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Discussion Starter #1
I've searched and looked through the forum on this topic, but couldn't find a different thread on the AWD system on the 2013 2.5i. I understand the marketing about symmetric AWD, but think I've read somewhere here that the AWD system is actually different on the 2.5i CVT, 2.5i MT, and 3.6R.

I have the 2.5i CVT, and down here in Texas, I've not had the pleasure of trying it out in winter weather yet. I do, however, have coming out of my neighborhood a patch of pea gravel at an intersection. When I launch, all wheels are in the gravel (in my truck, for example, I always break the gravel loose with the rear wheels), and what happens with the Subaru is that the front wheels break the gravel loose, but not all four wheels.

This tells me that the system on this car is a FWD biased system that delivers power to the rear wheels when needed.

Is this correct?
 

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I would think so, until one slips then it kicks in even in the rain thats the way I understand it.
 

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The AWD system in the 2.5 CVT is front biased.
The AWD system in the 2.5 6mt is split 50/50.
The AWD system in the 3.6 is rear biased.

Some say the system in the 3.6 is the most advanced while the system in the 2.5 6mt is the dumbest. I happen to have a 6mt and it does awesome in snow so if that is the worst, all of Subaru's systems have to be good to go.

Edited to Add: The 2.5 CVT and 3.6 has the ability to transfer more power front to back and back to front so it isn't a fixed front bias or rear bias AWD system.
 

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Discussion Starter #4
Pardon my lack of knowledge on this (my OB is my first AWD vehicle), but is the power shift from front to back or side to side managed by the CVT?
 

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Pardon my lack of knowledge on this (my OB is my first AWD vehicle), but is the power shift from front to back or side to side managed by the CVT?
Yes - front to back, side to side depending on numerous variables from a host of systems (Transmission, ABS, ECS, etc)

And symmetrical merely hints at the layout of the engine/transmission/differentials, not symmetrical in the sense of all wheels have the same amount of power at all times. As stated above, the different transmission (6MT, 5AT, CVT) all use different methods of shifting power. Most definitive answer I've seen for the CVT is 80F/20R split in normal conditions, with power being shifted as needed within milliseconds-seconds.

My previous Subaru was a 5MT WRX with the 50/50 locked split. I have to say, the Outback with CVT is actually MUCH better in inclimate conditions than my WRX ever was in terms of grip characteristics, so I'll take it.

 

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For the CVT front to back bias is managed by a clutch. I don't remember what the lower limit is on this system in terms of front to back bias but the best you can get is 50/50. It can not put more to the rear than the front from my understanding because with the clutch locked that just means both front and rear wheels are directly connected to the transmission's output. It can allow more power to go front than rear by allowing the clutch to slip, which is needed to allow the front wheels to turn at a different speed than the rear wheels while taking a turn.

For side to side power the Outback uses the brake at each wheel. When wheel slip is detected the system is supposed to apply the brake to that wheel. The result is more power applied to the wheel on the opposite side. I say "supposed to" because there have been a few people who didn't have this happen while stuck in the snow. They took video of this happening, posted it on YouTube, and Subaru fixed their vehicles in some way.
 

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This explained it for me.



Quote:
I mean sure you can say, If I pick the manual I get version A with a 50/50 split but if you upgrade to the CVT and end up with an 80/20 split is that an upgrade or downgrade?

Plain OM

Neither.

Okay, so the "active AWD" (multiplate transfer clutch --MPT) system is used with the 4 speed automatic, and in a similar format with the CVT, and those trannys are coupled with the 2.5 4 cylinder engine. The variable torque distribution (VTD) system is used with the 6 cylinder and the 4 cylinder turbo engines, both of which are paired with the 5-speed AT. Why this pairing? Could it have to do with the torque capabilities of the respective engines -- I think so.

The MPT uses a multi-plate clutch. Clutches depend on the pressure between the plates, and the friction surfaces, to transfer torque from the input side to the output. Pressure and friction area have limitations, and consequently the torque transfer capability is also limited. A larger set of plates (more friction area) and/or more pressure allows a clutch to handle more torque, but this also comes with a size and weight penalty. The MPT, as it is, is matched to the 2.5, 4-cylinder engine, but might not be adequate with the 6 cylinder or turbo.

The VTD AWD system uses a mechanical center differential to distribute power to the front and rear drives. This planetary gear type differential is the same type of planetary gear set with clutches and brakes that is used inside the transmission to determine the gear ratios, and to shift between them. They are relatively small, but quite rugged, and can handle the higher torque of the more powerful engines.

Can a VTD center differential be used with the smaller engines? Yes, and there are examples in Subaru forums where a VTD differential has been installed in the tail section of a 4-speed transmission, replacing the MPT clutch. But is it necessary? Probably not, and the higher cost of manufacturing the gear sets impacts on the overall cost of the car; not very helpful when manufacturing a car that is trying to meet a specific price point, as well as keep weight down for fuel mileage.

The MT uses a simple, open center differential. Why not an MPT or VTD? Because the MPT and VTD require a number of input sensors and a microprocessor control module, as well as a hydraulic system to provide and control the pressure for the clutch. Not very cost effective, and the pump and hydraulics both draw power from the engine, and increases operating costs, not to mention cost of manufacturing.

When it comes to performance, when working properly, they all work effectively. Don't fixate on the 50/50 or 80/20. The 50/50 is the nominal distribution of the MT differential, but guess what -- it's also the nominal ratio for any open differential, such as at the front and rear of the car. So this center differential sends power to the front and rear mechanically, in equal proportions when the tires have good grip.

The VTD has a nominal 45/55 ratio (or 35/65 in the STI version). It applies slightly more torque to the rear, but when all four wheels have good traction, the rears can't turn any more than the front, and so the internal differential carrier rotates, taking up the difference. Once again, all four wheels are powered and are pushing the car.

With the MPT, the clutch transfers power from the transmission to the rear drive. The transmission output is connected to the front differential through a set of "reduction" gears, so this is a mechanical, always-connected link. Full power is always applied to the front, while the power to the rear can be varied by the clutch. In order to maintain "full time" AWD, hydraulic pressure is always being applied to the clutch when the engine is running and in gear; measurements suggest it's around 40 - 50% torque transfer capability minimum, and, of course, can be increased to 100%, at which point, power transfer to the front and rear is equal, or 50/50. But even when the pressure is at 40%, the rear wheels are being powered and again, will push the car along with the fronts.

In other words, all three systems ensure all four wheels to be pushing the car, and for virtually all normal driving situations, there is no difference in the way the car handles.

The advantage of the AWD system is that if one wheel loses traction, there's up to three more to keep pushing. In a FWD or a RWD, if one of the drive wheels loses traction, the car slows or won't move (if there's no limited slip differential). With AWD, if the front, or rear, wheels lose traction, the other set can keep the car moving. Moreover, the control system, whether a viscous coupler (MT), MPT clutch (MPT) or the transfer clutch in the VTD, will force the spinning wheels to slow by locking the front and rear drives, and when the wheels slow, they are better able to gain some grip. This is how the basic AWD systems work on most cars. So again, there isn't as much real difference between the three as there appears to be from the nominal numbers.

In some special cases, such as racing, there could be situations where the driver can make use of the slight differences in nominal power distribution, such as the front bias with the MPT, the rear bias of the VTD, or the balance in the MT. But then again, if the car is being used in racing, there's probably more than just the transmission or AWD system needed to get the best out of the small differences.

I'll add that because Subarus now come with ABS/VDC, the AWD drive systems are even more sophisticated, particularly because of the additional control at each wheel. This has the effect of making the effect of the mechanical differences, as small as they are, even less significant.

If you buy a 2.5 automatic with CVT/MPT instead of a MT, you're upgrading to an automatic. If you buy the 6 cylinder with 5-speed tranny and VTD, you're upgrading to a more powerful version of the car, and in some cases, with more "bells and whistles". But they're all full-time AWD, and work remarkably well.

(Want to test this out? Ask a dealer for a test drive of each, and then try to identify if the different AWD systems -- not the engines, manual shifting, or accessories -- made any significant difference to your personal driving experience.)

http://www.subaruoutback.org/forums/375567-post44.html


I read in my FSM that the pressure is applied to the MPT for a 60/40 split under normal driving conditions.

If we can somehow tap into the AWD DUTY SOLENOID translate the data in real time to show us how much pressure is being applied at the MPT, that would be neat.

For example the new Nissan Pathfinder has this when put in Auto AWD mode.
 

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If we can somehow tap into the AWD DUTY SOLENOID translate the data in real time to show us how much pressure is being applied at the MPT, that would be neat. . . .
It's been done. See http://www.subaruoutback.org/forums/65-parts-accessories-performance/39426-freessm-complete-access-your-ecm-tcu.html?highlight=freessm.

In that thread we began by discussing two scanner programs, and progressed to running actual data logs, especially of the AWD control system. The text that you quoted from another of my posts was based, in part, on that.
 

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This explained it for me.
While most of this is close and it does explain the way each system functions, this description doesn't accurately describe how power is applied to the road in each case. One example is of the 80/20 split scenario. During dry road driving there will be 80% of the power to the front and 20% to the back. This means that when the engine applies power to the accelerate the front wheels will see more power than the rear will, even though all four wheels are turning at the same angular velocity. This is going to be most noticeable during a turn as the car will react more like a front drive car that wants to pull the front end around. In slick conditions the system will identify that the front tires are slipping some and apply more pressure to the AWD clutch pack, resulting in more power being applied to the rear wheels and the car hopefully moving forward.

In contrast, with the 40/60 split system as in the STI you will feel a slight difference in how the car reacts to the turn. This time the car will have two different reactions. One, the rear wheels will give the feeling of the rear end rotating around the front wheels (which is what they are doing to an extent). This is good as it provides more control in the turn and usually a faster cornering speed. Second, the front wheels will pull the front end through the turn. The combination of these two reactions makes for a planted feeling and faster cornering speed (assuming the suspension is correctly designed to handle these reactions).

Both systems will work well for getting you through the snow but the 40/60 split system provides better cornering and overall reactions for a sports car than one that can only provide up to 50% to the rear wheels. Now let's all be honest and admit that the Outback isn't meant to be driven like a sports car.
 

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While most of this is close and it does explain the way each system functions, this description doesn't accurately describe how power is applied to the road in each case. One example is of the 80/20 split scenario. During dry road driving there will be 80% of the power to the front and 20% to the back. This means that when the engine applies power to the accelerate the front wheels will see more power than the rear will, even though all four wheels are turning at the same angular velocity. This is going to be most noticeable during a turn as the car will react more like a front drive car that wants to pull the front end around. In slick conditions the system will identify that the front tires are slipping some and apply more pressure to the AWD clutch pack, resulting in more power being applied to the rear wheels and the car hopefully moving forward.

In contrast, with the 40/60 split system as in the STI you will feel a slight difference in how the car reacts to the turn. This time the car will have two different reactions. One, the rear wheels will give the feeling of the rear end rotating around the front wheels (which is what they are doing to an extent). This is good as it provides more control in the turn and usually a faster cornering speed. Second, the front wheels will pull the front end through the turn. The combination of these two reactions makes for a planted feeling and faster cornering speed (assuming the suspension is correctly designed to handle these reactions).

Both systems will work well for getting you through the snow but the 40/60 split system provides better cornering and overall reactions for a sports car than one that can only provide up to 50% to the rear wheels. Now let's all be honest and admit that the Outback isn't meant to be driven like a sports car.


The link psted by plain OM HERE is pretty fascinating, no one has yet done this on a 2010+ CVT but a lot of the information found can give you really good idea of how the power is distributed.


I think we can throw out the 80/20 ratio myth since it's pretty clear the percentage is greater, it's been mentioned that it's more on average to 40% rear, (under normal conditions) even then it's still constantly varying the ratio. I also believed to have read this on the FSM but I can't seem to find where it states this.The way I feel it is the OB has nowhere near FWD driving characteristics, especially in the turns. If you look at the data provided in the FreeSSM thread, Subaru's AWD system works incredibly fast. Far to fast for you to even notice front to rear loads under simple everyday driving. Of course it's not going to be like the STI but IMO it's far from your typical FWD behavior.
 

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By the way my old 5spd MT 50/50 Legacy GT would get scratch with the front wheels on a hard launch just enough to notice when your on some semi slick surface. Any AWD car will do this simply because a hard acceleration from stop the front tires are at a disadvantage regarding traction regardless of AWD unless the system is sending over 50% to the rear the front end will not have as much traction available to it and wheel spin is possible.
 

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It's been done. See http://www.subaruoutback.org/forums/65-parts-accessories-performance/39426-freessm-complete-access-your-ecm-tcu.html?highlight=freessm.

In that thread we began by discussing two scanner programs, and progressed to running actual data logs, especially of the AWD control system. The text that you quoted from another of my posts was based, in part, on that.
Much props to you and the guys who did this test I really hope we can get some more current data on our OB's. I'm going to think about doing this myself on my 12'.
 

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Much props to you and the guys who did this test I really hope we can get some more current data on our OB's. I'm going to think about doing this myself on my 12'.
Now get logging and post up some results for the 2010 +
;)
 

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The link psted by plain OM HERE is pretty fascinating, no one has yet done this on a 2010+ CVT but a lot of the information found can give you really good idea of how the power is distributed.


I think we can throw out the 80/20 ratio myth since it's pretty clear the percentage is greater, it's been mentioned that it's more on average to 40% rear, (under normal conditions) even then it's still constantly varying the ratio. I also believed to have read this on the FSM but I can't seem to find where it states this.The way I feel it is the OB has nowhere near FWD driving characteristics, especially in the turns. If you look at the data provided in the FreeSSM thread, Subaru's AWD system works incredibly fast. Far to fast for you to even notice front to rear loads under simple everyday driving. The data shows how even while turning the percentage of load transfer to the MPT increases. Of course it's not going to be like the STI but IMO it's far from your typical FWD.
This may be true but the Outback can't do more to the rear than 50%. In contrast an STI can easily surpass this to allow both a better launch (as subiesailor pointed out the rear wheels have more weight in a launch and therefore more friction) and the rear end to come around some in a turn. I never said the Outback behaves like a FWD car, just that the system in the CVT equipped car can not react like the 5 speed auto or STI systems do. Just like the article said, this is likely because the CVT system costs less and an Outback isn't meant to be a racer. In the snow all the systems should provide good results too.
 

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This may be true but the Outback can't do more to the rear than 50%. In contrast an STI can easily surpass this to allow both a better launch (as subiesailor pointed out the rear wheels have more weight in a launch and therefore more friction) and the rear end to come around some in a turn. I never said the Outback behaves like a FWD car, just that the system in the CVT equipped car can not react like the 5 speed auto or STI systems do. Just like the article said, this is likely because the CVT system costs less and an Outback isn't meant to be a racer. In the snow all the systems should provide good results too.
Actually, it can. As long as he transfer clutch is completely locked and the front wheels have no traction (i.e. ice or if they are off the ground for some reason), all of the torque is carried by the rear wheels.
 

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This may be true but the Outback can't do more to the rear than 50%. In contrast an STI can easily surpass this to allow both a better launch (as subiesailor pointed out the rear wheels have more weight in a launch and therefore more friction) and the rear end to come around some in a turn. I never said the Outback behaves like a FWD car, just that the system in the CVT equipped car can not react like the 5 speed auto or STI systems do. Just like the article said, this is likely because the CVT system costs less and an Outback isn't meant to be a racer. In the snow all the systems should provide good results too.
As RobMunach said, it can.

Here again this awesome video I'm sure we all have seen :) shows how the system effectively applies brakes to the spinning tires with zero traction thus all the force must flow towards the rear. 0:100 or 100:0.
Everytime I see this video reminds me of how fast it all works. If you look at our competitors almost all of them take longer to achieve the same results or they simply fail.

We need to remember our system isn't mechanical in a sense that it doesn't have a fixed center split ratio like some other AWD cars.

 

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So we are talking two different things here. You are correct that when the front wheels are on a slick surface they will not see the power that the rear wheels do. I am talking about a different situation where all four wheels have traction and you are doing something like taking a fun corner in a mountain pass. When all four wheels have traction a CVT Outback can direct up to 50% of the power to the rear wheels, no more. On the other hand, an STI's system can do a whole lot more to the rear. This rear bias allows more driver control through the turn because throttle changes can make the rear rotate about the front wheels slightly. Does this matter much to most Outback drivers who will never try to race through turns at maximum speed? I shouldn't. I am just point out that the different systems do result in altered driving dynamics. Again, in the snow or ice all these systems will work great.
 
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