Differential FAQ: Consolidated and made relevant for Legacy owners

This was taken from NASIOC Thanks to Unabomber there for making this post. I figured people here would want this info made a little more relevant for us legacy owners. So i added in some info.

 

Differential FAQ

 

What is a differential? A differential is a mechanical unit that allows a transference of power from one input source into two by various means. In the AWD world, it starts at the center differential first. Power hits the center differential and it determines the amount of power that goes to the front and rear differentials. From there, the front and rear differentials distribute the power to their respective wheels.

 

Where can I learn more general information about differentials?

 

http://auto.howstuffworks.com/differential.htm

http://en.wikipedia.org/wiki/Limited_slip_differential

 

What types of differentials come stock?

 

Viscous: Used as the center and rear differentials in the WRX/RS/LGT/06 Spec b, and center only of 07+ spec b 6spd boxes. (08+ WRX has an open rear diff). Varies power applied between two axles via fluid dynamics and discs. This type of unit is filled with a silicone based fluid that becomes thicker as the difference in input shaft and output shaft speed increases, thereby increasing the viscosity of the fluid and the grip between the input and output discs, which do not actually touch each other.

 

Suretrac: Only available on the 2004 STi. Manufactured by AP Racing, this unit comes stock in the front differential of the 2004 STi. It is a mechanical type differential that employs a set of specially shaped teeth to intelligently transfer power, unlike the bevel gears that are used in conventional LSDs. Very similar in operation to the torsen type, though through different mechanical means.

 

Open differential: Used as the front differential in the WRX/RS/LGT and in both center and rear of the Legacy 2.5i. In a nutshell, it’s one wheel drive. The wheel with the least amount of traction will have majority (if not all) of the vehicle’s power applied to it.

 

DCCD: Short for Driver Controlled Center Differential. Used on the STi. Planetary center differential in conjunction with an electronically managed continuously variable transfer clutch. And as the name suggests, it allows the driver to control the torque bias of the center diff by a turn of the thumbwheel.

 

Torsen type differential: Used as the front (2005+ STI) and rear differentials in the STi, and also in the rear of 07+ spec b Legacy. Short for TORque SENsing differential. It’s worth mentioning that though Torsen is a brand name, it is the most commonly used name for this type of differential. This type of unit is also known as a helical or mechanical type. It uses gears to split power between two axles. Once one wheel is off the ground or slips, it in essence, becomes an open diff or exhibits limited traction based on the torque bias of the unit. It has the added drawback of weight. The additional torque required to rotate a heavier differential will require more energy, energy is heat, heat is friction, friction and heat are wasted energy. It requires more energy to drive and this can be shown on a chassis dynamometer if same car is measured before and after the differential change. The other downside is that if the engine is quite powerful and extra special abuse is administered (high grip launches, donuts, etc.) they will explode. These broken gears will make their way through the case and can cause considerable damage. Keep this in perspective though, as very few cars exert this kind of power and those owners tend to understand what that power is capable of breaking.

 

What are the major upgraded differential types?

 

a. Aftermarket Torsen/Helical type differentials

b. Aftermarket Clutch type differentials

c. Aftermarket DCCD controllers or ECUs

d. Upgraded viscous differentials

 

 

What are Clutch type differentials? They use a series of plates to act as clutches to split power between two axles. Depending on how the clutch discs are setup, the friction (lock) will vary with the amount of differential torque applied to the axles. This diff type will produce increased noise at higher break torque settings (the amount of setting torque applied that opens the differential) during low speed, high differential torque conditions (i.e. sharp turns at low speeds). Pros of this unit are tune ability (through the stack order and number/thickness of plates in the stack), rebuild ability, low unit weight, and they operate in “lifted wheel” situations. On the flip side of the coin, they require maintenance (especially if aggressively set up and used in high traction environments, which generates more friction on discs and just like your clutch for your transmission, naturally wear) and low speed friction that can cause awkward engagement at low speeds (worsens in relation to break torque setting). In addition these types also offer end user tuning via changing the configuration. There are three different configurations, or “ways” for these LSDs. A 1 way differential means that the cam is shaped in such way as to have positive lock only when accelerating. The 2 way is constructed in a way to have positive lock motion in both acceleration and deceleration mode. The 1.5 way functions almost same as a 2 way but provides less lock when decelerating. The 1.5 way can provide more forgiving balance when braking than a full 2 way setup, although it is less effective for true racing applications, it provides easier operation for beginners in throttle off conditions.

 

So what should I upgrade my car with?

 

RS/WRX/Legacy front 5mt/6mt: Torsen/Helical or Clutch type. There is a JDM helical unit that fits the 5mt front. 6mt people can put in any front LSD available for the STI 6MT.

 

RS/WRX/legacy 5mt center: JDM STi 20kg center viscous unit. This increases the break away torque from the stock 4kg unit to hold rear traction longer.

Note(from rallispec.com): The JDM STi 20kg center viscous unit is Recommended for rally or drag racing use. Increases resistance torque from 4kgf-m/100rpm (stock) to 20kgf-m/100rpm. Improves response time and reduces heat generation during extreme wheelslip. Will increase understeer in tight cornering.

-Also available for the 5mt box is the Cusco Tarmac 35/65 f/r diff. This diff is a biased open diff so if you decide you want it, make sure you have a good front and rear LSD already.

-A third option is the PPG center helical/torsen type diff. To me this is the best option for a street/track/DD car.

-A 4th option is the JDM STI 5spd DCCD unit from the older 5MT STI. This will fit our 5MT but its rare and JDM and $$$$$$.

 

RS/WRX/Legacy R160 rear: Torsen or Clutch type For you 2.5i legacys, you can install the factory LGT viscous rear LSD in your car. It can be had for less than most of the aftermerket units.

 

STi/07+ spec b 6MT front: Torsen/helical or Clutch type

 

STi DCCD center: Keep what you have and upgrade it. It's cheap and effective in low traction situations and the only downside is that it remains open until there is a substantial slip or speed differential before it engages. That said, the best option for the center differential is a good center differential ECU and someone experienced in mapping this. The easiest route is to contact www.rocketrally.com as they have several maps available and can load multiple maps in one ECU that can be switched on the fly. MoTeC also makes a diff ECU, but it requires complex end-user tuning. While expensive, if you ever have a chance to drive a car with an aftermarket diff controller, you'll be hooked.

 

STi/07+ spec B r180 rear: Torsen/helical or Clutch type.

The 07+ spec b comes with a torsen/helical in the rear already. But if owners want, they can swap in the STI clutch type read Diff.

 

As well, both STi/07+ spec B and WRX/RS/legacy owners (Rear differential R-160 & R-180): Rebuilding/reshimming the factory rear diff is a good bet. Shim kits are under $300 and are available from your local Subaru dealer. Re-shimming requires specialized tools and knowledge to perform correctly. Shimming the rear to tight (high break away) can make the car very tail happy. The downside to this solution is the greater wear on the differential. In addition, there is a new STi pressure ring set for the R-180. This increases the pressure angles of the stock R180 plated LSD from 45 degrees to 60 degrees in order to improve throttle sensitivity, which ultimately allows the LSD to operate quicker and more effectively. The angle that the cam rides on can change the rate at which the differential locks when differential torque is seen, so basically, by increasing the angle, you increase the rate at which the center section initially expands, which will engage the clutches at a faster rate/time than a lesser angle (can be beneficial in gravel to help car rotate with lighter throttle inputs).

 

Imprezza/Legacy 5EAT/4EAT: AFAIK, there is no upgrade/aftermarket replacement for the front and center diffs. The rear diff is R160 IIRC so you should be able to use any option available for the wrx/lgt 5MT R160 rear ends.

 

I’m one of those guys that just wants to upgrade for upgrades’ sake, what is the best bolt on solution for me? Torsen type differential. Bolt it in and go, it’s the plug and play differential and notice that the STi’s “better” differentials use them. Most OEM differentials are of a Torsen type for a reason.

 

I’m one of those guys that wants the “best” of upgrades, what differential should I go with? Clutch type differential. You can tune them for your car, you can rebuild them for better handling characteristics, etc.

 

Which is better, Torsen or Clutch type? There really is no better. The best way to explain the difference is to use an analogy. Torsen type diffs are like the popular spring/strut combos like STi take offs + Pinks or KYB AGX + Prodrive springs. They are what they are and they do a great job. Clutch type diffs are like coilovers. They have end user adjustability and require more set-up, maintenance, and alignment. In the racing world, in theory a coilover equipped car has the advantage if perfectly driven and set-up, but they can and have been beaten by the spring/strut guys. In the end, it can be simply stated as do you want more or less hassle, adjustability, rebuildability, expense, or OEM feel?

 

How hard is it to swap out differentials? Installation of an LSD can be easy or hard depending on the location. Andrew Yates of www.gearboxtech.com has the opinion that the center and rear diffs on all transmission types are the easiest to swap out and may be accomplished with the service manual, tools, and know-how of the above average person. The 5MT/6MT front differentials should definitely be farmed out to a transmission specialist, as it requires a higher degree of precision and specialized tools that is outside the grasp of most people and even some certified mechanics. Rough professional installation pricing would be $300 for the center or rear differential and $900 for the 5MT/6MT front differential.

 

Who are the popular manufactures of Subaru differentials?

 

Cusco

Kaaz

MFactory

OBX Racing

PPG

Quaife

STi

 

Websites to visit for more information about differentials:

 

http://www.torsen.com/general/general_faq.htm

http://www.mycaterham.com/66828/1174...ession*id*val*

http://www.cusco.co.jp/english/e_lsd.html

http://www.billzilla.org/diffs.htm

http://www.houseofthud.com/differentials.htm

http://www.guardtransmission.com/velocity.htm

http://www.magnusmotorsports.com/tech/kaaztorsen.htm

http://www.quaifeamerica.com/differentials/diffs.htm

http://www.mrtrally.com.au/performan...s_coupling.htm

KAAZ LSD explained

 

Editors Note:

 

One of my favorite sayings is “Research twice, modify once”. When it comes to limited slip differentials, perhaps a paradigm shift is in order. After researching this topic, I’d advise people with interest to: Research twice, seek advice from vendors of all LSD types, seek advice from owners of all LSD types, research professional installers in your area, then purchase and install the unit(s). That’s a mouthful, but in this case, is critical to the success of this modification.

 

This post was created because I wasn't able to find a good differnetial FAQ. I came up with the text based on LOTS of searching here. Upon reading this you should have an idea of whether a differential upgrade best suits your needs or not. The manufacturer is up to you.

 

My thanks to Andrew Yates from www.gearboxtech.com and Franz Diebold from www.franzdiebold.com for providing valuable assistance in the formulation of this FAQ and for shedding some light on the world of differentials.

 

If you find an error in this FAQ, please PM me with factual details and I will update this post. Responses such as, "I have XXX's diffs and they're great!" or "XXX's diff broke after 1 month" are not appreciated here, that is what the Car Parts Review Forum is for.

 

 

 

Whitetiger: More comming.......

Edited May 18, 2009 by whitetiger

Excellent information.

 

For most people "upgrading" differentials is a waste of time and money, but that doesn't stop people and this is a great source of information.

Again, thanks to nasioc/unabomber.....

 

The following advice comes from Dave at Rallispec, one of the few people whom actually KNOWS differentials and I thank him for his contribution and advice as differential advice from people who REALLY know them is impossible to come by.

 

To add a couple points of information to this very cool thread:

 

1. Viscous LSD's are speed sensing. The work by reacting to speed differences between the two axles. They are limited in the time it takes them to respond and therefore they are not an ideal solution. They are designated by their torque reaction per difference in rotational speed. For example the stock unit is 4 kgf-m (torque reaction) / 100rpm (speed difference). Increasing this value increases how quickly it can respond because it will be able to redirect more torque for the same difference in shaft speeds. However, if the value is increased too much then a small difference in shaft speed (such as one that occurrs in a normal tight radius corner) will generate enough torque reaction as to create a noticeable understeer condition.

 

2. Gear type LSD's and clutch type LSD's are torque sensing. They respond to torque input and/or torque differences across the drive shafts. They do not need to be waiting for a speed difference (as in wheelspin) to be doing their job. These types of LSD's are designated by their bias ratios. Basically what ratio of the torque at the low traction side can be redirected to the high traction side.

 

3. Torsen, Quaife, and Helical LSD's are all very similar...all gear types that involve complex interactions between the gear teeth to do their job. Quaife and Helical LSDs both use helical type gears. Torsen uses a worm gear arrangement. These LSD's typically have bias ratios in the 1.5 to 4 range I think. However, most are not set up with any preload (although it is possible to build in some preload with these LSD's by shimming the end play of the gears against the case to a certain amount of interference fit). Therefore when the torque at one wheel is zero or very near zero (wheel off the ground or on ice) then the LSD cannot transfer any torque to the other wheel (anything multiplied by zero is still zero!).

 

4. Clutch type LSD's (Salisbury type...also known as plated diffs) use stacks of clutch plates alternately splined to the axle and the diff case. Pressure to apply the clutch packs comes from a set of pressure rings. The spider gear cross shaft rests on sets of ramps cut into the pressure rings. When torque is applied to the diff the shaft rides up the ramps and forces the pressure rings apart thereby applying the pressure to the clutch packs. Clutch type LSD's set up for racing generally have very high bias ratios (upwards of 6:1 I think). They also can be set up with significant preload if desired. The preload on the clutch plates ensures that the LSD can still do something even when there is no zero traction at one wheel.

 

5. The DCCD's open torque split is determined by the tooth arrangement of its planetary gear set. Unlike a typical diff's spider and pinion gear arrangement which gives a 50/50 split as long as available traction on both axles is equal, the planetary gears provide a torque split biased toward one end (be it 35/65 or whatever it is designed for) as long as available traction is equal. Attached to that planetary gear set is a clutch pack and a second clutch pack controlled by an electromagnet. The pressure on the clutch pack (which biases torque back toward a 50/50 split) is determined by the current flowing through the electromagnet. Therefore it is a clutch type LSD that can be electronically controlled to alter its operating characteristics.

 

In terms of tuning the clutch type LSD, your options are as follows:

 

A. Preload setting -- increasing preload increases its effectiveness in very slippery conditions. However, this preload must be overcome in order to go around a corner (which is why these LSD's when set up with any measurable preload will make noise). It will generate understeer. Preload is set both by a preload spring (some types use a bellville washer but Cusco's RS type uses coil springs) and also by the stack height of the clutch pack (the thicknesses of the clutch plates).

 

B. Ramp angles -- there are two ramp angles, one that influences on-throttle operation and one that influences off-throttle (decel) operation. When an LSD is described as 1way, 1.5way, or 2way they are talking about the off-throttle ramp angles. 1way types only offer LSD action on acceleration and have no ramps on the decel side. 2way types have equal ramp angles for both accel and decel. With 1.5way have steeper angles on the decel side vs. the accel side so it takes more torque to create the same pressure (basically on decel the LSD is working but less effective). Some manufacturers for some reason use other values such as 1.6way or 1.7way but they are just describing the relative angles from one side to the other.

 

C. Active friction surfaces (lockup percentage?) -- By altering the arrangement of clutch plates (outer splined vs. inner splined plates) you can alter the number of "active" friction surfaces in play. Reducing the friction surfaces reduces the bias ratio.

 

In terms of general recommendations....

 

Front LSD: for track, autox, and street use most people find that a gear type front LSD is most predictable and easiest to drive near the limit. It does not generate much understeer on corner entry due to the absence of preload and the low bias ratio under decel. A 1way clutch type LSD can be just as, if not more, effective when set up properly for the conditions and the driving style of the user. But if it is not set up well then it will cause difficulties for the driver. For slippery conditions such as rallyx, iceracing, rally, etc. it will be beneficial to run a 1.5way plated front LSD as it will help with braking stability. Preload should be set according to available traction and driver preference. For tarmac drivers having difficulty under braking it can also be worthwhile to try a 1.5way LSD with low preload.

 

Center LSD: I am very interested to try the PPG Torsen center LSD but have not yet had the chance. I suspect it may be an excellent track option. I would avoid using a 20kgf center viscous except for gravel, snow, or ice...or maybe straight line drag racing. On high speed tracks with no tight corners it may be OK but it will be counterproductive on tight tracks or autox. The Cusco Tarmac gear is worthwhile only when you have good front and rear LSD's to back up the fact that it is an open diff.

 

Rear LSD: There are very few instances for racing applications where I would not recommend a clutch type rear LSD. They are so tuneable that there is no reason not to use one (other than the need to rebuild it every season or every other season). The stock viscous unit used in the WRX and 2000-2001 RS and also in the Legacy GT models is essentially useless (the unit is so thin as its packaged off the side of a standard open diff and its capacity is very limited). The factory R180 clutch type LSD on the STI is set up extremely loose from the factory and should be rebuilt with appropriate preload and possibly different pressure rings if you want to use it for racing. For a street application the stock Torsen or a Quaife unit is usually preferrable from the standpoint of not needing to be rebuilt and creating very little noise.

Thanks, whitetiger for interesting info on diffs. I'm not a mechanic, but could use some info on center diffs. My '94 Legacy L wagon (5 speed manual)has an issue that occurs when the car has been driven for a while either in town or on the highway, It is not noticeable when the car has been sitting idle for a while or when first driven in the morning. It is particularly noticeable when manuevering in parking lots at low speeds. The car seems to clunk or chug like it has an axle problem which it does not. I've been told it is a bad center diff, which is rare in manuals, but can result from using mismatched tires for long periods. I bought the car a year and a half ago and the tires were matching and in fair shape. This issue has been present just about from the get-go, but it took me a few months to figure that it was a problem. First, does this sound like a symptom of a bad center diff to you? Second, if I replace the center diff, do I need to remove the tranny. How much time should this take? Third, would it be reasonable to install used center diff from a wrecker or used parts shop? What's a reasonable price on that? What are the chances that this issue has nothing to do with the center diff? Any info appreciated. Thanks

Wow Great information here. Thanks to clear up some grey areas on the LGT.

 

08+wrx open rear diff FTL!

Thanks, whitetiger for interesting info on diffs. I'm not a mechanic, but could use some info on center diffs. My '94 Legacy L wagon (5 speed manual)has an issue that occurs when the car has been driven for a while either in town or on the highway, It is not noticeable when the car has been sitting idle for a while or when first driven in the morning. It is particularly noticeable when manuevering in parking lots at low speeds. The car seems to clunk or chug like it has an axle problem which it does not. I've been told it is a bad center diff, which is rare in manuals, but can result from using mismatched tires for long periods. I bought the car a year and a half ago and the tires were matching and in fair shape. This issue has been present just about from the get-go, but it took me a few months to figure that it was a problem. First, does this sound like a symptom of a bad center diff to you? Second, if I replace the center diff, do I need to remove the tranny. How much time should this take? Third, would it be reasonable to install used center diff from a wrecker or used parts shop? What's a reasonable price on that? What are the chances that this issue has nothing to do with the center diff? Any info appreciated. Thanks

 

 

From what i understand, any center diff available for the 5MT should fit your car. IMHO, id just get a replacement OEM center diff for your car to save $$$. you dont need a performance center diff. Im not sure what the center diff would behave like if it was in the precess of failing, though a toally failed center diff would not send any power to the wheels at all.

sticky

Thanks, whitetiger for interesting info on diffs. I'm not a mechanic, but could use some info on center diffs. My '94 Legacy L wagon (5 speed manual)has an issue that occurs when the car has been driven for a while either in town or on the highway, It is not noticeable when the car has been sitting idle for a while or when first driven in the morning. It is particularly noticeable when manuevering in parking lots at low speeds. The car seems to clunk or chug like it has an axle problem which it does not. I've been told it is a bad center diff, which is rare in manuals, but can result from using mismatched tires for long periods. I bought the car a year and a half ago and the tires were matching and in fair shape. This issue has been present just about from the get-go, but it took me a few months to figure that it was a problem. First, does this sound like a symptom of a bad center diff to you? Second, if I replace the center diff, do I need to remove the tranny. How much time should this take? Third, would it be reasonable to install used center diff from a wrecker or used parts shop? What's a reasonable price on that? What are the chances that this issue has nothing to do with the center diff? Any info appreciated. Thanks

I'm quoting myself here from a previous message. Anybody familiar with the signs of a center diff that is either bad or going bad?

Sticky.

 

I have a question which I was going to ask in the other thread dedicated to front LSDs (read UNABOMBER's post this morning).

 

Answer this please: what does a front LSD do for you ?

I know you have one in your LGT.

Approach this question from the (aggressive) daily driving angle not from the competition use/rally cross/autocross.

 

I have some conceived ideas such as induced understeer; more front grip; being able to hit gas at or even before the apex and the torque just pulling you thru the corner etc. I may be right on some counts and wrong on most. I wanna hear it from someone who has the part.

 

I am interested in things like

- are you stressing your transmission more ?

- are you under-steering more ?

- are your front tires wearing out faster ?

- are you cornering/steering better ?

- is your car more stable on slippery surfaces (ice/snow/wet roads)

- are you less likely to spin on slippery surfaces (ice/snow/wet roads)

- do you feel "torque steer" when you launch the car ?

 

As Rob said, for most of us is a waste of money. But I am contemplating one; I am not alone in this as I know at least one other Chicago guy thinking of one - and i wanna safeguard my time and money by asking. And of course that is the angle from which I am interested. Some folks want to know how their times will drop at the drag strip or for auto cross use. Please provide insight into that as well. I think you recently won your first AX event.

 

Thanks.

 

I am sure WhiteTiger will get to it and edit the first post but this is

the other main thread on Front LSDs in a Legacy GT

http://legacygt.com/forums/showthread.php?t=99463

So I just picked up one of these....Subaru Front LSD

Edited May 20, 2009 by f1anatic

Sticky.

 

I have a question which I was going to ask in the other thread dedicated to front LSDs (read UNABOMBER's post this morning).

 

Answer this please: what does a front LSD do for you ?

I know you have one in your LGT.

Approach this question from the (aggressive) daily driving angle not from the competition use/rally cross/autocross.

 

 

well, aside from the obvious autox/track applications, the benefits of a front LSD really helps improve foul weather traction in rain and snow. IN a sense, the LGT is actually 3 wheel drive, not true AWD, due to the open front diff it comes with. the car will have much better control when driving in snow and you will be able to give more power in lower traction situations.

 

I have some conceived ideas such as induced understeer; more front grip; being able to hit gas at or even before the apex and the torque just pulling you thru the corner etc. I may be right on some counts and wrong on most. I wanna hear it from someone who has the part.

 

this is mostly correct except for the understeer if you have a torsen/helical in the front. As stated above in the FAQ, torsen/helical diffs have very little if any preload, so ther will be no increase in understeer.

 

I am interested in things like

- are you stressing your transmission more ?

 

for the most part, no.

 

- are you under-steering more ?

 

with the right setup for the application, no.

 

- are your front tires wearing out faster ?

 

All depends on how you drive. since a front LSD will stop the inside front tire from spinning under power in a corner, it may increase tire life.

 

- are you cornering/steering better ?

Absolutely. especially under power. grip levels are vatly increased.

 

- is your car more stable on slippery surfaces (ice/snow/wet roads)

 

yes. Very much so.

 

- are you less likely to spin on slippery surfaces (ice/snow/wet roads)

 

It depends on the tire, but it driven properly, it will make the car more predictable.

 

- do you feel "torque steer" when you launch the car ?

 

no. this would have nothing to do with the diff though. Our symetrical AWD takes car of any torque steer issues.

 

As Rob said, for most of us is a waste of money. But I am contemplating one; I am not alone in this as I know at least one other Chicago guy thinking of one - and i wanna safeguard my time and money by asking. And of course that is the angle from which I am interested. Some folks want to know how their times will drop at the drag strip or for auto cross use. Please provide insight into that as well. I think you recently won your first AX event.

 

Thanks.

 

 

Well, as far as drag racign goes, at autox, we do a drag style launch start. before the diff, i would always spin one of my front wheels before the center diff warmed up and started the car going. with the front LSD in. i get a 4 wheel spin/chirp and im off. a front LSD with def help drag launch traction. For autox, my car has never been this fast. IMO ther is a reason why the STI is classed different than the wrx/lgt. Aside from the power, the STI can put its power down much better than the WRX/LGT because the STI has a front LSD. Now with one in my car, i def can get on the power much eariler in corners. my times have def improved because of it. As i said in the other thread, its the best mod ive done to my car.

Awesome. Thank you very much for your time to answer these questions.

Fantastic and educational thread, and great timing, OP! I just came from http://www.legacygt.com/forums/showthread.php?t=111507&highlight=r160+lsd and the wikipedia entry about LSD's when I noticed this thread.

 

So my 5EAT LGT's VLSD is useless. Time for a Torsen replacement!

I'm quoting myself here from a previous message. Anybody familiar with the signs of a center diff that is either bad or going bad?

 

My BMW 325ix has a center diff that occasionally behaves like you described - the engine lugs down a bit in tight corners. It has always had matched tires (always bought sets of four, never had a flat, etc, etc) so while that may cause such behavior it's not a requirement.

 

Note that differentials can fail in different ways, so there are multiple unrelated correct answers to your question. In your case and mine, I think the differentials (strictly speaking) are working just fine, but the viscous couplings are failing.

 

I wouldn't be at all surprised if VCs also had a failure mode that makes them irrelevant, leaving the differential effectively just an open diff.

I wouldn't be at all surprised if VCs also had a failure mode that makes them irrelevant, leaving the differential effectively just an open diff.

 

I think the Wikipedia entry described a similar consequence for an aging VC diff-- it becomes an open diff.

 

For those who don't drive hard enough or in weather adverse enough, how would they determine for certain whether a VC rear LSD is acting like an open diff? Would I have to already be familiar with how the car reacts when I swing the rear out in order to gauge whether the VC rear LSD isn't being effective anymore?

 

I'm assuming the condition of the VC rear LSD could be also revealed by a mechanic + equipment.

With the car on a lift you can tell if the VC has gone completely open if you just spin one rear wheel by hand. If the diff is totally open, the other wheel will spin the opposite direction. If the VC still has some stick to it, both wheels will rotate the same direction (and it will take more effort, as the whole drivetrain as to move for that to happen). However that doesn't tell you if it will bind up properly when you've got one wheel with traction and one without. I don't know how to verify that except to be on a slippery road and feel/guess which wheels are slipping and how much.

From above....

 

The stock viscous unit used in the WRX and 2000-2001 RS and also in the Legacy GT models is essentially useless (the unit is so thin as its packaged off the side of a standard open diff and its capacity is very limited).

My BMW 325ix has a center diff that occasionally behaves like you described - the engine lugs down a bit in tight corners. It has always had matched tires (always bought sets of four, never had a flat, etc, etc) so while that may cause such behavior it's not a requirement.

 

Note that differentials can fail in different ways, so there are multiple unrelated correct answers to your question. In your case and mine, I think the differentials (strictly speaking) are working just fine, but the viscous couplings are failing.

 

I wouldn't be at all surprised if VCs also had a failure mode that makes them irrelevant, leaving the differential effectively just an open diff.

 

Thanks for the helpful info. I recall seeing somewhere that the failure could result in just leaving them open.

The stock viscous unit used in the WRX and 2000-2001 RS and also in the Legacy GT models is essentially useless (the unit is so thin as its packaged off the side of a standard open diff and its capacity is very limited).

 

2000-2001 RS and also in the....

 

a) ...2000-2001 Legacy GT?

b) ...2005-2009 Legacy GT?

 

I've spun both rears on slippery surfaces so it works like it should. It's there to maintain traction while driving (not racing) and it works fine for what it was designed for. I do look forward to getting a better front diff, and maybe a center as well (for more rear bias) but my rear diff will be the last one to get upgraded, if ever.

2000-2004 lgt and 2005+ legacy non-turbo has an open rear diff.

2005-2009 LGThas the viscous rear LSD

2007-2009 spec b/3.0r has the torsen/helical rear diff

 

All three i list here have the viscous center in the 5/6spd manual boxes.

2000-2004 lgt and 2005+ legacy non-turbo has an open rear diff.

2005-2009 LGThas the viscous rear LSD

2007-2009 spec b/3.0r has the torsen/helical rear diff

 

All three i list here have the viscous center in the 5/6spd manual boxes.

 

 

My 2000 BE LGT came with a rear VLSD.

2

2007-2009 spec b/3.0r has the torsen/helical rear diff

 

All three i list here have the viscous center in the 5/6spd manual boxes.

 

Are you sure about the 3.0R? I need to check it but I am surprised there is an auto with a helical/Torsen... it's not R180 is it?

^i could be wrong about that. I thought the 3.0r got the spec b rear end.

My 2000 BE LGT came with a rear VLSD.

 

must be just for that year then cause i had a 02 BE and it had a open rear.

^i could be wrong about that. I thought the 3.0r got the spec b rear end.

 

I very much doubt it. I'll verify this later.

you are prolly right about this. please verify.