What’s the diff? Open vs. limited-slip

There might be a whining, grinding or growling noise coming from the rear of your car. The noise may be constant or only when accelerating or turning. And it could be so subtle that you forget it’s there, but you really shouldn’t.

If you’re experiencing those types of issues, it’s time for a rear end service, and, possibly, a rebuild. And you’ll probably want to take your car to a specialist. After all, that’s what many full-service mechanic shops do.

Pro Gear has specialized in rear-end work since 1991. J. Daniel Jones

Pro Gear

Kraig Scott

7948 Ronson Road

San Diego, CA 92111

(858) 571-1158

“About 50 percent of our business comes from independent and dealer repair shops,” said Kraig Scott of Pro Gear in Kearny Mesa. They’ve been specializing in automotive rear ends since 1991.

Another sizable chunk of their business is from customers who gladly spend up to $1,500, simply because their rear end is working exactly as designed.

“Thirty percent of our business is people who want to switch from a perfectly good open differential to some kind of limited-slip version,” said Scott. And for the majority of those customers, the only real benefit is the ability to leave two black tire stripes on the pavement instead of one.

HOW IT WORKS: Within the rear end housing, power from the spinning drive shaft is diverted 90 degrees to the wheels and mathematically reduced through a beveled ring gear and set of pinion gears. Every complete turn of the drive shaft moves the rear wheels a fraction of a revolution. How much of a fraction depends on the desired balance between off-the-line acceleration and low rpm highway cruising.

The differential is attached to the ring gear as shown on this 2004 Jeep differential. J. Daniel Jones

Attached to the ring gear and transferring that spinning power to the axles is the differential. A differential is required because, in addition to driving in a straight line, cars need to turn — and sometimes rather sharply.

When turning, the wheel on the outside of the turn rotates more than the wheel on the inside. The sharper the turn, the greater the rotational difference. The differential is what allows the wheels to turn at different speeds.

The vast majority of rear-wheel drive cars have an open differential. This means that the rear wheels can spin independently of each other. The easiest way to tell if you have an open differential is to jack up the car and spin one of the rear tires. If the other wheel spins in the opposite direction, you have an open differential. If it spins in the same direction, you have a limited slip differential, or LSD.

When working properly, an open differential is the best riding, most comfortable option for everyday driving. The downside of the open differential becomes apparent when you are looking for maximum pull from the engine, as opposed to the highest rotational speed in the drive shaft.

An open differential always transfers an equal amount of power to both wheels. But if one wheel requires less power to turn than the other wheel, such as when one wheel is on dry pavement and the other on a muddy shoulder, it will take less power to turn the wheel in the mud than to turn the wheel on the pavement. The spinning wheel is getting the same amount of torque as the stationary one, but it’s getting the majority of the engine’s power because that wheel is easier to spin.

10 bolts on the cover most often signify an open differential. J. Daniel Jones

The alternative is a limited slip differential. The LSD senses when one wheel is losing traction and, through a variety of methods, connects the two wheels together. It won’t allow one wheel to spin significantly faster than the other; limiting the slippage between them. This allows the engine to move the car forward, even if one wheel has less traction than the other.

When both wheels receive enough power to break the bond between the rubber and pavement, the result is the hallmark of the high-performance car, the two-wheel burnout. Actually, under ideal conditions, an open differential can also do a two-wheel burnout. But it is rare because each wheel requires the exact amount of power to break loose the rubber.

The limited-slip differential, with brand names such as Positraction, Sure Grip, Anti-Spin or Safe-T-Track, was an essential weapon in the muscle-car wars of the 1960s. This is documented in the often misheard lyric in the Beach Boys classic, “Little Deuce Coupe.”

The actual lyric is, “There’s one more thing, I’ve got the big slip, daddy.” “Big slip” was ’60s hot-rod slang for a limited-slip differential.

Adjusting the rear end for smooth operation. J. Daniel Jones

THE SOLUTION: If you want to swap an open differential for an LSD, you have choices. There are multiple brands of new LSDs available for virtually any kind of automobile and its intended usage. And, if you’re a purist, you always have the option to go old school.

“It will likely be cheaper to rebuild your rear end with a new Auburn Pro or Eaton LSD. That runs in the $1,200 to $1,500 range,” said Scott. “But it’s not unusual for a customer to bring a used unit from a scrap yard and insist we use it. An old LSD in decent shape can certainly be rebuilt to as-new specs, but that usually adds a few hundred dollars to the cost.”

MAINTENANCE: A rear end and differential will last a long time with virtually no maintenance — if you use the correct oil and don’t abuse it.

“We recommend changing the oil every 60,000 miles, using a top quality conventional gear oil; not a synthetic,” said Scott. “Synthetics actually work too well, they are so slippery they don’t cling as well to the ring and pinion gears.”

The key to keeping your LSD in proper working order is to have matching tires with proper inflation. “If one tire is taller than the other, they are spinning at different rates even when going in a straight line,” said Scott. “This causes the differential to always be at least partially engaged. This will definitely make the differential wear out prematurely.”

As to an open differential, Scott’s advice is simple, “Don’t do one-wheel burnouts. That spins the spider gears far faster than they were ever designed to go. They will fail.”

Which, if it happens to you, might be a “posi-tively” perfect time for an upgrade.

Jones is a freelance writer, photographer and auto enthusiast in San Diego. His first novel, a fraud caper set in a New Orleans classic car auction, will be available soon. He welcomes topic ideas and shop suggestions; email him at