Changing the gears in a rear differential has been a popular modification for as long as enthusiasts have been working on cars. Mustangs are no exception. Gear swaps offer an extremely high bang for the buck in terms of performance. Changing the differential is another modification that’s also been popular, and selecting one that helps get better traction on the track and the street is often a necessity as horsepower increases.
For better acceleration and optimal traction while retaining good street manners, we installed a Motive Gear 3.73 ring and pinon set along with an Eaton Truetrac rear differential in our 2013 Mustang GT
Our Project Silver Bullet is rapidly increasing its performance output. While we’ve achieved some respectable numbers at the track, to reach our goal of a 10-second time slip, the original differential and gear set are going to have to go. In order to get power to the ground effectively we’ve turned to Eaton for a new Truetrac differential and Motive Gear for a new ring and pinion. Follow along as we upgrade the rear differential on our 2013 Mustang GT.
A Different Diff
If you’re wondering what gears your Mustang is equipped with from the factory you can check the ID tag located on the rear differential cover. Older cars will have a metal tag, while newer cars use this printed label. The 3.15 represents the gear ratio in this case.
The Eaton Truetrac is different than most differentials on the market, Barney Gwodz from Eaton tells us, “There are no natural wearing parts that require service with any Truetrac limited slip application. These automatic acting units have the capabilities to handle high torque loads and provide on demand traction.” That means there are no clutches to wear out, no biasing plates or springs to change, it’s simply mechanical metal parts working together.
According to Gwodz it’s when one wheel slips that the Truetrac gets to work quickly. Gear separation forces within the Truetrac work to send as much as 70% of available power to the wheel with the most traction. The helical cut gears within the differential mesh with increasing force to slow down the wheel that is losing traction. Once traction returns, the Truetrac goes back to normal operation supplying power to both wheels equally.
The Truetrac is ideally suited for a car like Silver Bullet. On the street in daily driving situations, the Truetrac is hardly noticeable as it operates seamlessly with the rest of the drivetrain. At the dragstrip, assuming we’re under ideal conditions with our Mickey Thompson drag radials proving traction, the Truetrac will send equal amounts of power to both rear tires. If we should break a tire loose though, the Truetrac will help us maintain control by slowing the spinning tire, and sending power to the one that still has grip.
Gwodz tells us that the Truetrac is well suited for Mustang owners who often drive their cars under various types of scenarios. “Mustang owners are just as eclectic as the Truetrac. These ponies are used in most high performance applications. Everything from cruisers, street/strip, drag racing, drifting, road course and circuit racing.”
Top Row: Left: The first step in disassembly is to loosen the cover bolts and drain the housing. Center: With the cover removed we then remove the center pin retaining bolt and pin. Right: We then turn our focus removing the brake calipers and rotors. Bottom Row: Left: We can now remove the C-clip that retains each axle. Center: The axles will now slide out. Right: We now disconnect the driveshaft from the pinion flange as well. The flange will also need to be removed from the pinion.
There are no natural wearing parts that require service with any Truetrac limited slip application. These automatic acting units have the capabilities to handle high torque loads. -Barney Gwodz, Eaton
The gear clusters that make up the inner workings of the Truetrac are constructed from 8620 steel, which is a high strength alloy. Gwodz says it’s difficult to place a power handling number on the Truetrac, as factors such as gear ratio, weight, and shock-loads, all play roles in the stress placed on any rear end components.
Gwodz did point us to an independent test where the Truetrac along with four other differentials was subjected to stress testing until failure. These tests subjected the differential to 3,200 ft-lbs of ring gear torque. Both the Truetrac, and the Eaton Posi differential were used as the baseline standards for the tests. The Truetrac performed best surviving over 600 cycles in durability and torque biasing tests. This gives us even more confidence that the Truetrac should stand up to significant abuse from the boosted Coyote engine lurking beneath the hood.
Left: The bearing caps can now come off, be sure to mark their side and orientation. Center: With the caps out of the way, we carefully pry out the differential assembly. Right: We can now remove the pinon gear after removing the driveshaft flange and the pinion nut.
The S197 platform marked the first time in over 30 years that Mustang owners could actually choose an option for the rear differential gearing. The introduction of the Coyote engine for 2011 sweetened that even further by offering up 3.73 gears as an option. Unfortunately if you’re an automatic owner you’re often stuck with the sluggish 3.15s.
Motive sent us a set of their 3.73 ring and pinon gears. The gears are precision cut and heat treated to ensure high strength, and should have no problem standing up to our street, strip and track abuse, while still providing quiet operation on the street.
Our gears are designed using the latest Gleason technologies, and utilize the most current cutting and heat treat processes. Ron Stobaugh, Motive Gear
Motive Gear supplied us with a set of 3.73 ring and pinion gears which we installed with our new Truetrac differential. These gears will make for a good compromise between hard acceleration on the track, and comfort when cruising on the interstate, without sacrificing much in the way of top end.
Motive has been in the drivetrain business for over 60 years. Motive’s Ron Stobaugh tells us, “Our gears are designed using the latest Gleason technologies, and utilize the most current cutting and heat treat processes.”
Top Row: Left: Motive sent us one of their install kits which includes bearings, seals, shims, crush sleeve, and gear marking compound. Center: It takes a little bit of math to determine the proper pinon gear shim. Right: The pinion shim determines the depth of the pinion and is critical in getting a proper gear mesh pattern. This shim goes behind the bearing. Bottom Row: Left: New ring gear bolts are included and must be used to install the ring gear to the differential. Center: We used a press to install the new bearings onto the differential. Right: The pinion gear was the last part out and the new one is the first part to go in.
Our 3.73 gear set was manufactured in Italy where Motive constructs all of their gear sets. Stobaugh says that each street car gear set is designed to be quiet running, and to set up correctly. This means a minimal amount of hassle for the person doing the installation, and a set of durable gears, that shouldn’t make us crazy driving the car in interstate traffic.
Motive also supplied us with a set of tapered roller bearings for this installation. Stobaugh tells us, “Motive Gear packages our bearing installation kits in house, ensuring high quality, correct components. We offer kits with Timken tapered roller bearings as well as high quality OE Koyo bearings”
Rebuilding the rear end on a Mustang rates fairly high on the skill ability chart. This job requires a few special tools including a press, a caliper for measuring shims, and a couple of torque wrenches. The entire job can take from as few as four to as much as eight hours of time for an experienced technician to perform.
If it’s your first time, we’d recommend planning for the job to take up your weekend, and get ready to do a little head scratching and careful contemplation along the way. The most difficult part of the job is setting pinion depth, and it must be done with extreme care, if not the gears will wear irregularly, could make excessive noise, or worse fail - so plan on taking your time. Additionally Motive offers some basic instructions on their web site, which offers good information on setting the backlash, and the rotational torque of the pinion.
Top Row: Left: Our next step is to carefully install the differential and bearing caps. Center: We then checked our gear mesh using the supplied marking compound. Motive has images on their web site of what a good pattern is. Right: With our pattern confirmed we final torqued our bearing caps. Bottom Row: Left: We can now install the axles, C-clips, and brakes. Center: Eaton includes a spacer that must be installed in the differential. Right: With the spacer in place the only thing left to do is seal up the differential cover, and fill it with fluid.
Fortunately for us the tech team in the Power Automedia shop has plenty of experience doing this type of job. As with all solid axle Mustangs, this entire job can be performed with the rear end assembly still in the car.
Once completed we updated the tune in our powertrain control module to reflect the changes in the rear end gearing. Failing to do this can cause all manner of headaches including the transmission shifting improperly, an incorrect speedometer reading, check engine light, ABS activation issues, and in a worst case scenario the computer can cripple the car by putting it into a limp-mode.
With everything set properly, we took the car out for a test drive. Immediately we noticed the additional grunt from the gears. Acceleration is a lot faster, and breaking the tires loose comes even more easily, at lower RPM due to the increased torque multiplication provided by the gears. Around town we’d never know the Eaton Truetrac differential was anything other than stock. However, as soon as we get into the power and the tires begin to fight for traction, the Truetrac kicks into action applying it’s magic, helping us keep the car straight, and applying power to the wheel that can use it best.
We went back to Irwindale Speedway to see what kind of a difference the 3.73 gears make. Obviously this improves our short times, especially with the help of our Mickey Thompson ET Street Radial II tires. With all of the previous upgrades we performed the 3.73 gears have us knocking on the door of that 10-second pass. We ran a best at Irwindale of 7.22 at 99 mph, which equates roughly to an 11.12 in the quarter-mile. 10-second ET’s are just a a step away from our grasp.
Overall we’re thrilled with how our Motive Gears and Eaton Truetrac differential perform on the street, and the strip. This provides us with the ideal combination of daily driver comfort, and quiet cruising with maximum traction and acceleration for trips down the track.