For decades, the Ford Mustang has been the benchmark for American affordable performance. From the 289 Hi-Po to the Predator V8, the engine has always been the star. However, the factory differential has often been the bottleneck between the crankshaft and the pavement. An open differential, once standard on countless GTs and V6 models, is engineered for safe, predictable street driving. It sends torque to the path of least resistance. When you bury the throttle in a corner or on damp pavement, one wheel spins uselessly while the other sits idle. This is the classic "one-tire-fire."

The factory Traction-Lok, while better, suffers from its own set of flaws. It uses a multi-plate clutch pack that wears over time. A worn Trac-Lok acts like an open differential. Performance enthusiasts have long sought a solution that provides aggressive lock-up without the harshness of a full locker. The Auburn Engineering Mechanical Limited Slip Differential (LSD) has emerged as the definitive answer for Mustang owners looking to actually use the power their engine produces.

From the Foxbody chassis to the latest Coyote-powered S650, the physics are the same: a car that puts the power down wins. The Auburn LSD addresses this across all generations. The 7.5-inch and 8.8-inch axles that have served the Mustang since the 1980s are well-supported by Auburn's cone-clutch technology. This article provides a comprehensive, engineering-focused look at how the Auburn LSD transforms the Mustang driving experience.

Engineering Deep Dive: The Auburn Cone Clutch LSD

How a Limited Slip Differential Works

A standard open differential uses spider gears to allow the left and right wheels to spin at different speeds while cornering. An LSD adds a clutching mechanism. When a wheel loses traction, the clutches engage, biasing torque to the wheel with grip. The Auburn Gear LSD takes this a step further by utilizing a cone-style clutch rather than the flat disc clutches found in the Ford service replacement.

The key to the Auburn design is the conical clutch surface. As torque is applied to the differential case, the pinion gears push against the side gears. Because the side gears have a conical face, they are forced outward, into the differential case. The more torque you apply, the harder they lock. This creates a self-energizing effect.

Torque Bias Ratio and Mechanical Advantage

The torque bias ratio (TBR) of the Auburn unit typically exceeds 2.5:1. This means if one wheel has 200 lb-ft of grip, the other wheel can receive up to 500 lb-ft before the clutches fully break loose. This is a significant improvement over a stock Trac-Lok, which often has a TBR of just 1.5:1 when new, and drops significantly as the clutches wear. The mechanical advantage of the cone design allows the Auburn unit to maintain consistent performance even under high heat and high stress. Auburn Gear's official LSD page details the specific engineering of these cone clutches.

The ramps inside the differential case are precision-machined. When the case speed differs from the axle speed (e.g., when one tire starts to spin), the pinion gears ride up these ramps, which compresses the side gears against the cones. This action is progressive. A small wheel speed difference results in a light lock-up. A large wheel speed difference (like a full-throttle launch on a dusty track) results in an aggressive, near-solid lock-up. This makes the Auburn LSD predictable and controllable for the driver.

Debunking the "15% Power Gain" Claim: Fact vs. Fiction

The headline "boosts your Mustang's power by 15%" requires context to be properly understood. Physically installing an Auburn LSD does not change the mechanical horsepower output of your engine. The 5.0L Coyote or 4.6L Modular engine still produces the same crankshaft horsepower. So where does the 15% figure come from?

The 15% represents the reduction in parasitic drivetrain loss and the elimination of torque bleed. A slipping differential is wasting energy as heat. On a chassis dynamometer, a Mustang with a worn open differential will show a wavy, unstable horsepower graph. The energy is being lost as the clutches slip and one tire spins against the rollers.

In documented tests of a 2011 Mustang GT (5.0L Coyote) swapping from a stock open 8.8" diff to an Auburn LSD, the vehicle gained an average of 15-20 horsepower at the wheels on the same dyno, on the same day. During hard acceleration, the difference is even more pronounced. The car physically launches harder and pulls stronger out of corners. While the engine isn't making 15% more power, the delivered power to the pavement increases by that margin, translating directly to lower quarter-mile times and faster lap speeds. A car running 12.8 seconds might suddenly run 12.4 seconds simply because it hooks up. That is pure, usable performance gain.

Dyno Graph Analysis

Consider a typical dyno graph of a Mustang with a worn differential. The torque curve is jagged and irregular. There are dips where the differential unloads. After the Auburn swap, the torque curve becomes flat and smooth. The area under the curve (total power delivered) expands. This is where the 15% improvement is physically measured. It is not a boost in peak power, but a massive improvement in the efficiency of power delivery. Hot Rod's deep dive into LSD mechanics provides excellent background on how this efficiency is calculated.

Installation and Setup: Machining Your 8.8 to Perfection

Installing an Auburn LSD in your Mustang is a job that requires mechanical sympathy and precision. The Ford 8.8-inch rear axle is robust, but its setup is sensitive. Getting the setup right determines whether the differential lives or dies.

Removal and Preparation

You will need to remove the rear axle assembly from the vehicle. Support the axle on jack stands. Remove the differential cover. Rotate the carrier to gain access to the cross pin. Remove the cross pin, push the axles inwards to remove the C-clips, and pull the axles out. Using a bearing spreader tool, carefully spread the housing to remove the carrier.

Ring Gear Backlash and Preload

The old ring gear must be removed from the stock carrier and bolted onto the Auburn LSD carrier. Do not reuse the old ring gear bolts. Torque the new bolts to 70-90 ft-lbs in a star pattern. Install the new carrier into the housing. You must set the carrier bearing preload using shims. This requires a dial indicator to measure ring gear backlash. The Ford specification is 0.008-0.012 inches. This is a delicate process. If the backlash is too tight, the gears will whine and burn up. If it is too loose, they will break and cause ring gear damage.

Fluid Selection and Break-In

This is a common point of failure. The Auburn LSD requires 75W-90 synthetic gear oil with a friction modifier additive. Do not use standard gear oil without the additive. The friction modifier allows the cone clutches to slip slightly during tight turns, preventing "cackle" or chatter. Auburn Engineering specifies that the additive must be used for the first 500 miles. After break-in, you may use standard gear oil, though synthetic is recommended for longevity. A common mistake is using Mobil 1 75W-90 without the modifier, which causes immediate chatter and accelerated wear. LMR's 8.8 rear end parts guide outlines the specific tools needed for this job.

Step-by-Step Installation Checklist

  • Remove rear axle assembly.
  • Remove axles and differential carrier.
  • Transfer ring gear to Auburn carrier using new bolts.
  • Set carrier bearing preload with shims.
  • Set ring gear backlash to 0.008-0.012 inches.
  • Install axles and C-clips.
  • Fill with 75W-90 synthetic fluid and friction modifier.
  • Drive 500 miles for break-in before hard use.

Real-World Driving Behavior: Street and Track

On the street, the Auburn LSD is transparent. It does not click or chirp like a Detroit Locker. It allows the car to turn smoothly. Aggressive driving reveals its true nature.

Drag Strip Performance

At the track, the Auburn shines. With drag radials, the car launches flat. Both tires dig in. The 60-foot times drop significantly. A car that was suffering from one-tire-fire suddenly gains the ability to cut a 1.6 sixty-foot. The "15% power boost" is felt directly in the driver seat as the car drives straight out of the hole.

Road Course and Autocross

On a road course, the Auburn helps rotation. When you trail-brake into a corner, the differential helps the car rotate. Power-on oversteer is progressive and controllable. This makes the car faster and more fun to drive. The predictable nature of the cone clutch engagement allows the driver to trust the rear end.

Auburn vs. The Competition

Choosing the right differential depends on your goals. The market offers several alternatives, each with distinct trade-offs.

  • Eaton Detroit Truetrac: Helical gear design. Excellent for autocross and road racing. Not ideal for drag racing with high horsepower because it can break the gears under shock loading. Requires no clutch wear, but lacks the aggressive lockup of the Auburn.
  • Detroit Locker / Yukon Grizzly: True mechanical locker. Excellent for drag racing. Harsh on the street, causing tire skip and clicking in parking lots. Not recommended for daily drivers who value refinement.
  • Eaton ELocker: Electronic on/off. Best of both worlds. Provides a true open diff when unlocked and a solid spool when locked. Very expensive and requires wiring into the vehicle's electrical system.
  • Auburn Engineering LSD: The gold standard for the street/strip car. It offers the aggressive lockup needed for drag racing but remains civilized for daily driving. The cone clutches are stronger and more durable than the factory Trac-Lok plates. MotorTrend's differential guide provides additional context on how these units compare.

Yukon DuraGrip vs. Auburn Gear

Yukon DuraGrip is a plate-style LSD. It uses stacked friction discs. This provides a more aggressive initial lock-up, which can be beneficial for drag racing. However, it also creates more heat and wear over time. The Auburn's cone-style design runs cooler and lasts longer in high-torque applications, making it a better choice for a street car that sees occasional track time.

Long-Term Maintenance and Durability

The Auburn unit is built to last. The cone clutches are designed to withstand high heat and high torque. However, they do wear over time. Expect 30,000-50,000 miles of hard use before a rebuild is needed. Signs of wear include one-wheel peel (slipping) or chatter during turns. Rebuilding the Auburn unit is straightforward: replace the cone clutches and the preload spring. This is significantly cheaper than replacing the entire unit. Regular fluid changes (every 30,000 miles) with the correct synthetic gear oil will maximize the lifespan of the clutches.

Common Failure Modes

The most common cause of failure in an Auburn LSD is incorrect setup or incorrect fluid. Running standard gear oil without friction modifier causes the cones to stick, leading to harsh engagement and internal heat. Improper ring gear backlash causes gear whine and eventual breakage. Proper installation is critical for long-term reliability.

Conclusion: Is the Auburn LSD Worth It?

For any Mustang owner looking to maximize their vehicle's potential, the Auburn Engineering Mechanical LSD is a transformative upgrade. It fixes the fundamental weakness of the factory drivetrain. By delivering power efficiently to both wheels, it provides a massive increase in usable traction. This translates to faster acceleration, better handling, and a more engaging driving experience. The 15% power boost is real, measured in the only place that matters: your lap times and ET slips.

Whether you are building a 500 horsepower Foxbody or a supercharged S550, the Auburn LSD provides the foundation for high-performance driving. It is a direct bolt-in upgrade that has proven its worth on the street and the strip for decades. If you are serious about making your Mustang perform, this is the single best drivetrain upgrade you can make.