Choosing the Right Differential Upgrade

Now that you’ve supercharged your truck, the next project to consider might be getting all that extra power planted to the ground!
Written by James Bertok – Guest Tech Guru

Consider the following:

  • Does your truck only spin the passenger-side tire when you mash the throttle?
  • Do you have trouble maintaining traction off the line?
  • Would it be nice if you didn’t have to keep rotating that bald tire around anymore?
  • Are you on a first-name basis with your local tire shop?

It might be time to consider a rear differential upgrade. Since truck owners all build and customize their rigs for different purposes, there is no “one solution fits all” when it comes to choosing an aftermarket differential. (One actor most trucks do have in common is weight distribution that doesn’t favor good traction out back where they need it most). Whether it’s dirt, dunes, mud, street or strip – this article is intended to help you make the best-informed choice of rear-end for your custom truck.

There are four major types of axle differentials. Their purpose is the same – transfer torque to the wheels while allowing them to spin at different speeds when going around bends or turning corners. Most trucks come from the factory with “open differentials.”

Open Differential

An open differential will always transfer the same amount of torque to both wheels when they both have traction.

(+) They are reliable, mechanically simple, and have very long maintenance intervals.

(-) The problem with open differentials is that they will allow all input torque to flow to the wheel with the least amount of resistance. Once a wheel breaks traction and begins to spin, its resistance decreases sharply. It’s a lot easier for the differential to send power to the wheel that’s spinning than it is to get the truck moving with the other wheel that still has traction.

On the street, the right side tire usually spins first because the rotational torque of the drive shaft actually shifts more weight onto the left side tire, giving it more traction than the right. Off-road, an open differential is your one-way ticket to getting stuck!

Good Looking G80!
Blown Out G80!
Possible Cause of Blown G80!

Locking Differential

A “Locking Differential” is mechanically locked up normally. It will unlock under low-torque conditions where slippage is desirable, such as when turning a corner. There are many manufacturers of aftermarket differentials. Each has a different method for building the lock-up mechanisms but their underlying function is basically the same. A locker becomes less likely to unlock and allow RPM differences between the rear tires as input torque increases. These are best for off-roading and rock-climbing but not the best choice for street since the mechanism on most can be felt and heard unlocking, like a ratchet, when turning corners. On slick surfaces a locker may not unlock at all causing the inside tire to squeal or chirp even at very low speeds. If your truck is used for off-road purposes, a locker is a very good investment to prevent yourself from getting stuck. On the street they will plant power to both rear tires and get you moving in a hurry.

Limited Slip Differential

A “Limited-Slip Differential” (LSD) is mechanically open normally. LSD’s are based on clutches between the left and right axles. The mechanism responds to input torque by tightening these clutches which limits the RPM difference allowed between the right and left. Clutch application is linear and smooth which makes these very predictable and mannered on the street. LSD’s also have preload which provides a predetermined amount of resistance between the right and left at all times; so they are never truly 100% “open.” On some units, preload is adjustable by changing springs. Most LSD’s have replaceable clutches. Limited Slip Differentials are very effective at preventing wheel slippage on take for great launches.

Manually-Controlled Differential

The driver controls a manually locking differential. Under normal conditions, these usually operate as an open differential. The driver from inside the vehicle actuates the lock-up mechanisms, which vary by manufacturer. These units use a vacuum line, an electrical circuit, or a cable depending on the design. Manual differentials are the best choice for extreme terrain where absolute traction for low-speed crawling or climbing is required. On the street, these might make a good choice for a supercharged truck if you want to retain the ability to leave a quarter-mile long burnout mark and still be able to launch well off the line when needed!

Inside a 10.5″ GM
Eaton Posi for GM 9.5″
Eaton Posi for GM 10.5″

GM Factory Locking Differential – a Special Note

GM’s locking differential factory option (a.k.a. “gov-lok” or option code G80) is a notable choice that fits somewhere between a true “locking differential”  and a “limited-slip differential.” Under normal conditions it behaves as an open differential. The mechanism inside uses a centrifugal flyweight that engages when a certain RPM difference between the right and left wheel is reached. Once engaged, a cam plate is forced to rotate in relation to one of the axle gears.

The more the wheel continues to slip, the more force will be exerted on the clutches until slippage stops. Once slippage has been stopped, the centrifugal flyweight resets itself. It is not uncommon for these mechanisms to engage and disengage in rapid succession during poor traction conditions. These differentials have clutch plates that look mechanically similar to a limited slip differential.

The factory GM locker is made by Eaton and should not be confused with the “Posi” (an LSD), which Eaton also makes. The factory GM lockers are all-around great performers. They work on the street and off road. They are also capable of pulling boats out of the water. Despite their apparent versatility, these differentials have limited usefulness in supercharged trucks. Due to this differential’s design it is possible to exceed the clutch’s ability to lock up against the slipping wheel if too much input torque is present. If this occurs, the cam plate can rotate far enough to literally explode the carrier in half.

Supercharger kits for GM trucks are designed with this and other powertrain limitations in mind. If your supercharged GM truck has additional performance parts and a factory locker, replacing the factory unit now with an aftermarket Limited Slip or Locking differential could save you time and transmission damage, in the future, if it should happen to fail.

Conclusion

If there is a differential upgrade in your future, it’s best to get it installed by a qualified technician. Besides the specialized tools required to set the proper torque and preload of the parts, it takes experience to recognize when the gear contact pattern is just right. Follow the manufacturer’s break-in procedure for the new differential. If a new ring and pinion are also installed, there will be additional break-in procedures to ensure their longevity as well. Change the fluid after the first 500 miles. Most aftermarket differentials have different fluid requirements than the stock part, so be sure to follow the new specifications, instead of your truck’s factory specifications, for the fluid.

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