Unabomber's Manifesto - Differential FAQ

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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?


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.

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?

OBX Racing

Websites to visit for more information about differentials:

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.