The differential is a necessary piece of equipment, though most drivers rarely think about it. Upgrading your Mustang’s differential becomes necessary if it breaks or if your goals can no longer be met by your current diff.
Through the years, the Mustang’s differentials have changed considerably. Understanding why and what the available options are can help you decide on the right differential for your Mustang.
What Does a Rear Differential Do?
Your differential is one of the last pieces in a chain that transmits power from the engine to your wheels. When the engine produces power, that power has to go through the transmission to the driveshaft and finally, to the differential. The differential takes one input, the driveshaft, and turns it into two outputs.
Without a differential, your rear wheels wouldn’t be able to travel at different speeds. This would make turning difficult since the outside wheel needs to rotate more than the inside wheel.
The ratio of the gears inside your differential is referred to as your “final gear ratio.” That’s because it’s the final gear between your transmission and your wheels. Changing your rear gear ratio can affect your Mustang’s performance dramatically. The differential itself can also be changed out.
Types of Differentials
Ford has built their Mustangs with both open and limited-slip differentials. But all of the below types have seen some use in different Mustang builds.
With an open differential there’s no mechanism dividing power between the wheels. That means that whatever wheel is encountering less resistance will spin faster. This can lead to a loss of traction, especially in slippery conditions.
A limited-slip differential tries to sense when a wheel is slipping and then limits that slipping. Limited-slip differentials lock the wheels together when one begins to slip. This improves traction and doesn’t negatively impact cornering.
When a wheel isn’t slipping, this differential functions the same as an open differential.
Locking differentials lock the wheels together so that they’re always rotating at the same speed. There are several different types of locking differentials, including automatic lockers.
Locking differentials are frequently used in four-wheel-drive vehicles like the Jeep Wrangler. A lot of Mustang drivers have installed these though. Depending on your build goals, a locking differential may make sense for you.
A spool is what’s used to refer to the absence of a differential. It’s a single, solid piece that connects the rear axle to a ring gear.
There are obvious practical issues with this setup. If your wheels can’t spin at separate speeds, turning becomes a challenge. This means that a car with a spool instead of a differential would have a hard time getting out of a grocery store parking lot. This is why spools are really only seen on track cars.
Does My Mustang Have a Limited-Slip Differential?
Mustangs have used limited-slip differentials on-and-off throughout the years. When the Mustang first launched, it came with the option of a limited-slip differential. This option wasn’t tied to a particular package, but was notated on the data plate. With an axle code, you can establish your Mustang’s original rear gear ratio and differential type.
Since then, limited-slip differentials have become a standard feature in Mustangs. GTs have had a standard limited-slip differential since 1986, and four and six-cylinder Mustangs have had one since 2011.
Traction-Loc vs Torsen
Ford’s limited-slip differential is called traction-loc (though the spelling has changed over the years). Trac-loc differentials use a mechanical clutch system to lock the wheels when necessary.
Mechanical clutch and hydraulic systems are the two most popular types of limited-slip differentials. There are pros and cons to mechanical clutches. Obviously, these parts can wear down overtime. But they can also be upgraded to increase the locking capabilities.
In the S550 generation, Ford also began offering Torsen differentials in performance packages. The Torsen is a worm-gear limited-slip differential.
Gear-based LSDs, like the Torsen, don’t wear out the way that clutch-packs do. They also aren’t as adjustable. Gear-type LSDs run a little cooler, which is why they’re seen as more desirable for high-performance applications.
How Do I Know If My Differential Is Bad?
Your differential needs minimal maintenance to stay in working condition. Check your owner’s manual to see how often your differential fluid needs to be changed. For most vehicles, that’s between 30,000 and 60,000 miles. 50,000 miles is the standard interval for changing differential fluid.
If your fluid change is overdue, the gears may become dry or dirty. If this happens, your differential will start to whine. As long as you change the fluid in a timely fashion, you should be able to keep your differential in working order.
Other signs your differential needs a fluid change:
- Howling or whining noises
- Whirring noise when slowing down
- Clunking sound when starting to move
- Vibration that increases with speed
Which Differential Is Right for Your Mustang?
Mustang owners often elect to upgrade their Mustang’s differential as part of a performance build. If you’re making a drift or drag car, then upgrading your Mustang’s differential will be critical.
It’s also not unusual for limited slip differentials to lose some of their locking capability over time. Replacing it with a new, or higher-end, limited slip can improve your performance.
Differential Type Pros and Cons
|Differential Type||Pros||Cons||Who Is It Best For?
||Simplest differential type.
||Can lose traction in slippery conditions.
||People who drive casually or show.
||Most versatile differential.
||Requires more maintenance than open differential.
||Most drivers, especially those who frequent curvy roads.
||Offers best performance.
||Not great for daily driving.
||Drift and drag Mustangs can benefit from locking differentials.
If you know you want to upgrade your differential, there’s some information you need to collect first. In addition to what type of differential you want, you’ll also need to know your rear axle size and spline count.
Replacing your differential is a complicated endeavor. Once you have the replacement part, it’s best to scout out a local mechanic or visit a mechanically-inclined friend.