Understanding Limited Slip Differentials

A limited slip differential (LSD) is a critical component in any performance-oriented vehicle, especially all-wheel-drive cars like the 2008 Subaru WRX. Unlike an open differential, which sends power to the wheel with the least resistance (often resulting in one-wheel spin), an LSD actively distributes torque between the two wheels on an axle. This allows the wheel with better grip to receive more power, improving acceleration out of corners, traction on loose surfaces, and overall stability under hard driving.

There are several types of LSDs: clutch-type (like the Cusco RS), gear-type (like the Eaton TrueTrac), and viscous (stock on many Subarus). The 2008 WRX came from the factory with a viscous rear LSD and an open front differential. The viscous unit provides some limited slip action through fluid shear, but it wears over time and reacts slowly to torque changes. Upgrading to a mechanical LSD like the Eaton TrueTrac offers more immediate and predictable lock-up, especially under heavy throttle.

Why Eaton for the Subaru WRX

Eaton’s TrueTrac differential is a helical-gear, torque-biasing design. It does not use clutch plates or friction modifiers, meaning it requires less maintenance and delivers consistent performance throughout its life. The gear action provides a progressive lock: it allows the inside wheel to rotate slower than the outside wheel in a turn (differentiation) but can transfer up to 3.5 times more torque to the wheel with more grip when needed. This is ideal for the 2008 WRX’s rear axle, giving the driver better corner exit speed and reducing understeer.

Another key advantage is compatibility with the stock Subaru R160 rear differential housing. The Eaton part number 916A564 (for the R160) is a direct fit for the 2008 WRX, requiring only the removal of the original ring gear and its installation on the new carrier. This makes it a relatively bolt-in upgrade, though careful setup of bearing preload and backlash is essential.

Benefits of Installing an Eaton LSD

Upgrading to an Eaton TrueTrac provides tangible improvements in both everyday driving and track conditions.

  • Improved Traction: The LSD actively transfers torque to the gripping wheel, reducing wheel spin in wet conditions, gravel, or during aggressive acceleration.
  • Better Handling: You’ll feel more rotation through corners; the car can be steered with the throttle, allowing earlier power application. Understeer is significantly reduced compared to an open differential.
  • Increased Stability: In slippery conditions (snow, rain), the LSD maintains a more predictable line, reducing the risk of a sudden loss of traction. The progressive lock-up also makes the car easier to control at the limit.
  • Durability: The gear-type design is virtually maintenance-free beyond fluid changes. There are no clutch plates to burn up or need periodic replacement, making it a long-term investment.

Cost Breakdown

Total cost for installing an Eaton LSD in a 2008 Subaru WRX ranges from $1,500 to $2,500 depending on labor rates and whether additional supporting parts are required. Below is a detailed breakdown:

  • Eaton TrueTrac LSD (R160): $800 – $1,100 (typical retail from vendors like 4 Wheel Parts or THMotorsports)
  • Ring & Pinion Installation Kit: $100 – $200 (bearing set, shims, crush sleeve, seal)
  • Differential Fluid (Motul Gear 300 75W90 or Subaru Extra-S): $40 – $60 (two quarts)
  • Gasket (OEM Subaru 41310AG010) or RTV: $10 – $30
  • Installation Labor (professional): $400 – $800 (including proper shimming and setup)
  • Optional: Rear Axle Shaft seals, stub shaft seals: $30 – $50

Note: If you plan to do the installation yourself, factor in the cost of a dial indicator, magnetic base, and a beam-type or inch-pound torque wrench for bearing preload adjustment. A shop press may also be needed to install the ring gear and bearings.

Installation Process

Installing an Eaton TrueTrac into the R160 differential of a 2008 Subaru WRX is a job best tackled by someone comfortable with differential setup. Below is an expanded step-by-step guide:

1. Preparation and Tools

  • Jack stands or a hydraulic lift (4 jack stands recommended for stability)
  • Drain pan, wrenches (12mm, 14mm, 17mm, 19mm), sockets
  • Torque wrench (ft-lb for axle nuts; in-lb for bearing preload)
  • Dial indicator with magnetic base (for backlash measurement)
  • Shop press or bearing splitter/puller
  • Marking compound (for gear contact pattern)
  • PDF copy of Subaru service manual (FSM) section DI-6 for R160 specifications

2. Remove the Rear Differential

  • Drain the differential fluid (remove fill plug first, then drain plug; torque to 50 ft-lb when reinstalling).
  • Remove rear wheels and unbolt the half shafts from the stub axles (two 12-point bolts per side, 42 ft-lb).
  • Disconnect the driveshaft from the differential (4 bolts, 14mm).
  • Support the differential with a floor jack, then unbolt the four mounting bolts (17mm, 54 ft-lb) and lower the unit.

3. Disassemble the Differential

  • Remove the rear cover (12mm bolts). Note the cover orientation for reinstallation.
  • Remove the snap ring on the pinion shaft and drive out the pinion shaft with a drift punch.
  • Remove the spider gears, side gears, and the original open differential carrier.
  • Remove the ring gear bolts (in a cross pattern, 60 ft-lb). Use a press or an impact driver carefully (avoid bending the ring gear).
  • Clean the carrier housing thoroughly.

4. Install Ring Gear on Eaton TrueTrac Carrier

  • Heat the ring gear evenly with a propane torch (to about 250°F) or place it in an oven at 250°F for 15 minutes. Do not exceed 350°F.
  • Position the ring gear on the Eaton carrier and install the ring gear bolts with Loctite 242 (blue). Torque to 60 ft-lb in a cross pattern in two stages (35 ft-lb then 60 ft-lb).
  • Allow the ring gear to cool before proceeding.

5. Set Up Bearings and Shims

  • Install the new bearings onto the Eaton carrier (inner races). The R160 uses two different bearing sizes: driver side (larger) and passenger side (smaller). Verify orientation.
  • Place the carrier into the housing with the original shims on each side. You may need to adjust shim thickness to achieve proper bearing preload (typically 5-15 in-lb of rolling torque with new bearings). This is measured with an in-lb torque wrench on the ring gear bolts while rotating the carrier.
  • Install the side bearing caps and torque them to 40 ft-lb. Use marking compound on the ring gear teeth to check contact pattern.
  • Adjust backlash by moving shims from one side to the other. Factory spec for R160: 0.003-0.005 inch (0.08-0.13 mm). Use dial indicator on a ring gear tooth.

6. Reassemble and Install the Differential

  • Once backlash and pattern are correct, lubricate all components with gear oil.
  • Install the stub axles and the half shafts (new o-rings recommended). Torque the half shaft bolts to 42 ft-lb.
  • Install the rear cover with new gasket or RTV (Subaru recommends ThreeBond 1217G). Torque bolts to 26 ft-lb in a star pattern.
  • Refill with 1.1 quarts (approx) of gear oil. Use a pump to fill through the axle hole or fill plug. Torque fill plug to 49 ft-lb.
  • Reinstall the driveshaft (4 bolts to 37 ft-lb), and lower the vehicle.

7. Test Drive

  • Take a slow test drive in a straight line to ensure no binding. Then perform figure‑eights in a parking lot (on dry pavement) to check for chatter or abnormal noise. The Eaton TrueTrac should be silent; any clicking or whining indicates incorrect setup.
  • After 500 miles, check the fluid level and retorque the ring gear bolts (a quick check of the bolts through the fill hole is not possible on the R160; the cover must be removed). Many builders choose to drain and refill after 500 miles to remove any fine wear particles.

Expected Results

After the upgrade, the difference is immediately noticeable. The car will put power down earlier in corners and feel more planted during acceleration. Here’s what you can expect:

  • Launch Performance: You can be more aggressive off the line without overwhelming the inside rear tire. The LSD helps both rear wheels hook up, reducing the tendency to spin one tire and bog the launch.
  • Cornering: Throttle application mid-corner is transformed. Instead of understeering wide, the car rotates slightly and you can steer with the throttle. On a tight autocross track, expect up to 0.5–1 second improvement per minute of driving, according to many forum reports.
  • Daily Driving: On wet roads or light snow, the LSD provides confidence. There is no clunking or harsh engagement because the helical gears are continuous. In parking lot maneuvers, the car behaves nearly like stock (no noticeable hesitation or brake-steer effect).
  • Durability: The Eaton TrueTrac is known to survive in high-horsepower builds (400+ whp) without failure, as long as the ring-and-pinion and bearings are properly set up. It’s a stronger design than the stock viscous unit and doesn’t lose its limited-slip ability with age.

One important note: the Eaton TrueTrac is a torque-biasing LSD, not a locking differential. If one wheel completely loses traction (jack stand scenario), the LSD may still transfer torque, but not as aggressively as a clutch-type. However, for 99% of real driving, this is not an issue.

Comparison to Other LSD Options

For the 2008 Subaru WRX, other popular LSDs include the Cusco RS (clutch-type), OS Giken Super Lock (multi-cone clutch), and the OEM viscous. Here’s how the Eaton stacks up:

  • Eaton TrueTrac: Minimal maintenance, smooth operation, good for daily and track. Not suitable for drifting (needs lock). Great for road racing, autocross, and spirited street driving.
  • Cusco RS: Adjustable preload (30-60%), more aggressive lock, can be noisy (clatter at low speeds). Requires periodic rebuild. Better for hardcore track use or drifting.
  • OS Giken: Very aggressive launch capability, strong design, but expensive and requires regular maintenance.
  • Stock viscous: Free (if you already have it), but weak and slow. Not recommended for performance driving.

For most WRX owners seeking a balanced upgrade without the noise and maintenance burden of a clutch-type, the Eaton TrueTrac is the best value. It’s a common recommendation on iWSTI and NASIOC forums.

Conclusion

Installing an Eaton Limited Slip Differential in a 2008 Subaru WRX is a proven modification that lifts the car’s dynamic capabilities without sacrificing daily drivability. With costs ranging from $1,500 to $2,500, it represents a significant but worthwhile investment for anyone serious about handling and traction. The installation requires patience and precision, but the end result is a car that corners harder, accelerates more confidently, and remains stable in all conditions. Whether you’re chasing cones on the weekend or simply want a more engaging daily driver, an Eaton TrueTrac is a top-tier choice for the WRX.