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Hi,

On the new 2009, 2010 4EAT models, does AWD "activate" at highway speeds.

For example, if I'm on the snow highway and at say 80 km/h and I need to pass another vehicle, would the AWD work and provide my vehicle traction to all 4 wheels ?

From what I understand AWD is mainly for traction from a stop or very low speeds....

Thanks in Advance,

"G"
 

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2007 Forester Sports XT 4EAT
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I see this is your first post, so welcome to the forum from Oregon! :biggrin:

Check out this thread. :wink:

Bobby...

My MODding Journal
 

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Ah crap.. I'm back with my 05 XS
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Right, it's all the time but the split is different depending on the conditions and gear for 4EAT. Most of the time, dry land split is more like FWD on a 4EAT. But you'll always have some power to the rear wheels, just more when you need it more, and up to 50/50 split (theoretically).
 

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2009 2.5X EJ253 Manual
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On a new Forester w/ auto trans you will have AWD at 60/40 front/rear torque split up to maximum vehicle speed. You can rocket down that snow highway full throttle and have 60/40 AWD in overdrive gear. Don't rocket along at full throttle though. That would be dangerous. Just illustrating a point. Love Subaru.
 

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99 UK S-turbo
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No, SH are back to MPT and don't have VTD......

Back to AWD 101 for you LR

Simon
 

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Ah crap.. I'm back with my 05 XS
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Here's a decent description of MPT:
All-Wheel Drive (Automatic Transmission)

Active all-wheel drive is a term coined by Subaru to differentiate the all-wheel drive system in the automatic transmission 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.

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
 

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Ah crap.. I'm back with my 05 XS
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Mmm 01 I think? AFAIK, this is basically how the MPT functions in my 05.
 

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99 UK S-turbo
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VDC+MPT is nothing at all like (as good as) VTD, but yes I guess VTD was replaced with the cheaper MPT when the car got VCD.

Simon
 

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How old is this info and where'd it come from ?

From Subaru's Canadian website
Mmm I do recall reading that somewhere. So they've changed the splits to be closer to 50/50 @ 60/40 instead of the old style where it was mostly 90/10 in cruising, 100/0 under hard braking, 50/50 only when locked in 1 & 2.
 

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I guess VTD was replaced with the cheaper MPT when the car got VCD.

Simon
The vast majority of pre-MY09 auto Foresters also had MPT - it was just carried over to MY09 and did not replace VTD.
 

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I thought that the '09+ years have VDC which kinda reduces the need for VTD (or I guess acts like VTD)?
VDC won't reduce the benefit of VTD over MPT. VDC has two functions, but they do not replace that of MPT or VTD:
  • In the case of wheelspin, apply the brake to the spinning wheel(s)* or in extreme cases reduce torque from the engine
  • In cases of oversteer or understeer, apply the brakes to wheels selectively to 'steer' the car in the intended direction or in extreme cases reduce torque from the engine to reduce the vehicle speed

*This is the traction control component of VDC at work. It is mainly of benefit in cases of cross-axle (side-to-side) wheelspin. The MPT or VTD should sort out front-to-rear wheelspin without need for intervention from the VDC, hence the functions of the VDC and MPT (or VTD) are quite different.
 

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2009 2.5X EJ253 Manual
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I thought we hashed out in the last AWD How Does It Work thread that the SH (2009+) models with auto trans according to Subaru themselves, ran at a default 60/40 torque split and could vary to 50/50 as needed.
 

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No you were corrected in that thread as well....I take very little notice of marketing departments (who have no technical knowledge and frequantly for all companies have no clue what they are talking about) especially when its obvious they are wrong, MPT cannot maintain a 60:40 split, the clutches would be trashed in no time.

Simxs, yes I know the majority of SG's, even when VTD was available were still MPT.

Simon
 

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... MPT cannot maintain a 60:40 split, the clutches would be trashed in no time....
So 60/40 in the 2010 Subaru of Canada brochure is wrong, and the split is really 90/10?
Why would a 90/10 split be better for the clutches?
 

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Because the load on the clutches is reduced they will wear less/stay cooler due to the rotation across them that wiil inevitably be happening all the time.

Simon
 

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No you were corrected in that thread as well....I take very little notice of marketing departments (who have no technical knowledge and frequantly for all companies have no clue what they are talking about) especially when its obvious they are wrong, MPT cannot maintain a 60:40 split, the clutches would be trashed in no time.

Simxs, yes I know the majority of SG's, even when VTD was available were still MPT.

Simon
Wrongo. I was corrected for saying it was 90:10. In the thread I am referring to, proof was shown from Subaru that it is 60:40. Here is the post with the direct quotation from Subaru:

http://www.subaruforester.org/vbulletin/f89/subarus-awd-system-best-65170/index5.html#post722056

Here is the entire thread:

http://www.subaruforester.org/vbulletin/f89/subarus-awd-system-best-65170/

Here is the one where I mistakenly mentioned 90:10 split and was taken to task:

http://www.subaruforester.org/vbulletin/f89/how-awd-distributes-power-61686/

I'm sticking with the Official Subaru Information posted by Crewzer in the referenced post above, until you can back up your assertions with Official Subaru docs or press releases.
 

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Stick with what you like, its wrong! As an engineer I've never ceased to dispare at what marketing departments have said about what what my AOS does and doesn't do, trouble is once printed they won't change it until the next MY, so you're stuck with rubbish info going out for a year! They also like to 'twist the truth' to prevent it looking like something is being downgraded (and MPT is a downgrade from VTD for sure!) After all they say its 60/40 for best FE, when best FE would be at 100/00, any drag in the centre clutch at all is wasted energy, even if it is only worth 0.1%

Also don't confuse VTD which is 60:40 with MPT which isn't.

Simon
 
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