Page 1 of 4 123 ... LastLast

Thread: Which late-model Subaru has the best off-road AWD system?

  1. #1
    Join Date
    Dec 2010
    Location
    Rubicon Trail, California
    Posts
    209

    Default Which late-model Subaru has the best off-road AWD system?

    Old Subarus had low-range gearing and a proper part-time 4WD system, making them way better off-road than anyone ever gave them credit for. Modern-day Subarus have multiple different AWD systems of varying degrees of off-road competence, but they're all lacking the low gearing that allows the driver to ease the vehicle through tougher sections of trails.

    I much prefer manual transmissions, but everything I've read says the automatic transmissions come with the superior AWD systems. So, which late-model Subaru has the best AWD system? Which distributes torque the best? Obviously, the STI's is the most advanced but that car isn't meant for high-clearance trails; I'm more interested in the Outback or Forester.

    My frame of reference is hardcore rockcrawling 4x4's. I've always enjoyed automatic lockers and air lockers, low gearing, and a proper 100% front/rear lockup in the 4WD system. My daily driver is a 2-inch-lifted Suzuki SX4 with oversize tires and lots of skidplating. Its manual transmission gives me a deeper 1st gear than the automatic provides, but at high altitudes on steeper trails, the clutch takes a lot of abuse because of the 2-liter's meager off-idle torque. The Suzuki's AWD system works better than one would think, but its computer-controlled wet clutch (mounted ahead of the rear differential) can eventually overheat and auto-shutdown to allow it to cool after extended use in deep sand or snow where it's really fighting hard to distribute maximum torque from the spinning front tires to the gripping rear tires. The ESP system does a surprisingly good job of simulating a pair of limited-slip diffs by applying brakes to the spinning tires, but with extended use it'll build up enough heat that it starts boiling the brake fluid.

    I guess I work my cars a bit harder than the manufacturers intended.

    I'm not criticizing the SX4's AWD system, I just like to keep things simple and as effective as possible. I like low gearing (or high-stall torque converters) to allow big torque at slow speeds. I like mechanical limited-slip differentials (or even better, full differential lockers) to allow consistent, effective power transfer to the tires with traction. My SX4 goes about this the fancy-schmancy way, but it works - and my addition of the lift kit, skidplates, and aggressive A/T tires was a huge improvement.

    I already have a couple of hardcore rockcrawlers for the extreme trails. What I'm wondering is which Subaru AWD system is best for high-mountain trails? Fire roads with rocks strewn about, deep washes that can get 1 or 2 tires up in the air, and steep climbs at high altitude where off-idle torque is of critical importance? Here are a couple photos of the types of roads I take my SX4 and on which I would take a Subaru:












  2. #2
    Join Date
    Mar 2011
    Location
    Los Angeles
    Posts
    70
    Geoff, while I can't offer any input on your question, I'd love to hear you detail the modifications (parts and sources) you made to your SX4, and too, how you feel about it as a daily driver in all conditions.

    I've been keenly interested in this vehicle as an economical daily driver that can still handle mild off road driving comfortably. A good friend spoke strongly of his--and it's available with a manual transmission. One of my reservations with it is its FWD bias. I've never liked FWD cars. You sound very dialed-in to the dynamics of your vehicles, so I'd appreciate hearing about this too.

    BTW, where are the pictures from? Very nice stuff!

  3. #3
    Join Date
    Dec 2010
    Location
    Rubicon Trail, California
    Posts
    209
    The SX4 is an awesome car. I've owned it since new (2008 model) and I've been very pleased with it. I'm not looking to replace it with a Subaru, I'm merely adding to my fleet with another car for me or my fiancé to drive.

    The lift kit is from Rocky Road Outfitters. The skidplates are from Primitive Racing. I upgraded from the terrible OEM tires (205/60-R16) to General Grabber AT2 (215/70-R16). I also installed some upgraded Monroe shocks for the rear, as the OEM shocks don't provide enough damping for hauling loads or towing my trailer.

    Any concern about the drivability with these mods are completely unfounded. The only difference you would notice compared to a stock SX4 are the slightly higher ride height and a mild increase in tire noise. I still get high-20's MPG on the highway, and the car actually handles better on-road because these all-terrain tires have more traction than the OEM tires. The upgraded rear shocks help counter any additional body lean from the higher center of gravity, and provide much better chassis control over midcorner bumps. Off-road, the differences are night-and-day.

    I was skeptical of the FWD-biased AWD system at first, but it's proven to be all that it's hyped up to be. The car's off-road prowess is not limited by the AWD but rather the limited ground clearance and too-tall gearing. Sure, driving on bottomless snow would benefit from difflocks front and rear, but the AWD system seems perfectly capable of apportioning sufficient torque to the rear wheels.

    The photos I posted are all from California's Sierra Nevada mountain range. I keep my hardcore rockcrawler for the Rubicon and other challenging trails, but the SX4 is a lot more fun and comfortable for backcountry exploration on easier trails in the mountains and deserts. It also makes for a wonderful daily driver. I have nothing but respect for this car, I'm just curious to hear from the Subaru experts which particular year/model/transmission/option package is considered the "ultimate" for off-road use. I'd love to have the STI's AWD system in a lifted Forester with 30-inch tires and the low-range-equipped foreign-market manual transmission, but that's not going to happen. What's the best U.S.-market late-model Subaru for rough-road backcountry exploration?

  4. #4
    Join Date
    Jul 2009
    Location
    San Diego, CA
    Posts
    1,174
    As for manual vs. auto? Stick to the manual. The auto uses the ECU controlled clutchpack and I know a couple of guys who managed to overheat theirs while wheeling a bit beyond what Subaru recommends. They'd have to stop and wait for things to cool, then continue on. The manual with the Viscous center diff is much more "fluid" in that it seems to react with a bit more elasticity . It will go from 50/50 to 10/90 or 90/10, IIRC, which works just fine.

    I haven't done it myself, but when I was using my "outback-ified" WRX as a "trailhead approach" vehicle, I did research the Forester/Outback transaxles from Australia. Basically, they're more like the "old" subaru transaxles with low-range included - but they're longer and the gears are much wider than the old EA81-series stuff so they can handle the power now. Of course you'd want a manual-shift in this anyway because the US ECU won't know how to talk to the AUS TCU, and the AUS ECU wouldn't pass US Emissions - as far as I can tell the manual dual-range tranny should bolt right in, with the only unsolved issues being the shift linkage and rerouting of the clutch slave cylinder and/or lines.

    Hope that helps.

    Edit: Incidentally, I had my WRX on roads about like you picture (except for the first/steepest one) without problems and only a *teeny* amount of lift from the Outback springs. If I were going to get serious, a strut-mount lift and the aussie tranny would be things to look at.
    Herbie - K6ZMB
    "My minivan is cooler than your bro-truck"
    2003 Chevrolet Astrolander 4WD Van - DIY GTRV PopTop, 4" Lift
    2002 Subaru Impreza WRX "Outback" - a.k.a Frankenstein's Trailhead Vehicle. Lifted, WRX-TR Brakes, STi Seats + More

  5. #5
    I haven't been keeping up with the latest developments so I don't what's changed in the latest models (<5 years), but I believe the only difference between the manual and auto is the center diff. Manual is a viscous coupling and auto is a clutch pack. There are some differences in the distribution of torque front/back, from my memory, the auto starts out 90/10 and is able to shift 50/50 front/rear, while the manual is always 50/50. But from a real world perspective I don't think there's enough of a difference to be noticable.

    When I was off-roading with some friends with manual subarus vs. my auto, the key difference is the auto's ability to deliver power to the ground a lot more smoothly than the manual. I think that alone trumps any type of drivetrain differences. In slippery situations you don't lose traction by spinning the tires and you don't need to slip the clutch. Two footed driving in an auto is a lot easier than 3 feet in the manual.

  6. #6
    Join Date
    Jul 2009
    Location
    San Diego, CA
    Posts
    1,174
    Quote Originally Posted by ducktapeguy View Post
    I haven't been keeping up with the latest developments so I don't what's changed in the latest models (<5 years), but I believe the only difference between the manual and auto is the center diff. Manual is a viscous coupling and auto is a clutch pack. There are some differences in the distribution of torque front/back, from my memory, the auto starts out 90/10 and is able to shift 50/50 front/rear, while the manual is always 50/50. But from a real world perspective I don't think there's enough of a difference to be noticable.

    When I was off-roading with some friends with manual subarus vs. my auto, the key difference is the auto's ability to deliver power to the ground a lot more smoothly than the manual. I think that alone trumps any type of drivetrain differences. In slippery situations you don't lose traction by spinning the tires and you don't need to slip the clutch. Two footed driving in an auto is a lot easier than 3 feet in the manual.
    The VC diff in the manual will go from 50/50 to about 90/10 or 10/90 - if it was locked at 50/50 they wouldn't need a diff.

    My experience is actually opposite of yours - I found my VC to be a bit smoother and not "shock" the wheels as power transferred around, whereas my friends with the auto/clutchpack would get the "thud" as the ECU ramped up the duty cycle on the diff to move power rearwards.

    The one other difference is in the "package" nature that Subaru sells - more of the Manual tranny cars include a torsen rear diff which at least helps a little, while the auto-tranny cars tended to have pure open diffs since the torsen made them push on the street. In fact, I think only the H6 autos as of late got the torsen rear diff, none of the 4-cylinder cars.
    Herbie - K6ZMB
    "My minivan is cooler than your bro-truck"
    2003 Chevrolet Astrolander 4WD Van - DIY GTRV PopTop, 4" Lift
    2002 Subaru Impreza WRX "Outback" - a.k.a Frankenstein's Trailhead Vehicle. Lifted, WRX-TR Brakes, STi Seats + More

  7. #7
    Join Date
    Dec 2010
    Location
    Rubicon Trail, California
    Posts
    209
    Quote Originally Posted by ducktapeguy View Post
    When I was off-roading with some friends with manual subarus vs. my auto, the key difference is the auto's ability to deliver power to the ground a lot more smoothly than the manual. I think that alone trumps any type of drivetrain differences. In slippery situations you don't lose traction by spinning the tires and you don't need to slip the clutch.
    Well that all boils down to the driver's abilities. The driver of a manual transmission has a lot more control over the torque that gets sent to the tires versus the driver of an automatic. But if the driver is lacking in skill, then you're absolutely right - an automatic will "smooth over" the unskilled driver's erratic torque delivery.

    But that's not my concern. I'm thinking more along the lines of a situation like this:

    The road is very steep. 1 front and 1 rear tire are on ice. The other 2 are on dry asphalt. The car is at a dead stop, and the AWD system is trying to apportion torque to the wheels to get the car moving. Which system is better?

    Here's another hypothetical situation:

    The car is crossed-up after trying to diagonally cross a deep ditch, with 1 front and 1 rear wheel in the air with zero traction. Which modern-day Subaru is going to be the best at getting moving?

    Obviously, if any come with physical limited-slip differentials, those will be at the top of the list. Does one AWD system (the one in the manual vs. the one in the automatic) perform better than the other in these situations? Does an Outback with an automatic and VDC perform better than one with a manual transmission? What's the best off-road Subaru AWD system?

  8. #8
    Join Date
    Sep 2007
    Location
    Western Colorado
    Posts
    115

    Default 2005 GT Limited

    My 2005 Legacy GT was purchased to replace my 88 BMW 32iX. The iX was an absolute hoot to drive in the snow, stable, docile, with phenomenal traction. Punch the throttle to kick the back out, keep your foot in it and zip on down the country lanes.

    Based on Subaru's information I chose the 5 speed auto box because . . .

    "Variable Torque Distribution (VTD) Models equipped with 5-speed automatic transmission utilize an electronically controlled variable transfer clutch in conjunction with a planetary-type center differential. Rear-wheel-biased torque distribution normally configured at 45/55-split front-to-rear. 2.5 GT models also feature a viscous-type limited-slip rear differential." (https://www.subaru.com/content/downl...005_legacy.pdf)

    The BMW had something like maximum of 35% torque transfer to the front through a viscous unit and the Subaru has behaved very similar.

    Probably more information than you are looking for but if you look at the attached pdf from above (pdf page 19) you will see there is a difference between the manual an automatic transmissions. I prefer a manual but chose the automatic due to driving characteristics (The rear torque bias and LS rear diff).

    Hope this helps

    Added: The "Sport" selectable autobox has worked well for downshifts, there is some delay, and ecu safety overrides for rev's but with my experiences it behaves like most selectable automatics.
    Last edited by Shiryas; 04-13-2011 at 09:54 PM. Reason: Added info

  9. #9
    Join Date
    Dec 2010
    Location
    Rubicon Trail, California
    Posts
    209
    Does any late-model Subaru offer the combination of a manual transmission and a rear limited-slip differential?

    Or does anyone make an aftermarket LSD for it? I'd love a torsen-type (Quiafe or TrueTrac) or even a selectable locker (ARB air locker, Eaton E-locker, etc.) but even a marginal clutch-based limited-slip diff would be better than an open diff.

  10. #10
    Join Date
    Jan 2011
    Location
    Austin, TX
    Posts
    36

    Default a 3rd AWD system - 5EAT

    So there is a 3rd AWD system in the subaru vehicles. I have an '08 Outback 2.5XT. This is a 2.5L Turbo motor with the 5EAT - 5 speed automatic. This transmission has a 'normal' torque split of 45/55 (F/R). The AWD is a more advanced 'predictive' clutch pack. The rear also contains a Viscus LSD. Overall the system worked great for me on my recent trip to Big Bend when paired with Yokohama ATS tires.

    http://www.expeditionportal.com/foru...ad.php?t=59337

    I don't have any experience with a Manual but the gearing on the Outback seemed adequate. The only times I saw some issues were steep descents where there was limited braking (with the sport-shift in 1st) and the ABS would kick in a little sooner then I would like.

    Here is an excellent write-up from LegacyGT.com:

    http://legacygt.com/forums/showthrea...2.html?t=48112


    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..._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..._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:

    Subarus without Rear LSD:

    Impreza 2.5i (5MT and 4EAT)
    Outback Sport (5MT and 4EAT)
    Legacy 2.5i (5MT and 4EAT)
    Legacy 2.5i Ltd. (4EAT)

    Subarus with Rear LSD:

    Impreza WRX (5MT and 4EAT)
    Outback 2.5i (5MT and 4EAT)
    Outback XT (5MT and 5EAT)
    Outback 3.0R (5EAT)
    Legacy GT (5MT and 5EAT)
    Spec.B (6MT)
    WRX Sti (also has Front 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/?sr...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:

    DCCD with Front and Rear LSDs - WRX Sti
    VTD - 4EAT on WRX and 5EAT on Outback XT, Outback 3.0R and Legacy GT
    Active AWD with Rear LSD: 4EAT on Outback 2.5i
    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.

Page 1 of 4 123 ... LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •