Choosing the Right Differential for Your Strip Burner

 

3.55 Posi rear8

We’ve gone through engine sizes and mods for your strip burner. We’ve discussed the pros and cons of the different types of fuel delivery systems. We’ve even discussed transmissions for your strip burner. The last thing we need to talk about is the differential, also known as the rear-end.

 

What Is a Differential?

Almost every motor vehicle made these days requires a differential. Every vehicle with an engine and transmission needs one. The differential, in a read drive vehicle, translates the longitudinal rotation (more on that later) of the driveshaft to the axial (later on for this too) rotation that the wheels need to drive the vehicle forwards or back. The differential is also responsible for sending the engine’s torque to the drive wheels.

Vehicle engines are mounted one of two ways in the vehicle: axially, for front wheel drive, and longitudinally for rear drive vehicles, with the front of the engine being at and pointing at the front of the car. Thus, longitudinal rotation is rotation along the lengthwise-front-to-rear- axis of the vehicle. In front wheel drive vehicles, the differential is an integral part of the transmission and the full assembly is called a transaxle.

 

What Types of Differentials Are There?

There are basically two general types of differentials: those built for passenger vehicles and light trucks and those built for big heavy-duty trucks. Passenger and light truck differentials can be further broken down into three categories: Limited slip differentials, positive traction differentials, and lockup or lockers.  There are some people that categorize limited slip and positive traction (posi-trac) differentials in the same category and add another, the open differential. Racers don’t.

 

The Limited Slip Differential

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One type of limited slip differential

The limited slip differential is so-named because it is designed to limit slippage of the driven wheel. In other words, when the differential “senses” that one wheel has lost traction, it will send the torque to the wheel with more traction. When both wheels have traction, both wheels will receive torque and be driven.

A good example of this can be seen when a car equipped with a limited slip does a burn out. You may see tread marks from both wheels or you may see marks from only one. However, you may also see the burnout marks swap sides. This is because the diff “senses” the wheel burning rubber losing traction and sends torque to the other wheel, but then that wheel breaks traction. The process can be repeated until the vehicle’s velocity catches up to the applied torque, or the diff can go into posi-mode and deliver torque to both wheels.

Another reason that we have limited slip differentials is about physics. When two wheels on a single axle track through a corner, the wheel on the outside of the turn (example: The left wheel when turning right) has to travel farther. If the differential delivers the same amount of torque to both wheels, the inner axle has a good possibility of stripping gear teeth or snapping.

 

The Locker or Spool Differential

This is a locking differential from ARB. Notice the solid carrier.
This is a locking differential from ARB. Notice the solid carrier.

The differential types listed above all have one drawback when considering them for use in our strip burners. They both introduce or allow some loss of engine torque through internal slippage. This isn’t good for our strip burners. We need full power transferred to the ground all the time. Since we aren’t doing any cornering at speed, we don’t want that slippage.

So what’s a drag racer to do? Install a locking differential. These are known as lockers, locking diffs, spoolers, or spooling differentials.

 

The Positive Traction Differential

 

This is an example of a newer-style Eaton Posi-Trac diff carrier. Notice the heavy duty springs that allow a very limited slippage to control wheel hop and axle strain while cornering.
This is an example of a newer-style Eaton Posi-Trac diff carrier. Notice the heavy duty springs that allow a very limited slippage to control wheel hop and axle strain while cornering.

This type of differential is commonly referred to as a posi or posi-trac differential. This is the kind of differential that most race vehicles have, whether straight liners or circle/oval track racers. Oval and circle track racers usually use tires of differing sizes to make up for the difference in distance travelled by the right wheels.

A posi rear-end delivers torque to both wheels, regardless of whether either has full traction or not. You can see this when a dragster does its warmup burnouts. Both tires throw off huge amounts of smoke and leave tire marks (although you won’t see them if the track is “seasoned.”)

 

The Parts of a Differential 

A cutaway view of a standard differential.
A cutaway view of a standard differential.

In my more than 40 years of working on cars, I’ve heard some of the major components of a differential given slightly different names, so where a part has multiple names, I will use the ones I know.  The two major components of a differential that you can see without opening it are the axle housing and the cover.

Most GM and Ford rear-ends have their cover on the rear of the diff. This cover is usually secured to the housing with anywhere from 10 to 15 bolts, give or take. These differentials can be worked on without removing the driveshaft or dropping them from the vehicle. Most older Mopar axles, however, require you to remove the driveshaft because the central portion of the axle housing itself is two-piece.

The driveshaft connects to an input yoke which is connected to a shaft that passes through a seal into the axle housing. On the other side of that shaft if the pinion gear. The pinion gear drives a large round gear that is sometimes called the ring gear, while others call it the pumpkin gear.

Bolted to the backside of the ring gear is either the carrier or the differential housing. This component is known as the differential housing because this is where the action of the differential, allowing one wheel to turn faster or slower than the other, occurs. I was taught this was the carrier, while others also call it the differential cage.

Inside the carrier are two more sets of gears. Two of the gears are physically connected to the carrier and rotate with it. These are called either spider gears or planetary gears. I learned both terms at an early age. Planetary gear is usually used because they rotate around the two end gears which are connected through internal splines to the axle shafts.

There are a number of other parts inside the differential such as bearing caps, retainer pins and/or caps, shims, and washers. Some differentials are also equipped with shims which help with the differential effect.

 

The Meat and Potatoes-Gear Ratios

 

The gear ratio of the differential refers to the ratio between the number of teeth on the ring and pinion gears. This ratio is obtained by counting the number of teeth on the driven gear, the ring or pumpkin, and dividing that number by the number of teeth on the drive gear, the pinion. The higher the number the better. For example, we have a ring gear with 37 teeth and a pinion gear with nine teeth. Our gear ratio is 4.11:1. This means that for every full revolution of the ring gear, the pinion will turn 4.11 full revolutions.

Waitaminit there Mike!?!?! We want more turns of the axle for every turn of the pinion!  Yes, yes you do. However, the gear ratio is also a type of mechanical advantage, a torque multiplier if you will. The higher the number/ratio, the more the torque is multiplied. If your engine is putting out 500 pounds-feet and you have a 4.10 rear gear, the final torque applied to the wheels will be in the area of 2050 lb-ft. However, if you’ve got 3.83 gears installed, you’re only putting 1915 lb-ft to the ground. Those 3.83 gears? Strictly street use, my friend.

One thing I’ve got to mention here is that differential gearing is counter-intuitive. A higher number is a lower gear ratio. This means that the 4.11 gears you want for your dragster are lower than the 3.83 gears you want for your street car. I know, it doesn’t make sense, but it is what is.

Ok, now throw all that out the window. Those 4.10 gears are for the engine that makes most of its power in the upper RPM ranges. Some engines, such as some from Buick, Olds, and Pontiac make most of their power in the lower ranges. Cars with these engines, and dragsters that are nitrous-equipped are going to want higher gears, possibly as high as 3.50s or 3.73s.  These higher gears help spread the torque range out some and allow you to make use of every bit of torque all the way through the traps.

 

Use a Gear Ratio Calculator to Choose the Right Gears

There is no hard and fast rule on determining what gears you’re going to want other than use a gear ratio calculator. Exactly what ratio you need for maximum strip performance will depend on your engine’s output, the top speed you’re shooting for, and the size (external diameter) of your tires at speed. That last part is important. Whether you’re running a set of street-legal Goodyear

Eagles or a set of Mickey-Thompson or Hoosier slicks, your tires will expand as their rotational speed increases. This affects the final drive ratio of the rear-end.

Watch an NHRA race with dragsters using those large slicks. As the driver does his burnouts and his launch, pay attention to the rear tires on the car. You’ll notice that the outer diameter of the tire can increase by as much as a foot. The taller the tire is, the more distance it will travel per revolution. For this reason, when building a strip burner, don’t use tires that will completely fill the wheel well if you’re building a closed wheel racer. Remember tire expansion.

 

Who Makes the Best Racing Differentials and Axles?

What day of the week is it? Who is making the recommendation? My favorites are Strange Engineering and Dana/Spicer. However, some of my racing friends also rear-ends and axles from Wallace Racing and Moser Engineering.

 

 

About Mike Aguilar 202 Articles
Mike's love of cars began in the early 1970's when his father started taking him to his Chevron service station. He's done pretty much everything in the automotive aftermarket from gas station island attendant, parts counter, mechanic, and new and used sales. Mike also has experience in the amateur ranks of many of racing's sanctioning bodies.
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