What Makes the ACR Limited Slip Differential Stand Out?

The ACR Limited Slip Differential (LSD) is engineered specifically for the S550 Mustang platform (2015–2020), offering a significant upgrade over the factory open differential or even the stock limited-slip unit found on performance pack models. While the stock differential provides some slip limitation, it uses clutch packs that wear over time and can deliver inconsistent lockup, especially under hard acceleration or sustained track use. The ACR LSD employs a more robust mechanical design that delivers predictable, consistent power transfer to both rear wheels, reducing wheel spin and putting more power to the ground. This makes it a favorite among drag racers, road course enthusiasts, and street drivers who want maximum traction without the harshness of a spool or full locker.

How It Works

At its core, an ACR LSD uses a combination of ramped clutches and preload springs to manage torque distribution. When one wheel begins to lose grip, the differential transfers torque to the wheel with more traction. Unlike open differentials that send power to the path of least resistance, the ACR LSD ensures both wheels contribute to forward motion. The result is sharper corner exits, more stable braking, and reduced one-wheel-peel launches. The ACR design is tuned for the Mustang's weight distribution and power delivery, offering a balance between aggressive lockup for performance and acceptable behavior for daily driving.

Key Design Features

  • High-Strength Steel Case: The ACR LSD uses a heat-treated steel housing that withstands the higher torque loads from modified engines and sticky tires.
  • Carbon Fiber Clutch Plates: These plates provide smoother engagement and longer life compared to standard paper-based clutches, reducing the need for frequent rebuilds.
  • Adjustable Preload: Some ACR models allow you to adjust the breakaway torque by swapping springs or shims, letting you tailor the behavior from street-friendly to track-focused.
  • Direct Fitment: The differential is designed as a direct replacement for the factory unit, requiring no custom brackets or axle modifications on S550 Mustangs.

Performance Gains for the S550 Mustang

Installing an ACR LSD transforms how your Mustang puts power down. The improvements are noticeable across different driving disciplines, from straight-line acceleration to high-speed cornering. Below is a breakdown of what you can expect in various scenarios.

Drag Racing and Launch Performance

On the drag strip, traction is everything. Even with sticky tires, an open differential will often spin the inside wheel during a hard launch, wasting precious time. The ACR LSD locks both rear wheels together under acceleration, providing a two-tire grip that significantly reduces 60-foot times. Many S550 owners report dropping 0.15 to 0.25 seconds off their quarter-mile time simply by upgrading from the stock unit. Combined with a good set of drag radials and a tuned engine, the ACR LSD helps achieve consistent launches without the unpredictable wheel hop that plagues factory differentials.

Road Course and Autocross Handling

When cornering at speed, the ability to apply power earlier in the corner exit is a huge advantage. The ACR LSD allows you to get back on the throttle sooner without the inside wheel spinning and causing understeer. This improved corner exit speed translates to faster lap times and more confident driving. The diff's smooth engagement prevents the snapping or sliding that can occur with aggressive clutch-type differentials, giving you a more predictable rear end to work with as you approach the limit of grip.

Daily Driving Considerations

One concern many Mustang owners have is whether a performance LSD will make the car unpleasant to drive around town. The ACR LSD is designed with a relatively mild preload in its standard configuration, so you won't hear constant clicking or feel excessive binding during low-speed parking lot maneuvers. The carbon fiber clutch plates also reduce chatter, a common complaint with aftermarket differentials. However, you will notice the diff working when you accelerate hard from a stop or corner aggressively—the car feels more planted and less likely to break traction unexpectedly.

Before You Buy: Compatibility and Options

Not all S550 Mustangs are built the same. The 2015–2020 models came with different axle configurations, gear ratios, and electronic stability systems that can affect LSD compatibility. Before purchasing an ACR LSD, verify your car's specific details to avoid installation issues or parts that don't fit.

Axle Configuration Differences

Mustangs with the standard 8.8-inch rear axle (non-PP and GT Premium) share the same carrier size, making the ACR LSD a direct swap. However, cars equipped with the Performance Pack or higher-output models (like the Shelby GT350) may have different axle tubes or brake proportions. The ACR LSD is designed to fit the standard S550 8.8-inch housing, but you should always check the manufacturer's fitment list to confirm your specific model year and build date. If your car has a Torsen-style differential from the factory, the swap is still possible, but you may need adapter rings or shims to ensure proper gear mesh.

Gear Ratio Considerations

The ACR LSD can be installed with any factory gear ratio (from 3.15 to 3.73, or even the 4.09 found on some GT350 models). If you plan to change your ring and pinion at the same time, it's more efficient to install the LSD and new gears together since the differential must be removed anyway. For drag racing, many owners prefer a 3.73 or 4.09 gear set to maximize acceleration. For road course use, the factory 3.31 or 3.55 ratio provides a good balance of top speed and torque. The ACR LSD handles all of these ratios without issue, as long as the gear set is properly set up by a qualified technician.

Comprehensive Installation Guide

Installing an ACR LSD is a moderately difficult mechanical job that requires attention to detail and proper tools. If you have experience with rear axle work and a well-equipped garage, you can tackle this project in a weekend. If not, hiring a professional mechanic is strongly recommended to avoid damaging the differential or compromising your safety. Below is a detailed installation guide with tips to get it right the first time.

Tools and Parts Checklist

Before you start, gather everything you'll need. Running to the parts store mid-installation wastes time and can lead to mistakes. Here is a complete list:

  • ACR Limited Slip Differential (specific to your Mustang's axle code)
  • Full set of metric and SAE sockets (including deep sockets) and ratchets
  • Torque wrench capable of 35 to 150 ft-lbs
  • Jack and two jack stands (minimum)
  • Drain pan for gear oil
  • Gasket scraper or razor blade
  • New differential cover gasket (or RTV sealant)
  • Gear oil: 2.5 to 3 quarts of 75W-90 or 75W-140 (check ACR specifications)
  • Friction modifier additive (if required by the LSD type)
  • Breaker bar for stubborn bolts
  • Safety glasses and mechanic's gloves
  • Marking paint or a scribe for alignment marks

Step-by-Step Installation

  1. Prepare the Vehicle: Park on a level surface, chock the front wheels, and safely lift the rear of the Mustang with a jack. Place jack stands under the rear axle tubes for stability. Never work under a car supported only by a jack.
  2. Remove the Differential Cover: Using a socket, remove the bolts securing the rear differential cover. Carefully pry the cover loose (it may be stuck by old RTV). Let the old gear oil drain completely into a pan. Dispose of old oil at a recycling center.
  3. Mark the Driveshaft and Pinion Flange: Use a scribe or paint to mark the relationship between the driveshaft and the pinion flange. This ensures the driveshaft goes back in the same orientation, preventing vibration. Then, remove the driveshaft bolts and slide the driveshaft forward slightly to disengage it.
  4. Remove the Axle Shafts: Unbolt the four retaining bolts on each axle flange at the differential housing. Slide the axle shafts outward carefully—support them to avoid loading the CV joints or wheel bearings. Tie them up with zip ties or hangars to keep them out of the way.
  5. Remove the Carrier Bearings and Caps: The stock differential carrier is held in by two bearing caps on either side. Remove the bolts and caps (keep them organized—they are usually marked with arrows or numbers). Use a small pry bar or bearing puller to remove the carrier bearings from the stock case. You may need a tool like a two-jaw puller.
  6. Remove the Old Differential: Carefully lift the stock differential carrier out of the housing. Be mindful of the shims that may be present between the bearings and the housing—these set the gear mesh and must be reinstalled with the new carrier.
  7. Transfer the Ring Gear: Unbolt the ring gear from the old differential carrier. Clean the ring gear and the mounting flange of the new ACR LSD. Apply thread locker to the ring gear bolts and torque them to the manufacturer's specification (typically 65-85 ft-lbs).
  8. Install the ACR LSD: Place the new differential carrier into the housing, ensuring the ring gear aligns with the pinion gear. Reinstall the bearing caps and torque them in a crossing pattern to the correct specification (check your service manual—usually 55-70 ft-lbs).
  9. Check and Adjust Backlash: This is the most critical step for longevity. Use a dial indicator to measure the backlash between the ring and pinion gears. Factory specification for S550 Mustangs is typically 0.006 to 0.010 inches. If the measurement is out of spec, you'll need to move shims from one side to the other, which requires pulling the carrier out again. If you're unsure, have a shop perform this setup.
  10. Reinstall Axle Shafts and Driveshaft: Slide the axle shafts back into the differential and bolt them to the flanges. Reattach the driveshaft, aligning your paint marks, and torque the bolts.
  11. Clean and Reinstall the Differential Cover: Scrape off old gasket material or RTV from both the cover and the housing. Apply a thin, continuous bead of RTV (or use a new gasket) and bolt the cover on in a crisscross pattern. Torque to specification (usually 25-30 ft-lbs).
  12. Fill with Gear Oil: Remove the fill plug (usually on the side of the housing) and add gear oil until it starts to dribble out of the hole. Install the fill plug. If your ACR LSD requires friction modifier, add it before the final amount of oil.
  13. Test Drive: Lower the car, start the engine, and check for leaks. Drive gently for a few miles, then perform several figure-eight turns to distribute the oil and allow the LSD clutches to seat. Listen for unusual noises—whining or clunking may indicate incorrect backlash or a poorly seated bearing.

Common Pitfalls to Avoid

  • Skipping the Backlash Check: Installing the carrier without checking ring and pinion clearance can lead to rapid gear wear or noise within a few hundred miles.
  • Using the Wrong Gear Oil: Some LSDs require synthetic oil with specific friction modifiers. Using standard oil can cause chatter or reduce the life of the clutch plates.
  • Over-tightening Bearing Caps: The bearing caps are made of cast iron and can crack if over-torqued. Always use a torque wrench.
  • Reusing Old Gaskets: The differential cover gasket or RTV must be fresh. Old gaskets can cause slow leaks that may not be noticed until the differential runs low on oil.

Detailed Cost Breakdown

The total investment for an ACR LSD upgrade varies depending on whether you do the work yourself or hire a professional, as well as the specific parts you choose. Below is an itemized breakdown of all potential costs.

DIY vs. Professional Installation

Doing the installation yourself saves $500 to $800 in labor, but it also requires mechanical confidence and specialized tools you may not own. If you need to buy a torque wrench, dial indicator, and bearing puller, that adds another $150 to $250 to your tool collection. However, those tools can be used for future projects, so many enthusiasts consider it a worthwhile investment. Professional installation ensures the job is done right the first time, especially the critical backlash setup, which can be tricky for a beginner. Most shops will charge between $500 and $800 for differential replacement, depending on your area and shop rates.

Parts and Materials

Here is a realistic pricing range for each component:

  • ACR Limited Slip Differential: $1,200–$2,000 depending on the specific model (street, drag, or adjustable versions may cost more)
  • Gear Oil (2.5–3 quarts): $50–$100 for high-quality synthetic 75W-90 or 75W-140
  • Friction Modifier Additive: $10–$20 (if required)
  • Differential Cover Gasket or RTV: $15–$30
  • New Axle Seal (recommended): $20–$40 per side
  • Miscellaneous Supplies: $30–$60 (brake cleaner, thread locker, shop towels)
  • Tool Investment (if DIY without existing tools): $150–$250 (torque wrench, dial indicator, bearing puller)

Total Estimated Cost Range

When you add everything up, the total cost for upgrading to an ACR Limited Slip Differential on a 2015–2020 Mustang falls between $1,450 and $2,700 for a DIY installation (assuming you already own basic tools), and between $1,950 and $3,500 if you pay a professional shop to do the work. The higher end of these ranges applies to cars that need additional parts such as stronger axle shafts or upgraded bearings, or that require custom shimming due to wear in the differential housing. To keep costs manageable, shop around for the best price on the LSD itself and consider buying a kit that includes the differential, bearings, shims, and gasket in one package.

Final Thoughts

Fitting an ACR Limited Slip Differential to your S550 Mustang is one of the most effective upgrades you can make for traction and cornering performance. Whether you plan to drag race at the strip, attack corners on a road course, or simply enjoy more confident driving on the street, the ACR LSD delivers measurable improvements that justify the cost. The installation is not a beginner-level job, but with careful planning, proper tools, and attention to detail, a capable DIY mechanic can complete it successfully. For those who prefer not to turn the wrenches themselves, having a reputable shop handle the swap ensures peace of mind and correct setup. Once installed, you'll immediately notice the difference in how your Mustang puts power down—less wheel spin, earlier throttle application, and a more connected feel from the rear axle.

For further reading on differential setup and Mustang axle specifications, refer to the How a Car Works guide to differentials or the official Ford Performance parts catalog. You can also find community feedback and installation experiences on Mustang6G forums, where many owners share their torque specs and troubleshooting tips. For sourcing the ACR LSD itself, check reputable Mustang parts suppliers such as AmericanMuscle or Lethal Performance for current pricing and fitment verification.