rear LSD:

As we all know that the center differential can split the torque in between the front and the rear axles depending on the front/rear wheel slippage. But its the Front and the Rear LSDs which can split the power between left and right wheels. So it is very advantageous to have at least the Rear LSD if not the Front LSD. If the left rear wheel slips the Rear LSD can transfer the power to the right rear wheel.

Subaru does not offer Rear LSDs in its lower models except in Outback 2.5i!! which is understandable since it is designed for off-road conditions. (This might explain the mpg difference in Outback 2.5i and Legacy 2.5i)

Which is Better?

Now that brings us to the next point of debate as to which one is better?

So here's an excerpt from an article I found:

(Source: http://www.cars.com/carsapp/cars/?srv=parser&act=display&tf=/advice/shopping/4wd_demystified/all_wheel.tmpl)

"Simpler AWD systems “bias” the power to the front or the rear in this way and react to slippage when it occurs. Subaru's “continuous” AWD system is this type. More advanced systems are designed to be proactive rather than reactive. For example, Subaru’s “active” AWD is claimed to anticipate and prevent slippage for a seamless driving experience. Audi's quattro and related Volkswagen 4MOTION AWD systems route power based on vehicle dynamics: Rear tires have greater grip during acceleration, so during that action the rear wheels receive more of the engine’s power. In turns, the outside front wheel has the most grip and thus gets the most power, followed by both rear wheels.

With advanced AWD like this available on cars such as the Porsche 911 Carrera 4, it’s clear that four driven wheels are no longer the province of high-ridin’ mud buggies. For surefooted handling during acceleration — even on bone-dry roads — AWD is tough to beat."

As per the author of this article the Continuous AWD system found on Subarus with 5MT and 6MT is a simpler system in that it is more reactive than proactive. Whereas the Active AWD systems (with Variable Transfer Clutch) found on 4EAT Subarus and the VTD (which is an advanced Active AWD system) found on 5EAT Subarus are proactive in predicting wheel slippage conditions and hence react faster. But obviously the Continuous AWD systems have the advantages of being less complex and hence less prone to failures and low repair costs.

So again which system is the best? I would say the order could be as follows:

1. DCCD with Front and Rear LSDs - WRX Sti
   
2. VTD - 4EAT on WRX and 5EAT on Outback XT, Outback 3.0R and Legacy GT
   
3. Active AWD with Rear LSD: 4EAT on Outback 2.5i
   
4. Tie. Depends on what you need more? proactive AWD system or the Rear LSD (if offroading)
   • Active AWD without Rear LSD: 4EAT on Impreza 2.5i, Legacy 2.5i and Legacy 2.5i Ltd
     
   • Continuous AWD with Rear LSD: 5MT on WRX, Legacy GT, Outback 2.5i, Outback XT and 6MT on spec.B

[*]Continuous AWD without Rear LSD: 5MT on Impreza 2.5i and Legacy 2.5i

Subaru saves its best AWD system for the Impreza WRX Sti which has the DCCD system with front and rear LSDs.

I wasnt aware that the Outback 2.5i has the Rear LSD. Thats an advantage over the Legacy 2.5i.

Disclaimer: These are my deductions from what I have read on the internet and limited by the capability of my 3lbs. So please no one should get offended . Also, if you feel any of this information is incorrect and needs to be updated then please feel free to enlighten all of us.

Nice post. Why do you think that a continuous 50/50 split in the 5MT's is one of the worst?

I'd figure that all wheels driven all the time is the BEST. Oh well, that's all my 3oz can discern...

How is the torque split accomplished in the passive (viscous-coupled) system? If it is simply a differential, then it seems to me that the torque split isn't really being varied in quite the way that the author of the linked article stated.

Rather, it is 50/50 under normal conditions (like with any regular diff) and 50/50 when the VC binds up (since that pretty much turns the diff into a solid axle (not quite, but close enough)).

I can see how planetary gears can create a non-50/50 split (my old car works that way), but with a regular diff, I don't get it... But it could just be that I have more to learn. Anyone know how torque split really gets varied with Subaru's passive system?

Forester XT auto has VTD

Well I totally ignored the Foresters, Baja and the Tribeca. I was just planning to incude the legacys and its cousin - the outbacks....but also included the imprezas just to have something to compare with. But yaa.....I will try to include the rest also in these lists. appreciate your input.

Nice write up.

For clarity purposes it should be noted that the Spec.B also uses a 5MT so that no (less?) confusion is created amongst the uninitiated.

All LGTs have rear LSD including the 07 Spec except that its differential is a Torsen system rather than VLSD.

Y'all can disregard my previous question, I just found that it's been thoroughly covered in another thread:

http://www.legacygt.com/forums/showthread.php?p=866792#post866792

Nice post. Why do you think that a continuous 50/50 split in the 5MT's is one of the worst?

I'd figure that all wheels driven all the time is the BEST. Oh well, that's all my 3oz can discern...

It's all situational I'm sure, but generally speaking it makes sense that the proactive system would be the better one. Prevent slippage before it happens. Where as our 5MTs only attempt to regain grip after grip is lost.

So under hard acceleration on wet roads, I might expect the 4EAT system to do better, shifting torque to the rear wheels where it's needed... but for "suprises", say maybe black ice, I'd expect the 5MT system to be better.

If you hit that black ice while casually accelerating in a 4EAT, with the majority of torque to the front wheels, and you hit a patch of ice now you've got 80% of the power being routed to the worst possible wheels, and now that proactive system is having to react, and more so at that, needing to send far more torque to the rear wheels then would have been necessary in the 5MT, of which only 50% would have been to the slipping wheels, meaning more traction and less torque shifting necessary.

The 5EAT is an intersting one though, with a 45/55 default split... honestly, I wouldn't be surprised if it was better in just about every situation.

"Prevent slippage before it happens" sounds suspiciously like "see into the future." What evidence does the system have of poor traction, other than historical evidence in the form of slippage?

By guessing of course. I think Tophaholic post covered it pretty well.

There are certain car dynamics, that under normal circumstances will cause slippage.

Accelerating too hard, cornering with too much gas, braking too hard, etc... by shifting the torque as necessary, it's right there where it's needed, before it's actually needed.

IMO, it makes a lot of sense. But like I said ealier, I can see some potential down falls to such a system, especially with a 90/10 split.

It's all situational I'm sure, but generally speaking it makes sense that the proactive system would be the better one. <snip>

Depends on what you want your AWD to do for you. I really didn't buy my Legacy just for bad weather. Personally, I find he 50/50 split of my 2.5i manual to greatly enhance the daily driving experience. It virtually eliminates torque steer and the handling is very neutral, with responsive steering and is "European" feeling. I live in Maryland, where winters are not that hard, so having AWD for bad weather traction is more of an advantage, not a necessity. I pick my cars for 10/12 of the year. Having even a slight advantage in the other 2/12 of the year is just fine with me. Heck, after all, the dang car only sits a few inches off the ground to begin with anyway.

I should say that since the continuous AWD it is a 50/50 split, power is right there when you need it. There is no waiting for one set of wheels to slip before power is transferred to the other set. As soon as say the fronts begin to slip, the VC immediately works to maintain the 50/50. It doesn't have to shift from 90/10...

Right, which goes back to my original comment, and an obvious advantage of the 5MT system over the AT style

Would this mean that a Legacy 5MT with the continuous AWD and no LSDs wouldn't be able to climb a hill where one side of the road is tarmac and the other is wet ice?

Will it just spin the side with the ice, and with no possibility of transferring power to the side with grip?

Using studless tires, of course.

Yes, it's an extreme scenario, but I've been there. :-/

Also, does anyone know how the EDM/JDM Legacy's with the 5MT and a lowrange ratio works? I guess it's the same as the continuous AWD with no LSD.

How is the torque split accomplished in the passive (viscous-coupled) system? If it is simply a differential, then it seems to me that the torque split isn't really being varied in quite the way that the author of the linked article stated.

 

Rather, it is 50/50 under normal conditions (like with any regular diff) and 50/50 when the VC binds up (since that pretty much turns the diff into a solid axle (not quite, but close enough)).

 

I can see how planetary gears can create a non-50/50 split (my old car works that way), but with a regular diff, I don't get it... But it could just be that I have more to learn. Anyone know how torque split really gets varied with Subaru's passive system?

Wow, somebody read the humor thread, hehe.

OK, so it's active. Now what about the rear LSD reference? Do you know?

Active AWD, I had one.

 

And yes, that does make quite a difference too. For fun one day I popped in the AWD fuse (turning it into a FWD car) and took it for a spin around the block in the rain. There is one turn at the end of my road where I turn right onto a hill, makes for a great traction test. With the fuse in, the car really spun the front wheel under acceleration as the weight shifts off of the inside wheel to the outside wheel making that corner. With all four wheels engaged in combat with the road, it gripped like it was dry

 

 

And I would say that the XT with the Viscous diff in the middle handles that just as well, even with the extra power.

An open differential will transfer power to the wheel with less traction, so in your case, the open front and rear diffs would both transfer torque to the wheels on ice.

 

Now, depending on the market (and I see you are in Norway) I can't say if your car is equipped the same as our base models. One would hope that if it is an area that gets lots of snow, Subaru would put a LSD in the rear.

I was afraid of that.

I'll check with Subaru Norway about the rear LSD, but I seriously doubt that it's fitted on the 2.0R. It's only standard on the 3.0R, unfortunately.

Kinda sad when you think of it.

Oh well, better be careful about what I try to climb, even with Subaru AWD.