American High Performance
From Identifying to Building

By Matt Strong

Whether your car is a daily driver or a blazing hot racing machine, the right differential makes all the difference! Author Matt Strong covers everything you need to know to fix or replace your existing differential. It all comes down to your real needs in strength and performance and how to build one at a price you can afford. This book contains enough pictures and illustrations to introduce the six different American High Performance Differentials and actually teach you to rebuild one. No mystery, just great DIY information.

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I have tried to avoid all the mistakes and follow the successes. I think I have, although I did sneak in a few big words. I didn’t exactly write this book in English though, instead I talk like you were right next to me and we were bench racing. (I really wish we were.) I may have placed a few reminders in parts about important things to maintain at the forefront of your mind when working on differentials, but that’s all. If you find that certain things are repeated several times, it is because they are really important.    

There is no engineering or formulas on any page. None of us care to work through them anyway. I have left out excessive technical data, although I put some in where it is interesting and I know you will enjoy it. (Look at the chapter on axles and alloy cocktails.) I have left some of my California jive in the book, words like bitchin, boogie, and the like, even though I left that paradise for the Catskill Mountains of New York ten years ago. Boy do I love this place three seasons of the year.  Just fifty miles north of New York City, it is a hotbed of racing, hot rodding and endless car gatherings from spring through October. The weekly show at Bear Mountain enjoys a minimum of 600 cars and several times in beautiful weather as many as 1,200.

I truly love cars and have since I was five and started designing them with crayons. I had my parents send them to GM. I got back form letters, but they excited me. I have been a gearhead since. My objective is to give back to those that truly love cars today by transferring some of my knowledge. I hope you’re one of them and have bought this book because you have a “differential project.” If you did, thank you, but not for the money. Your purchase means I’m doing my job as a writer. 

Enjoy, Matt Strong

From Chapter 1: Why We need Differentials

The rear axle differential does two basic jobs: It changes the direction of the torque of the engine/transmission from side-to-side to back-to-front so it can power the drive wheels, and 2) it allows one wheel (outer) to turn faster than the other (inner) as you go around a turn regardless of direction. These sound like simple tasks, but are very important to the performance of a vehicle on the street. If you were to lock a car’s differential action (permanently) the thing would be dangerous; it would hop around a turn, not go through it smoothly. It would quickly wear out the rear tires and axle bearings. And, even in the slightest amount of rain the car would skid out of control as it tried to navigate a corner. As you can see, the differential is one of the most important components in a vehicle.

When cars started showing up, the roads were still dirt and often mud. When more cars were traveling on them, these major routes were paved either with asphalt or by simply grading, adding some gravel, and a coat of oil. Once cars were running on paved roads, they needed a differential action for the rear wheels. The first cars used chain drive connected to a really large rear sprocket that gave the vehicle the benefit of serious torque multiplication, but nothing else. Considering the first cars were lucky to deliver 25 lb. ft. of torque to the rear sprocket, you can understand why torque multiplication was critical. These cars had the same problems as a go-kart, the only way to take a corner was to skid the back tires through the turn or to engineer the chassis so only the outside rear tire is on the ground in a turn. But on dirt it didn’t matter. Once cars started to generate more than 50 horsepower and an equal amount of torque to the rear, differential action became critical to vehicle operation. 

The 1928 Ford Model A wasn’t in any way a luxury automobile. They were not well equipped or handmade like the Cadillac of the day, but they only cost about $400 compared to several thousand. The differential used splash oiling and the engine only delivered around 40 HP. If more power were available, it would have broken this simple yet effective rear end.

Looking at a vehicle from the front, the crankshaft rotates from passenger side to driver side. Don’t think left or right; think passenger side to driver’s side and you’ll never be confused.   

Early differentials were crude things, but they worked fine until the advent of “serious” horsepower. Engineering began to lead the way with development of the hypoid differential, improved bearings, and higher strength components. The pinion gear was supported by bearings and the ring gear acted as an oil pump to push fluid through the housing, sending it to the pinion and its two major supporting bearings. Without this oil flow and the cooling effect it affords, gears and bearings would quickly fail with anything more than 50 HP. The hypoid differential was a significant breakthrough and is still used today in one form or another, except in high-technology transaxles where the gear oiling is handled through high-pressure lubrication.

Viewing a car from the front, the crankshaft, transmission and driveshaft will always rotate from the passenger’s side to the driver’s side. As you face the car from the rear, the rotation will still be from the passenger side to the driver’s. The ring and pinion gears convert the side-to-side rotation from the engine, transmission and driveshaft to back-to-front rotation in all forward gears. In reverse gear, the rotation is opposite, or from front to back. 

The Posi-traction was a big hit and a very popular option on factory orders because it was an OE-only option and there was no aftermarket competition. Although new, this unit is essentially the same as the early models. Friction materials and spring technology have been the major changes through the years. The onset of CAD/CAM drawings and CNC automatic machining has made a big difference in quality. (Photo - Eaton Corporation)

The famous Ford-fleshed Detroit Locker differential was the toughest, baddest traction device available in the 60’s and it is still king for all-out racing regardless of venue. We see how it works using springs to keep the locking plates on each side pressing against the center plate as driven by the ring gear. When enough torque is going in different directions at different speeds, it will unlock and allow the vehicle to make a turn without hopping to the outside of the turn as it would with a locked differential. (Art – Eaton Corporation)

The open differential is the most efficient of all types, when the weather is dry and there is no snow, ice, dirt or sand on the road. When both tires are on the same type of surface traction will inherently be equal. The open differential simply sends equal amounts of power to each drive wheel as long as they are both spinning at the same speed. But, when one loses traction, the differential automatically senses that speed difference and sends all available power to the spinning wheel. If the car is on snow or ice it cannot move, even if one wheel has good traction. It’s the Jekel/Hyde personality of the open differential that led to the development of traction control.

What about traction/torque-sensing devices that can be placed inside the differential? These devices assure that the spinning wheel isn’t the only one with power going to it. These performance aids were developed during the 1950’s and were adopted by OE manufacturers for their high-performance models in the late ‘50s. The Posi-traction was used on GM cars, while Ford and Chrysler used similar systems with different names. Positraction is still going strong today, greatly improved with better spring and friction materials, better metals in the castings and, most importantly, rebuilding kits.

Ford went from nothing to serious traction control. Their first product they called Traction-Lok, a friction-lock differential for street vehicles. Then the infamous Detroit Locker for hardcore racing vehicles. NASCAR, desert racing, Trans-Am sports car racing and other venues, took to this product like ducks to rain. The Locker is capable of handling brutal power without failure. The big three NASCAR series and big truck Off-Road Racing have a serious demand for this kind of strength (about 850 HP and close to 700 lb. ft. of torque.) The philosophy of the three types of locking differentials (Eaton Posi-traction, Auburn Gear Cone Clutch, and Detroit Locker) is that they all act like locked differentials until the car making a turn can generate enough torque to force the springs to release the axle gears.   

This rear axle assembly is in a restored 1928 Cadillac Dual Cowl Phaeton, one of the most beautiful cars ever built. The differential was quite advanced and far superior to that of the Model A Ford. Since Cadillac built some of the most expensive automobiles of the time, it could afford to invest more in engineering time as well as the best components. It used an early hypoid design that incorporated the ring gear teeth to pick up oil and send it through a channel in the housing to feed the oil to the pinion bearings and teeth. The engine was rated at 90 HP so it required an advanced differential to survive the 110 lb. ft. of torque the motor delivered.

This drag racing spool kit contains two large, high spline-count axles, the spool, long racing wheel studs, and heavy-duty retainer plates to keep the axle inside. A dirt track spool kit would look similar, but the axles wouldn’t have a flange, as they must be of the full floating type. Since the ring gear is mounted on the spool, it always turns each axle the same amount regardless of traction conditions.

Some racecars don’t use any type of differential. Dedicated drag race cars and circle track dirt cars, regardless of class, use a racing spool. The spool effectively locks the two drive wheels together into a solid axle from side-to-side, as driven by the ring gear, so there isn’t any differentiating action at all. But, these cars are rarely, if ever, driven on the street.   

Although this might look like an all-show and no-go street-car, it is anything but. I built it with a 450+ HP Ford 400ci Cleveland engine, a Ford C6 transmission, and a 3:50:1 ring and pinion with a Posi-traction differential in the stock Ford 9-inch carrier. This car can embarrass an unsuspecting Porsche as well as many other hot street machines. What looks like a ‘79 Thunderbird is actually a Ranchero with the T-Bird’s grafted on. (Photo – Felipe Gonzales)

When selecting a differential for your street car, pay close attention to the gear ratio, differential type, and suitability to your application. The transmission gear ratios will define rpm drop between gears, while the ring and pinion gears will define the RPM you have at cruising speed and the maximum top speed of the vehicle. Whether you are drag racing or going flat out at Bonneville, the decision is yours, but your selection of a ring and pinion ratio will be drastically different for each venue.

To give the best all-around performance, applications for the street will be a compromise between these two extremes. The type of differential housing must be based on the punishment you plan to wreak. If you are going to cruise, the stock differential housing/carrier with a performance gear set and traction-control device should suffice. A Posi-traction clutch type, or a worm-gear type like the Eaton TrueTrac, will do a good job also. But, if you have a high HP/torque engine and are going to drag race or road race that’s a totally different story. Just remember any Posi-traction type of traction control device acts like it is locked all the time, until turning a corner forces the clutches to release.

If you want a traction device that acts like an open differential and provides equal torque delivery, you’ll want some gear-type such as the Eaton TrueTrac or Autotech WAVETRAC® from Moser Engineering. The WAVETRAC thinks it is an open differential except when one wheel starts taking more torque than the other, then it locks up. Gear-type devices are a great way to have a hot rod with lots of power and still be able to cruise and even get better fuel economy versus using an Eaton Posi-traction, Auburn Gear, or Eaton Detroit Locker.

The Salisbury type axle, also called an integral carrier housing, features a cover at the rear to access the differential. The high-performance differentials of this type are the GM 10-bolt (8.5-inch ring gear), GM 12-bolt (8.75-inch ring gear), Dana 60 (9.75-inch ring gear, Ford 8.8, and the differentials on most full size domestic full size pickup trucks.    

In racing, where the car is tuned for each track by switching rear gear ratios, teams will have a series of prepped third members or drop-in carriers ready for quick installation based upon the gearing required. Regardless of how the gears reside in the housing, for all high-performance work the differential must be changed from an open type to a traction-control differential or to a spool.  


There are 13 more chapters!

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The paths of drive wheels going through a right or left turn make it clear why a differential is required on all vehicles. The outer wheel goes much farther than the inner wheel. If there were no differential, the wheels would hop to the outside of the turn causing wear and tear on the entire vehicle. (Art – Eaton Corporation) 

This is the basic layout of a hypoid integral carrier differential. The ring gear opposes the pinion and gear oil is pumped through the case to lube the pinion gear and its bearings. The ring gear goes through the pool of oil below it in the case and creates a splash system - throwing oil on the pinion gear, bearings, and carrier bearings. (Art – Eaton Corporation)

This image portrays the pinion gear (the one with the long shaft) and how it intermeshes with the ring gear. Without these gears your car would not be able to go anywhere.       

A traction-control differential (right – Eaton TrueTrac) and an open differential (left – GM 10 bolt)  show two important things, 1) You cannot see inside the traction control differential and 2) there are no planetary/spider gears inside the traction control differential. Both differentials are resting on their side where carrier bearings go.

This is the proper orientation of the carrier, looking at it as if you were in front of the car. Taking a closer look at the open differential, you can see how simple it is and the spider gears that are prominent above and below the axle side gears. Also note the side gears and spider gears are always in contact and the spiders are retained by a large pin held in place by a bolt.

In this exploded view you can see the axle side gears with friction plates and steels behind them on both sides. They form a strong clutch on either side to make sure both axles turn at the same speed. The spring pre-load keeps them locked together in all straight driving, but when the axles turn at different speeds, as in a turn, the springs release and allow the axles to do exactly that. If you have ever seen the friction packs inside an automatic transmission or oil-bath clutches found on most motorcycles you will quickly understand this design. (Art – Eaton Corporation)

Auburn Gear was also a competitor in the heyday of racing in the 60’s and still makes its unique Cone Clutch traction-control differential. Although it looks and is different than the Posi-traction, it works by the same principals. The Auburn’s side axle gears are machined in the shape of a cone with oil groves on the cone surface.  The inside of the case (a reverse cone shape) also has oil groves. When these two are pressed together by the pre-load springs they lock up delivering equal torque to each wheel. Oil cools them and keeps them from welding together. The Auburn unit cannot be rebuilt but Auburn sells a new one for about the same price as Eaton’s rebuilding kit (if replacement is required during the first four years of ownership). (Art – Auburn Gear Corporation)

Looking at the Locker from the outside doesn’t give much of a hint regarding its performance potential. But, you can tell by the tough case and large fasteners it’s designed to take hellish punishment.  The Eaton/Detroit Locker is as popular today as it was 40 years ago. (Photo – Moser Engineering)

A chrome cover on a Salisbury differential/integral carrier rear axle housing looks good, but provides no structural enhancement. (Moser Engineering)

An aluminum cover improves the structural rigidity of an integral carrier with a very small weight penalty. These covers have adjusters located right over the center of the carrier caps inside. They are adjusted to 5 in/lb contact and then locked in place.  (Photo – Moser Engineering)

The big Ford 9 differential features a removable carrier that is separate from the housing. After removing the axles and the carrier case nuts, it comes off the front of the housing. This Moser nodular iron case is stronger than anything Ford ever offered. The pinion support and bearing cartridge are inserted in the big hole in the front. Steel shim gaskets setting the pinion depth go over the cartridge studs before it is installed. (Photo – Moser Engineering)
If your GM vehicle is domestic and your rear axle gear carrier has a removable center section it’s from the ’50 s. The drop-in center section holds the ring and pinion and traction control system. Chrysler used drop-in center sections well into the 70’s and Ford even longer.

For about thirty minutes of work, and that includes jacking up the vehicle and placing it on safety stands, that Ford 9 carrier comes right out of the housing. Though the Ford and the Chrysler 8.75-inch axle housings/carrier cases are quite strong, they can be improved further with billet steel caps, performance gears, traction-control differentials, and larger axles. You can see that the bolts go completely through caps and then the case and fastened at the front of the section. This aluminum carrier case also has a matching aluminum pinion support cartridge. (Photo – Moser Engineering)  

Not all removable carriers are Ford. This one is for the Chrysler 8.75 differential/ring and pinion. Just like the Ford 9, it is easily removed and allows the user the option of several cases ready for racing or to have different gear sets for cruising and long range missions alike. (Photos – Moser Engineering)

128-page Full color glossy manual. Staple/soft bound.
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