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OK so it is my understanding that the 08 WRX doesn't have a Limited Slip Differential but it does have stability control and traction control.
So my question is with the little VDC TC button turned off what stops all of the power going to any one wheel. There obviously is some other type of differential but does it control slippage.
Also what are the pros and cons for having only VDC TC over a LSD and didn't the 07 have both?
 

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Well, on the track, the VDC controls your car extremely well. Just turn your steering wheel, and you'll expect your car to turn for you. Its almost like what the evo has in terms of electronics.

in my opinion, it was a good move towards VDC.
 

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OK so it is my understanding that the 08 WRX doesn't have a Limited Slip Differential but it does have stability control and traction control.
So my question is with the little VDC TC button turned off what stops all of the power going to any one wheel. There obviously is some other type of differential but does it control slippage.
Also what are the pros and cons for having only VDC TC over a LSD and didn't the 07 have both?
All AWD Subarus have a limited slip diff (of some sort) in the center. Non-STI manual transmission cars have a traditional center differential that has a viscous locking component to transfer power once slip occurs. This viscous unit is only ever called upon when there is a significant difference in speed between the front and rear axles.. such as when one end loses traction. When there isn't any slip.. the differential acts normally and distributes the torque 50/50 (f/r). Since it's very hard for a Subaru to spin wheels on pavement with this configuration.. the viscous portion of the center LSD does nothing 99% of the time. Off-pavement and in extremely slippery conditions, the viscous LSD keeps the front and rear axles turning at roughly the same speed whenever possible. So.. even with the VDC turned off.. the center LSD is still working to make sure you have power going to at least two wheels. ;)

Most Subarus have had open front and rear diffs... yet, they're still notorious for being unstoppable snow machines. In the US, the Legacy 2.5GT, some Legacy Outback models, 00-01 2.5RS, 02-07 WRX and STI have all had a limited slip rear differential. Except for the STI, this rear diff used a similar setup to the manual transmission's center LSD.. with a viscous slip-limiting unit working in conjunction with a traditional differential. Just like the center viscous unit, the rear LSD doesn't do anything when there's decent traction. Under power, the rear axle has more traction as weight transfers to the rear.. as such, a Subaru is more likely to lose traction on a front wheel in higher grip situations rendering the rear LSD useless for most performance applications. The rear LSD does make the rear of the car easier to 'kick out' on loose surfaces... but, as a side effect it can also make the car more twitchy on snow and ice.

As for the pros & cons of having only VDC over an LSD... well, as I mentioned, the rear LSD can make the rear of the car twitchy which doesn't usually sit well with something designed to make the car more stable. In addition, the TC can do a better job of taming a slipping rear wheel than a viscous LSD can.. and it can still transfer torque by braking the slipping wheel.

No, the '07 Impreza did not have VDC or TC of any sort. This is a first for the Impreza.

FYI: The STI still has mechanical limited slip front, rear and center diffs that are more predictable, progressive and smooth than the viscous LSDs.. so they probably work better alongside the VDC.
 

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the VDC in snow is a big help. i drove in snow with it on and off. with it on it will keep you at a steady pace and not allow the tires to break loose and has prevented me from seeing a ditch a couple of times. with it off thats when all the fun begins.
 

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Stability control systems (like VDC) function in similar concept to ABS, but in reverse. ABS will sense a locked wheel and selectively release individual brake pressure at each locked wheel (pulsing you feel) to modulate the brakes.

VDC works in reverse...if it senses a combination of wheel slippage vs throttle position vs steering angle (actually, much more complex than this, but for the sake of explanation) to detect the possibility of the car going off intended course. When such wheel slip is sensed and the computer thinks you are going off the intended course, it will selectively apply brake individual slipping wheels to improve traction and get the car stabilized.

Now what does this have to do with LSD's?

Well the LSD essentially assures power is distributed evenly between both wheels on the same axle. This style differential (similar to a locked diff in some 4x4's) will allow power to both wheels, but will slip (unlike locked diffs) while coasting through a corner to save your tires (wheels spin at different speeds through a corner). Under power though through a corner, power is equally distributed left/right so that power is still transferred to the loaded wheel, usually the outside rear when powering through a corner. The STi has this feature.

An open differential (like on the 08 WRX), when placed in the same situation as used before (powering through a corner), the differential will transfer power to the wheel with the least amount of load. Imagine a car stuck in the mud/snow...the buried wheel will stay stationary while the unloaded wheel spins like crazy. This causes a problem in a racing environment when accelerating through a hard corner because the outside rear wheel is loaded and not receiving as much power to push you through the corner as does the unloaded inside wheel.

This is where the VDC comes in....by using the concept of selective braking, the VDC system is able to simulate the LSD in some scenarios. Say in that same corner, you accelerate to track out and the inside wheel starts to lose traction and power is reduced from the outside wheel...you are about to have an a$$ puckering moment. The VDC will gently brake the inside slipping wheel only...this adds load to that wheel (similar to a wheel stuck in the mud) so mechanical power is now transferred to the OUTSIDE wheel where you want the power!

This sounds great and all, but unfortunately, these systems also tend to cut power on traction loss, etc etc that actually hinders performance as it tries to help. This is why LSD's are still great to have on rear-drive axles. HOWEVER, I am excited to see how Whiteline's Black Box performs on our cars. It essentially tunes the VDC system to tell it when we want it to selectively brake, and on which wheels, to help induce oversteer, understeer, etc. Should be interesting. But I am also looking out to see if we can get an STi LSD or aftermarket installed on our cars.


Hope this helps.
 

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I know this thread is dead, but THATS why our brakes wear out so quickly. Meh, brake pads are cheap compared to LSD's, practically the same function except electronic instead of mechanical?
 

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I know this thread is dead, but THATS why our brakes wear out so quickly. Meh, brake pads are cheap compared to LSD's, practically the same function except electronic instead of mechanical?
If you are using the VDC all the time, perhaps so! I find the limit is pretty high with the Subaru system, can't imagine anyone kicking it in on a daily basis. Even then probably only a couple seconds at a time to get you back on course. Snow might be a different matter.

The interesting thing is that technically you are right about the functionality of VDC vs an LSD. I recall back in my Neon SRT-4 days (stock LSD up front) that the message boarders were furious the new Caliber SRT-4 did not have an LSD. Luckily the SRT engineers were always active in the community, they explained that they were using a brake based system via stability control, and that this system worked as well, if not better.

The poster a few posts above does not understand how and LSD works, it does not "distribute" power evenly. There is a ratio, i.e. while a wheel is spinning or about to slip (depends on type of LSD) only some of that power will transferred. For instance the Quaife diff in the SRT-4 was something like a 4:1 ratio, so while a wheel is slipping, only 4 times that power can be transferred. This is the achillies heel of the Quaife/Torsen style diff, put one wheel in the air with 0 traction...well guess what 4X0 is. With a VLSD like Subaru, it is not the same but still similar, only some of the motion of that spinning wheel is causing transfer of power to the other wheel.

Getting back to the SRT engineers, they pointed out that the brake-lock method can transfer up to 100% of the power from the spinning wheel to the other wheel. In this particular case they programmed the system for performance and it worked at up to about 80mph (most TC systems only work at lower speeds) so it was useful on a race track.

Now the problem is these systems are normally programmed very conservatively in conjunction with engine power limiting. In the WRX this is the case. The system can be turned on or off only fully. There is no option to use the VDC without power limiting, though in the STI there is a secondary setting of the VDC with this option. The function of the VDC could be as good as an LSD if it was programmed that way.
 

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Mr. Power, very informative post!

I'd now like to see the whiteline 'blackbox' out for the WRX to take the traction control OUT of the VDC...at least that is my understanding of what that does. Hmmmm....

j
 

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I know this thread is dead, but THATS why our brakes wear out so quickly. Meh, brake pads are cheap compared to LSD's, practically the same function except electronic instead of mechanical?
I have gotten 22k miles out of my padsand still going. And that is with 7 autocross races in which I went for 6-8 runs each race.
 

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i just picked up a older wrx vlsd and i am very excited to get back home from work to install this piece. i know it is not as good as a mechanical unit but it will still be miles ahead of the open diff that is in the car now. chunks of gears were found when changing out the rear end gear oil and i am simply not going to replace the rear end w/ the same parts. i also have a new pinion shaft and gear ring so it is time to upgrade...



once i find another rear end housing i will build up a complete unit and change out the whole rear end then. i just don't want my car decommissioned for too long. i know it is way easier installing a rear end out of a 06 or 07 automatic wrx but they are getting hard to find fast. first, were not a whole lot of auto wrx's sold and secondly, even less of them are wrecked already. i will keep you informed when i get the parts together and maybe a diy for the not so faint of heart. :)
 
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