Rearend Makeover: Project Red Dragon Gets New Gears And A Diff

One of the most difficult tasks for a car that sees both street and track duty is that of how to strike an acceptable balance between performance and street manners. These two options really don’t live in the same circles, and you usually have to sacrifice comfort for track performance if you want to lay down some impressive elapsed times. For Project Red Dragon, we decided to give up just a little bit of performance to gain a better street-driving experience by switching to a Detroit Truetrac limited-slip differential unit from Eaton, and a 3.50:1 rearend gear from US Gear.

Originally, the Red Dragon came with a 10-bolt rearend from GM with a 2.73:1 gear inside. The 10-bolt rearend isn’t known for being the strongest to come from GM, so as the car has progressed with the addition of more horsepower, the 10-bolt was on the list to go. The circumstance arose to purchase a used Moser 9-inch rearend for a great price, which was equipped with a spool, 31-spline axles, and a 3.75:1 gear. This prime opportunity was seized and the rearend was bolted into the car, so when the nitrous was added there wouldn’t be any issues of a rearend failure.

The spool that was in the Red Dragon was great for strength, but really wasn't fun to drive on the street all the time. The decision was made to give up the nearly indestructible spool for something a little more friendly to drive to car meets.

The problem is, this Moser rearend was built for a car that made more power and was set up differently from the Dragon, so it wasn’t a perfect fit for the Trans Am. Sure, it was plenty strong, and didn’t over-rev the car on the freeway, but driving the car with a spool through parking lots wasn’t the best experience, and the gear ratio wasn’t ideal at the track for the power the Red Dragon makes. To solve these problems, some give and take needed to be made between the spool and gear ratio to provide a better overall street-driving experience, while not taking too much performance away at the track.

Getting The Gears Going

Based on our elapsed time goals and the need to drive the car on the street, we opted to go with a 3.50:1 gear ratio for the Red Dragon. US Gear suggested we go with a ring and pinion from their Lightning line of gears, and they shipped one out to us right away. The great thing about this kit from US Gear is that it comes with everything you need to install the gears from shims to bearings.

The way that you gear the car is just as important as the modifications you make to the engine. – Donavan Handley, US Gear

The best way to start the process of selecting a gear is to begin with a plan for how the car will be used, and a final goal in mind. A dedicated track-only car will have much different requirements for gearing than a street car. You also have to consider the transmission and torque converter combination when calculating what gear ratio you want to use because each has a large impact on the final gear ratio you select.

US Gear provided the full kit to bolt their gear set in our rearend. You get the ring and pinion gear along with fresh bearings, shims, seals, and bolts you need for installation.

Donovan Handley, Product Manager at US Gear, is able to add some details on how you should look at selecting your rearend gears.

“The way that you gear the car is just as important as the modifications you make to the engine. For a street/strip application, you want to make sure you’re in the power band as much as possible during a run, but you also want to be able to launch effectively. A ratio from 3.42 to 3.90 would be great for a street and strip car, whereas 4.10 and higher ratios would be the best option for a track-only oriented vehicle. Last, you’ll want to determine if you want to use a regular performance street gear or a competition pro gear. The latter of the two is only recommended if you don’t drive the vehicle on the street. Otherwise, the regular performance gear is your best option,” he says.

When you’re looking at all the different numbers for gear ratios, your head can begin to spin, but there’s a simple way to look at it. You can break gears down into two different categories: tall gears and short gears. Tall or higher gears will have a lower number like 2.73, 3.00, 3.25, while the short or lower gears will have a higher number like 4.11, 4.56, 5.13 and so on. What these ratios reference is as follows: let’s say you’re driving at 60 mph in high gear at a 1:1 output ratio. In the process, for every one rotation of the rear tire, your driveshaft (and the pinion) is spinning a number equivalent to the gear ratio. So for a set of 4.11 gears, the driveshaft has to spin 4.11 times for every rotation of the tires.

The Isotropic Super Finishing that US Gear adds a great look but also increases the durability of the gears.

Based on the gear ratio, you can see how short and tall gears will dramatically change the application of power for any combination, along with how the car will behave when driving. Taller gears are a great fit for street and highway use since they don’t tax the engine as much, but they lack in acceleration. A shorter set of gears will give you more punch down low, but will run out of breath at the top end of the track. This is where the use of the car will play into how you gear it based on the application.

The ultimate goal is to have a gear ratio that keeps your engine in a happy power range based on how you drive it. A smaller displacement engine that spins big rpm will want a set of lower gears to make use of its powerband, while a larger engine needs a taller gear to help it use all its power correctly.

The ring gear bolted to the Eaton differential unit (left) and the pinion added to the Moser case (right).

Taking all of this into account, it made the most sense to go with a gear ratio that will benefit the combination in the Red Dragon at the track, while still allowing it to take long trips on the freeway without any issues. Handley sums up our course of action the best in this situation on why we went with a 3.50:1 ratio.

“For a street/strip application, you will need to run street gears and not the pro-style gears to prevent wear issues. As far as the ratio, you will want to think about a shorter gear since it can be used for more spirited street driving, but not too short that you’d hurt your fuel mileage, and not be able to maintain a tolerable rpm while driving to the track.”

Rotating Tires With The Detroit Truetrac

Having a spool in the Red Dragon was both a blessing and a curse at the same time. It was great to know the spool could deal with just about any level of power we wanted to put down for a street car, but the downside was the constant binding of the suspension on the street, making tight turns difficult at best. The only real solution was to install a differential that can deal with some abuse, while allowing the car to be more street friendly.

The Eaton Truetrac is packed with super-tough gears that can deal with street/strip abuse.

Barney Gwozdz from Eaton expands on why you want to look at a high-quality differential for a vehicle that will see heavy street use.

“Spools cause you to drag the tire every time you turn. The amount of stress that puts on the rest of your rearend depends on the size of tire you’re running. The bigger the tire you have, the more friction there will be between the tire and road, therefore, you can expect more stress on parts. The weight of the car and how sticky the tires are also play a role in how the spool behaves. It will put extra stress on the drivetrain, but a lot of the damage will be done to your tires.”

A spool is nothing more than a solid piece which rigidly connects the axles of the car, where a differential is filled with clutches and/or gears to allow for limited slip as the vehicle is turning. The key to a quality differential is the gears that are inside and how they’re made. To make a highly functioning differential unit, Eaton uses net-forged spider gears inside that add additional durability and torque load carrying capacity to increase its strength.

“All limited-slip units, no matter if they’re cone-driven, helical gear-driven, or clutch-driven, rely on some type of natural gear separation forces. The gears will either separate and wedge clutch disks together like the Eaton Posi, wedge a cone into a side of the case, or force the gears into the case like a helical gear Trutrac design,” Gwozdz explains.

What makes a limited-slip differential such an attractive choice for a street/strip car is the efficiency that it can offer. The ultimate goal of a limited slip differential is to get the power generated and transferred ground as quickly as possible.

All of that efficiency has to come from somewhere, and according to Gwozdz, it’s based on how the unit is designed.

“The unit is made efficient by the materials, its design, the pre-load that’s in the differential, and the tolerances to how the units are assembled. By working with these, that’s how you make the differential more efficient. The more efficient you can make the process happen inside the unit, the better the unit will react.”

An exploded view of the Trutrac unit.

Selecting the correct differential is just part of what it takes to make your rearend combination work flawlessly. You also have to make sure everything is set up and maintained properly. Taking the time to ensure the ring and pinion have the correct tolerances and torque specs is the best way to guarantee it can deal with abuse at the track. Equally important is that you observe the prescribed maintenance intervals to eliminate lubrication problems and other issues.

The finished product after GM World Class Technician and co-pilot of the Red Dragon, Scott Cordell, finished the assembly process. Cordell made sure to check the tolerances and gear mesh carefully to provide a correct assembly.

“Because the Truetrac unit is a limited slip differential and uses a helical gear design, it doesn’t have any naturally-wearing parts inside. That’s what really sets the Truetrac apart is that there’s no real service required for the differential. It’s designed to have a lifespan that will last as long as the vehicle.”

To make sure all their products, including the Truetrac, are ready for the customer when released Eaton has developed their own proving grounds at their engineering facility in Michigan. The concept behind these proving grounds is to provide products to customers that are already sorted out and ready for any level of abuse.

“As these units are developed, we do real-world testing, in real-world environments at our proving grounds to be sure they are ready. We have our own differential dyno where we can test anything we develop as well. We’re able to test and re-test in these products real-world situations to make engineering adjustments as needed to really refine each product”, Gwozdz explains.

With the new gears and posi-unit the Red Dragon was able to drive an hour to Kil-Kare Dragway without any issues and make a low 11-second pass in some brutal heat!

By making the change to the Truetrac unit and rearend gear, our goal to make the Red Dragon more fun to drive on the street has been achieved. We can take corners in the car easier now without extreme care or planning, and driving on the freeway is not an issue since we’re only spinning the engine to about 2,700 rpm. The rearend gear will also pull the car down off the rev-limiter at the top end on the 28-inch tall tires, and that is key to some of the future plans we have for the Red Dragon!

Article Sources

About the author

Brian Wagner

Spending his childhood at different race tracks around Ohio with his family’s 1967 Nova, Brian developed a true love for drag racing. When Brian is not writing, you can find him at the track as a crew chief, doing freelance photography, or beating on his nitrous-fed 2000 Trans Am.
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