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Old 10-31-2006, 10:15 AM   #1
Unabomber
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Exclamation Differential FAQ: Read if you are thinking of upgrading!

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

What types of differentials come stock?

Viscous: Used as the center and rear differentials in the WRX/RS (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. 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.

DCCD controllers:

OEM DCCD
DCCDPro
MoTeC
Neetronics
GEMS
Rocket Rally
MAPDCCD

Torsen type differential: Used as the front (2005+) and rear differentials in the STi. 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 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 front: Torsen or Clutch type

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

RS/WRX rear: Torsen or Clutch type

STi front: Torsen or Clutch type

STi 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 rear: Torsen or Clutch type

As well, both STi and WRX/RS 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).

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.
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Last edited by Unabomber; 12-27-2012 at 03:53 PM.
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Old 10-31-2006, 10:18 AM   #2
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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 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 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.

I guess if anyone has any questions they can email me (please don't PM me I don't check them very often). I also have a somewhat complete JDM transmission chart if someone needs info on factory diffs, gear ratios, or whatever. Me and Jeff actually cross checked some of our info at one point.

Last edited by Unabomber; 01-20-2009 at 11:20 AM.
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Old 10-31-2006, 10:51 AM   #3
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Hey Ron - when does that issue with your article hit the shelves?
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Old 10-31-2006, 11:10 AM   #4
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In theory, it should be my very first Feature Article in the NEXT issue of Subiesport. My last copy arrived at the house a few days ago and I think that is the Nov/Dec issue, so I think it will be the Jan/Feb 2007 issue.
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Old 10-31-2006, 12:56 PM   #5
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Any info on diffs for us 4eat guys?
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Old 10-31-2006, 01:17 PM   #6
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Great job Ron!

Couple of notes, for a center diff upgrade, you may want to add in the 5mt DCCD from STI/RA transmissions, and locked center diffs with rear bias like the Cusco Tarmac.

It's taken too long to get good info on diffs for our cars. AWD is the core of Subaru's success, and more people on this forum know more about turbos than they do differentials! Great job.
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Old 11-01-2006, 11:22 PM   #7
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Quote:
Originally Posted by Unabomber View Post
What is a differential? ... From there, the front and rear differentials distribute the power to their respective wheels.
A little fix for you.

Quote:
Originally Posted by Unabomber
Open differential: Used as the front and rear differential in the WRX/RS.
As discussed elsewhere (I think you were party to that thread, and now I don't remember where it was...), the '02+ WRX has a viscous rear LSD, and it was optional (I think?) on the '99-01 2.5RSs.

Quote:
Originally Posted by Unabomber
Torsen type differential: Used as the front (2005+) and rear differentials in the STi. 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 (Quaife, which also makes differentials for Subaru applications, is another torque biasing differential similar to the Torsen.) 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.
Actually, the torque bias won't affect anything once one wheel is off the ground. The diff must be pre-loaded in order for it not to become an open diff in that situation. Torsen sells the T-2R for some applications, which in addition to having a higher torque biasing ratio (TBR) than the T-2, also (in some applications) is pre-loaded. The purpose of the pre-load is to keep the diff from becoming open if one wheel becomes unweighted.

Quote:
Originally Posted by Unabomber
It has the added drawback of weight.
I'd be curious to see some weight numbers of, say, a Torsen/Quaife vs. a Cusco Type RS clutch-type rear diff. I'd be willing to bet the difference is just about nothing. Whatever difference there may be I don't think is responsible for any additional heat.

Quote:
Originally Posted by Unabomber
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.
Heat is friction? Friction creates heat, but the opposite doesn't happen. And again, I don't think whatever marginal weight difference there may be between mechanical and clutch type diffs is going to create additional heat in the diff.

However, a mechanical diff does generate more heat. The way it works involves a bunch of gear interactions (which you can read about if you go to the Torsen FAQ), and that's what generates the heat. Again, going back to my Mustang, guys who open track with Torsen diffs have a lot more issues with the rear diff puking fluid (due to the fluid overheating) than with the stock clutch type Traction-Lok LSD. The guys with the Cobra IRS have even more issues, as they don't have the big, heavy live axle housing to dissipate all that heat.

Quote:
Originally Posted by Unabomber
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.
You may have already seen this, but here's some good discussion of 1 vs 1.5 vs 2-way diffs from Dave @ Rallispec (via email to me). http://forums.nasioc.com/forums/show...2&postcount=67

Quote:
Originally Posted by Unabomber
RS/WRX center: JDM STi 20kg center viscous unit. This increases the break away torque from the stock 4kg unit to hold rear traction longer.
As already mentioned, the Cusco Tarmac Gear, although expensive, is an option here, and a damn good one. 35% front, 65% rear fixed torque split - it really changes the way the car feels pretty drastically.

Quote:
Originally Posted by Unabomber
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?
I guess tune-ability is nice, but I think it's a heckuva lot smarter to go with the most predictable, repeatable diff (that isn't going to wear out and should never need to be rebuilt/reshimmed/repacked) and tune the handling of the car with the much more readily altered components of the suspension (springs, struts, swaybars). In other words, go with a mechanical TBD at each end if you can. I just don't see the tune-ability aspect of the clutch type diff being a big selling point, as the hugely overwhelming majority of people here are not going to go through the trouble of removing a diff to change the % lock-up or the 1/1.5/2-way setting to change their car's handling.

That comes from a guy who has a Cusco RS clutch type in the rear right now. Hindsight being 20/20, I probably would go with the Quaife instead.

Last but not least, I think there are some good morsels of clutch type vs. mechanical diffs in my post here - http://forums.nasioc.com/forums/show...9&postcount=41 (in case you hadn't found that one in your searching).

Pat Olsen
'97 Legacy 2.5GT sedan

Last edited by Patrick Olsen; 11-01-2006 at 11:36 PM.
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Old 11-03-2006, 11:01 AM   #8
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Thanks for the feedback Pat. Most of it I can't use as I'm trying to be vague with this FAQ and not state which should be used, only highlight things to consider. Advice on differentials is terribly hard to come by and I've talked to several titans of the industry and they all say different stuff. I pointed out the 20kg center viscous unit, the aftermarket DCCD controllers, and the rear end diff shimming/R180 angle change as they are unknown to most, and not so much as things to do as to things to research since they aren't commonly known. Heck, the R180 angle change thingy was "discovered" only 2 weeks ago by Franz when he was helping me. Now he's all giddy to try it out.

I do highly agree with your opinon of diffs though that predictable is best, but I put in all the clutch type stuff as many here like the "bling" of running them. I also appreciate your comments about the Quaife unit. As with most of my FAQs, I try to remain brand neutral and you know more than the average bear it would seem as you found the parallel between my text and the Quaife units. Once again, I leave this up to the end user to research as while Quaifes are damn nice, they aren't for everyone.
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Old 11-03-2006, 12:46 PM   #9
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Old 11-03-2006, 04:58 PM   #10
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Quote:
Originally Posted by Patrick Olsen View Post
As discussed elsewhere (I think you were party to that thread, and now I don't remember where it was...), the '02+ WRX has a viscous rear LSD, and it was optional (I think?) on the '99-01 2.5RSs.
A VLSD was standard on the '00-'01 2.5RSs. I found this thread by stimpy about swapping differentials and a referenced thread also by stimpy about which models had LSDs. Apparently the '99 2.5RS had the VLSD as an option.
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Old 11-05-2006, 01:31 PM   #11
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what do you need to put the 5mt dccd on an 03wrx? ima doing my diffs after the new year so i was wondering. i was thinking of running the tarmac diff, and recently became aware of the 5mt thing.. yeah im pretty new to subaru still. but i always find good info on here. if you could pm me about this... anyone. it would be appreciatted


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Old 11-05-2006, 09:55 PM   #12
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I am not finding this FAQ to clear up a lot of my confusion regarding the various diffs that have been supplied stock in the USDM STi.

The 2007 apparently have a Torsen in the rear, as a new thing. I've noted a few posts talking about this like its a really significantly great improvement for some reason. But according to your FAQ, 2007 is the third model year for the USDM STi to have torsens in both front and rear - the 2004s had a suretrac unit in front.

As near as I can tell, it went like this:

2004 - Suretrac front, "mechanical" rear
2005 - "Helical" front, "mechanical" rear
2006 - "Helical" front, "mechanical" rear
2007 - "Helical" front, "Torsen" rear

Now, if really, for all intents and purposes, Torsen = helical = mechanical, I'd like to hear you explain why that is, but me and a lot of guys like me are still standing around out here wondering what the tiny little differences between these various units are, or where we can go to research it ourselves. There is a lot less general info about diffs out there than there is, say, suspension or forced induction concepts.

Thanks!
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Old 11-07-2006, 04:51 PM   #13
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A great FAQ writeup as always! And just in time for my transmission & differential project... You can never have too much information on this stuff!
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Old 11-08-2006, 05:24 PM   #14
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Quote:
Originally Posted by Unabomber View Post
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.
Nope. Differentials split torque, not power. (Not a big deal, but this is just the beginning.)

Quote:
Originally Posted by Unabomber View Post
Viscous: Used as the center and rear differentials in the WRX/RS.
Nope. Those are viscous limited-slip devices, not viscous power couplers (which are more like differentials). Viscous power couplers are used in cars like VWs (4-motion) and Merecedes (4-matic), but not the WRX or RS. If you can't keep straight which component is splitting and transferring torque and which is limiting slip, you are in serious trouble.

Quote:
Originally Posted by Unabomber View Post
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.
One of the most common misconceptions is that the DCCD controls torque split. It does not. It controls the locking of the differential. You would not be doing anyone any favors by spreading the idea that the DCCD controls the torque split, since it is widespread enough as it is.

Quote:
Originally Posted by Unabomber View Post
Torsen type differential: Used as the front (2005+) and rear differentials in the STi. 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.
All (real) differentials use gears. (The exception, mentioned above, is the viscous power coupling, but those aren't really differentials; I was just being nice.) More important: Torsens cannot change the native torque split any more than a DCCD can. What a Torsen really does is lock as a function of the difference in transmitted torque (or "reflected torque," is you prefer). Type-1 Torsens do this by using the one-way behavior of worm gears. Type-2s (aka helicals) do this by using the side-force generated by spiral-cut gears to wedge the pinions against the ends of the pockets in which they live.

Quote:
Originally Posted by Unabomber View Post
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?
You have got to be kidding. Please actually learn how the two diffs work and explain the difference, instead of using a misleading analogy.

Torsens respond to differences in transmitted torque. They react. They don't lock until they need to.

Clutch-packs lock as a function of input torque. They don't wait for a loss of grip; they lock in advance.

Quote:
Originally Posted by Unabomber View Post
One of my favorite sayings is “Research twice, modify once”.
[edit] I won't mention my favorite sayings, but will say that you haven't done anything close to enough research to be writing a primer on this issue.

- Jtoby
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Old 11-08-2006, 06:00 PM   #15
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^^^ An entire post on technicalities? Yeah buddy, you didn't really help much at all. A "primer" is really only an introduction. He didn't have to write a book to explain exactly how each different type of differential worked.
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Old 11-08-2006, 09:49 PM   #16
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Quote:
Originally Posted by jtmcinder View Post
Nope. Those are viscous limited-slip devices, not viscous power couplers (which are more like differentials). Viscous power couplers are used in cars like VWs (4-motion) and Merecedes (4-matic), but not the WRX or RS. If you can't keep straight which component is splitting and transferring torque and which is limiting slip, you are in serious trouble.
In the interest of providing useful information to this thread (rather than just derisively pointing out how stupid and wrong Unabomber is), could you explain what the difference is between a viscous limited slip device and a viscous power coupler? Or at least provide a link to something that explains what they are? Subaru (and other auto manufacturers) seems to think those viscous things are differentials, I'm curious what the difference is.

Quote:
Originally Posted by jtmcinder
One of the most common misconceptions is that the DCCD controls torque split. It does not. It controls the locking of the differential.
Does the amount of differential locking directly affect the torque split, or how are they related?

Quote:
Originally Posted by jtmcinder
Torsens respond to differences in transmitted torque. They react. They don't lock until they need to.

Clutch-packs lock as a function of input torque. They don't wait for a loss of grip; they lock in advance.
This is the opposite of what I've always understood. I thought that a clutch type differential, or a viscous diff, required some slippage to occur in order for it to start to work. Hence the term "limited slip" - it's not a "prevent slip" differential, because there has to be some slip in order for it to begin to work.

Pat
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Old 11-09-2006, 11:52 AM   #17
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I have already posted links to the detailed primers that I have written on both differentials and limited-slip devices. Stop whining and do your homework.

As to Torsens and clutch-packs: you are correct that in both cases, no resisting torque is produced until some slippage occurs, but that misses the deeper difference between the two. Type-2 Torsens don't even start to lock until there's an imbalance in the amounts of reflected torques from the two outputs. Until that moment, the helicals are happily sitting in the middles of their pockets, with no (serious) force against the end walls. In contrast, modified clutch-packs (aka Salisburys) add clamping force on the plates as a function of input torque. This means that they are often already locked before either output starts slipping. It also means that they are often clamped solid when you have no wheel slip, which is a serious negative when powering out of a corner.

This is a huge difference. It causes a helical to be almost invisible until the moment that it is needed. Yes, a helical can't handle situations where one output reflects no torque at all (since helicals are only capable of a certain ratio of output torques and anything times zero is still zero), but the fact that a helical stays out of the picture until it is needed makes it ten times better for the front of a car (and if you really are lifting a wheel, then your problem is suspension, not drivetrain).

In contrast, because a clutch-pack is already locked when it needs to be (and doesn't snap open when you lift a wheel), it is a much better option in the rear (or anywhere in the system if all you do is drag-race). It takes less than a tenth of a second to snap-spin an AWD car. You can't afford a rear diff that is only going lock when a problem arises; you'll already be looking back at where you just were. You want the rear locked in advance. A little under-steer due to it being locked when no wheel slip is happening is a small (and affordable) price to pay.

- Jtoby
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Old 11-09-2006, 05:37 PM   #18
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Quote:
Originally Posted by jtmcinder View Post
I have already posted links to the detailed primers that I have written on both differentials and limited-slip devices.
You have? Where did you post these links?

Quote:
Originally Posted by jtmcinder
Stop whining and do your homework.
Since when does our homework involve searching back through your old posts to find what you may or may not have said or posted links to in the past? Based on what you've said in this thread thus far I have no doubt you know what you're talking about, but that doesn't mean you should expect others to go searching out your previous posts.

Quote:
Originally Posted by jtmcinder
In contrast, modified clutch-packs (aka Salisburys) add clamping force on the plates as a function of input torque. This means that they are often already locked before either output starts slipping. It also means that they are often clamped solid when you have no wheel slip, which is a serious negative when powering out of a corner.

In contrast, because a clutch-pack is already locked when it needs to be (and doesn't snap open when you lift a wheel), it is a much better option in the rear (or anywhere in the system if all you do is drag-race). It takes less than a tenth of a second to snap-spin an AWD car. You can't afford a rear diff that is only going lock when a problem arises; you'll already be looking back at where you just were. You want the rear locked in advance. A little under-steer due to it being locked when no wheel slip is happening is a small (and affordable) price to pay.
So is a locked rear clutch pack a "serious negative", or is it a "small (and affordable" price to pay"?

I'm having a hard time visualizing what you mean when you mention that one can quickly snap spin an AWD car. Maybe it's just a matter of handling balance or my driving style, but I can count on one hand the number of times I've ever managed to spin my car in 6+ years of auto-xing, and 1/2 of those times were in the rain. Or are you referring to the snap spin as a result of lifting the inside wheel if one had a helical diff in the rear?

Pat
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Old 11-10-2006, 01:58 PM   #19
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http://dsmtuners.com/forums/showthread.php?t=177117
http://dsmtuners.com/forums/showthread.php?t=177206

When I said that clutch-packs are a serious negative, I was referring to the front, not the rear. (A locked front causes much more understeer than a locked rear.)

Many people snap-spin their AWDs (or FWDs) at corner entry when set up with lots of rear roll stiffness and too much trail braking. With the inside rear in the air, a quick stab at the throttle probably won't save you with an open rear diff (incl. a Torsen). If you have never snap-spun at entry, I would suggest you try some more trail-braking. (If you are that one-in-a-hundred that learned trail-braking without ever making any errors, please disregard.)

- Jtoby

Last edited by jtmcinder; 11-10-2006 at 02:03 PM.
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Old 11-10-2006, 04:27 PM   #20
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Thanks, looks like I've got some reading to do.

Quote:
Originally Posted by jtmcinder
When I said that clutch-packs are a serious negative, I was referring to the front, not the rear. (A locked front causes much more understeer than a locked rear.)
Ahhh, gotcha.

Quote:
Originally Posted by jtmcinder
Many people snap-spin their AWDs (or FWDs) at corner entry when set up with lots of rear roll stiffness and too much trail braking.
Roger that. I don't have gargantuan springs or swaybars (still my daily driver), and perhaps I'm not that aggressive on the trail braking.

Pat

Last edited by Patrick Olsen; 11-10-2006 at 04:32 PM.
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Old 11-10-2006, 05:34 PM   #21
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Very good reading and closest to "engineering level" explaination I've seen on forums. Please continue to school us... it's very educational and being appreciated.

So, from reading the links from the DSM forum, it sounds like the helical LSD will wear out faster than the clutch ones (generally speaking, as there are obviously many variables: clutch pressure, clutch material, helical teeth angle, etc)?

Are the clutch LSD's then the the only way to have an electroncally controlled LSD?

Is the VD chosen for regular cars (like the WRX) because it is the most economical/long lived LSD for an AWD system and not because it's a "best choice" for the general use application?

On a side note, why did Lotus feel that the Elise did not need an LSD and autox'ers who run the Elise think it does?
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Old 11-11-2006, 10:19 AM   #22
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Quote:
Originally Posted by chimchimm5 View Post
So, from reading the links from the DSM forum, it sounds like the helical LSD will wear out faster than the clutch ones (generally speaking, as there are obviously many variables: clutch pressure, clutch material, helical teeth angle, etc)?
Helicals don't wear out that quickly. At least, modified clutch-packs wear out faster. The grinding against the end of the pocket might wear down the pinions in a helical, but there's plenty more. But a scuffed-up clutch is toast.

Quote:
Originally Posted by chimchimm5 View Post
Are the clutch LSD's then the the only way to have an electroncally controlled LSD?
Yes. But keep in mind that electronic clutch-packs can use any of a variety of signals to increase the clamping force, while a Salisbury only uses input torque. Note, also, that Subaru and Mitsu (both using electronic center clutches) have developed very different approaches. Most off all, the Mitsu center is all about how quickly the center should return to open when no longer needed; the Subaru center is all about being locked in advance.

Quote:
Originally Posted by chimchimm5 View Post
Is the VD chosen for regular cars (like the WRX) because it is the most economical/long lived LSD for an AWD system and not because it's a "best choice" for the general use application?
As you know, most of the top-end AWDs (and high-power FWDs) have switched from open fronts to helical fronts. In the rear, you don't often find helicals; rather, you find VCs and clutch-packs. My guess is that this comes down to cost. Assuming you aren't some slimey Asian company that rips off the designs of others ... OBX, cough, cough ... then adding a helical adds more than a $1000 to the cost of the car, while a clutch-pack or VC is a lot less.

In the center, you see that Subaru and Mitsu have taken different routes in recent years. Subaru (which has been producing planetaries for years, as in the VTD automatics), went with a planetary for the STi, while Mitsu stayed with a 50/50 spider. At the same time, Subaru stayed with relatively standard rears, while Mitsu created the AYC rear. Well, I think that the jury is in on the initial decisions, what with Subaru backing off the torque split, mostly because the people who bought the 35/65 version didn't seem able to use the gas pedal in any way other than an on/off switch. Whether the lower-split version will work out is yet to be seen. People who known how to drive are often putting 35/65s into their new STis. Several people have also put 35/65s into the older AWD-turbos, such as the 35/65 in Charles Moss' 2G DSM.

Quote:
Originally Posted by chimchimm5 View Post
On a side note, why did Lotus feel that the Elise did not need an LSD and autox'ers who run the Elise think it does?
Autocrossing is weird. Rarely are cars asked to produce lots of lateral and longitudinal acceleration while moving slowly (and having tons of available wheel torque via gearing). If you aren't trying to turn sharply when on the gas, both rear wheels (esp. in a rear-engine car) will have enough grip to make an LSD unneeded. But asking the same car to power out of a tight turn might shift enough weight off the inside rear to make some sort of limited-slip device necessary. Also, keep in mind the rather low power of the Elise. If you modify the engine for more low torque, you'll need a limited-slip device of some sort. Until then, however, it's better to leave the diff open, as any and all forms of limited-slip cause some understeer and the key to the Elise is its nimbleness. Leave the wallowing power approach to the 'Vettes.

- Jtoby

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Old 11-26-2006, 03:53 PM   #23
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Originally Posted by bWRX View Post
I am not finding this FAQ to clear up a lot of my confusion regarding the various diffs that have been supplied stock in the USDM STi.

The 2007 apparently have a Torsen in the rear, as a new thing. I've noted a few posts talking about this like its a really significantly great improvement for some reason. But according to your FAQ, 2007 is the third model year for the USDM STi to have torsens in both front and rear - the 2004s had a suretrac unit in front.

As near as I can tell, it went like this:

2004 - Suretrac front, "mechanical" rear
2005 - "Helical" front, "mechanical" rear
2006 - "Helical" front, "mechanical" rear
2007 - "Helical" front, "Torsen" rear

Now, if really, for all intents and purposes, Torsen = helical = mechanical, I'd like to hear you explain why that is, but me and a lot of guys like me are still standing around out here wondering what the tiny little differences between these various units are, or where we can go to research it ourselves. There is a lot less general info about diffs out there than there is, say, suspension or forced induction concepts.

Thanks!
I was sitting there thinking "why did Subaru say that the rear diff was changed to a Torsen diff for 2007 if there is no actual difference? Thanks for confirming that.
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Old 11-26-2006, 09:25 PM   #24
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Torsen, mechanical, and helical are all terms that mean the same thing. Yes, there are subtle differences between manufacturers, but it's easier to say they are all the same.

My apologies to those of you that know more about differentials than I. This FAQ, like all of mine, are not the end all/be all of information, merely a jumping off point. One day I'll go through this and update it. While the information posted is not, nor is it intended to be 1000% accurate, I hope it stands as a good base. I could argue until I'm blue in the face about many of the points brought up here, but this is a BASIC post, not advanced. Want advanced? Call some Pat at Rocket Rally, or GEMS, or Franz Diebold, or people like that.

Don't like my FAQ? I challenge you to write one better.
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Old 11-26-2006, 10:15 PM   #25
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Originally Posted by JiveMasterT View Post
I was sitting there thinking "why did Subaru say that the rear diff was changed to a Torsen diff for 2007 if there is no actual difference? Thanks for confirming that.
They changed the rear diff to a DIFFERENT helical/mechanical, and the new one happens to be made by Torsen, which is a big name in the field and as far as I know (which is NOT very far) was one of the first companies to develop mechanical LSDs for performance applications. I know Lancia and Audi had them on their hot AWD cars of yesteryear, and they were what the Group B cars ran.

I wish there were a book with very many, extremely large, colorful pictures, that explained all about diffs for performance applications.
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