When you’re building a project car, you really need to have a defined goal in order to make sure the project stays on the right path. However, even the best laid plans are not life-proof. This 1994 Mustang GT was originally on the path to being built as a single-digit car for the True Street class. As such, some pretty stout competition components were chosen for the car along the way.

However, life happens. Before moving across state lines, I had four dragstrips within an hour’s drive. Now I am quite a distance away from any dragstrips at all (along with a mountain range in between either option). After the disbanding of the series your author was planning on running, the motivation to do much with the half-finished car dwindled.

Motivation eventually returned and the car was brought to a running and driving state, with the intention of using it as a fun cruiser. It was quickly realized that for the car to be a tolerable cruiser — let alone a “fun” one — some of those race car parts were going to have to be replaced.

While part of the beauty of the Ford 9-inch design is that you can pull the center section with the rearend housing in the car, as you can see, the housing needed some cleaning and new paint.

Taming the Tail-End

The largest barrier to comfortable street driving came in the form of the car’s rearend. As the original plan called for the car to have to drive a mere 30 miles on the street, and then hold up to three back-to-back passes (with violent nitrous-assisted launches thrown in), the rearend was overbuilt. A Currie Enterprises fabricated 9-inch rearend with 35-spline axles was installed along with an Eaton Detroit Locker and 275 drag radials.

It only took about 100 miles of attempting to drive that combination around our little mountain town before it was decided that something had to change. Between the poorly maintained roads, the abundance of roundabouts, and frequent weather changes, the locker (along with the solid rear control arm bushings and drag radials) was just too aggressive, and had to be replaced.

Thankfully, the aftermarket support for both the Ford 9-inch and the 1979-2004 Mustang is among the strongest in the industry, and there were options available that would allow us to keep the 9-inch housing, and restore daily-drivable street manners. Enter Eaton Corporation.

One of the things that we needed to address on the housing was the solid aluminum bushings we installed, originally (left). We went the polar opposite this time, installing factory Ford Performance 8.8 replacement bushings (right).

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Eaton to the Rescue

While the Detroit Locker is the ultimate in strength when it comes to something other than a spool, since the parameters of the project have drastically changed, so have the differential’s priorities. “Generally speaking, a locking device such as a Detroit Locker will have higher torque-handling capacity than the associated Detroit Truetrac application, but may not necessarily be the best choice depending on the vehicle’s intended use,” says Jeff Saxton, product manager for the automotive aftermarket at Eaton.

That left two options on the table for us: the iconic Eaton Posi clutch-type limited-slip differential, and the wildly popular Eaton Truetrac helical gear-type limited-slip differential. “Many of our customers tend to apply the Eaton Posi clutch-plate type limited slip differential to moderate torque/horsepower applications where a ‘traditional’ limited slip function and vehicle ‘feel’ is preferred,” Saxton says of the Posi. “Given that the Posi is both tunable (via the available various pre-load systems) and rebuildable, it is an excellent choice for limited-slip performance in applications such as on-street performance, road/autocross racing and drifting.”

In this exploded view of a Truetrac differential, you can see that there is no friction material to possibly wear out, like there is in a clutch-type limited-slip differential. The Truetrac relies on the binding of the outer helical gears to transfer power when a wheel is overspeeded due to loss of traction.

However, for bigger-power applications, the Truetrac reigns supreme. “The comparatively higher torque-handling capacity of the Detroit Truetrac makes it an excellent choice for demanding applications such as higher-horsepower street vehicles, drag racing and even towing and off-roading,” says Sexton. Ultimately for us, the decision was an easy one: the F9 rearend uses 35-spline axles and only the Truetrac was available in that application. Short of replacing the axles, the Truetrac was our only option.

The Detroit Truetrac’s helical gear-type design offers a number of extremely desirable features for a street-driven car of any power level. The geared internal design is maintenance-free, meaning you’ll never have to replace wear items, or worry about fiddling with any preload. The helical gears make for quiet operation and seamless engagement, which is unnoticeable by the driver — it operates quietly in the background, allowing you to forget it’s even there. That smooth operation belies its significant torque-biasing ability. The Truetrac can transfer up to 3.5 times the torque to the tire with more traction in limited-traction situations.

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Gearing Up

In addition to replacing the differential, we had another part that wasn’t appropriate for the street — a set of “pro gears” made from 9310 steel, which aren’t recommended for street duty. Luckily for us, Eaton has a new line of ring and pinion gears spanning a wide range of applications and ratios.

“9310 steel has the highest overall nickel content, giving it maximum impact resistance, making it suitable for specialized applications such as dedicated drag racing,” explains Saxton. “This material should not be used for street applications, however.” As such, Eaton’s 9310 gears are part of its “Competition” line. We were interested in their “Street” line, which is made up of 8620-steel, and 4320-steel gearsets.

“8620 steels are an excellent material for general purpose street use as it demonstrates a good combination of impact resistance and durability for long-term/high mileage use at a moderate cost,” says Saxton of the most traditional ring and pinion street alloy. “4320 steel contains a slightly higher nickel content which increases its impact strength, making it suitable for higher horsepower/performance vehicles where aggressive acceleration is desired.”

Eaton’s new line of ring and pinion gearsets are available in both 8620 street gears and 9310 competition gears. They are also available in standard finish and their micro-polished Super Finish, which offers increased efficiencies and does not require an extended break-in process. We chose to stick with the same 3.70 ratio we had before, as it seems like a solid performer on the street.

Since we’re not planning on any hard launches at the strip anytime soon, we went with the 8620 gears, with a little twist — no pun intended. In addition to the standard gearsets, Eaton also offers them with the “Super Finish” applied. “The Super Finish process is also known as ‘Isotropic Super Finishing,'” explains Sexton.

“This is an additional/final surface finishing process that is applied to the gearsets after all machining and heat-treating operations have been completed. The process is intended to reduce the micro-roughness of the gear surface, bringing it to an industry-leading level of smoothness and consistency. That results in precise tooth-to-tooth mesh, reduced friction, wear, noise, and heat generation.”

The Super Finish is advertised as causing a 3 to 5 percent reduction in the power needed to turn the gears, along with the elimination of any length break-in period, normally needed with a set of new street gears. “Mechanical system break-in, also known as ‘run-in’ is the general practice of allowing moving parts to achieve their final size, shape and surface finish in the initial stages of their intended use,” Saxton says. “By producing a highly precise gearset to begin with and applying the Super Finish option, the tooth-to-tooth mesh is very nearly at its optimal and final shape and finish, so break-in is no longer required.”

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While there are instructions included with both the differential and ring and pinion for those of you who are comfortable with setting up a differential, that isn’t something we are comfortable with. We gave Arizona Differential in Phoenix, Arizona a shout and scheduled the two-hour trip down into the Valley of the Sun.

Thanks to the modularity of the Ford 9-inch design, rather than having to haul the whole car down the mountain, or even the entire axle assembly, we were able to pull the third member out of the axle, drop it into a transport container and toss it in the back seat of the daily.

Here, you can see the Detroit Locker and 9310 gears that came out of the third-member (left), compared to the Eaton TrueTrac helical gear-type limited slip differential.

The crew at Arizona Differential made the whole teardown and setup process look deceptively simple. In addition to the new differential and gearset, we needed a new bearing kit as well. Even though the condition of our old parts was “almost new,” reusing bearings when setting up the third member is a shortcut you probably shouldn’t take – so we didn’t.

After a quick question and answer session about the car’s use, weight, power, and our preferences – ensuring the third member was set up specifically for our use – Matt went to work pressing bearings on, torquing the ring gear to the Truetrac, and setting, checking, and then resetting the proper pinion depth with only one .002-inch adjustment needed from the first shot.

Once the wear pattern got the final stamp of approval from the shop owner, Nate Warren, we loaded up the third member in its travel case, put it back into the back seat and drove back up the mountain.

Another reason we went to the pros at Arizona Differential instead of even attempting to set the third member up on our own, is how easy they make it all look. Our tech, Matt, had the gear painted, checked it, made one small adjustment, and was all set faster than we could point the camera at the wear pattern in the paint.

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Putting the Pumpkin Back In

Reinstallation of the third member was quite simple, especially since the rearend was out of the car during the process. It’s simply dropping the pumpkin into the housing, with either a gasket or some black RTV to make a fluid-tight seal between the two components.

Because we subscribe to the ethos of “if a little is good, a lot must be better,” (and because that’s how Currie did it originally) we used both a Fel-Pro gasket from the local auto parts store, and some Right Stuff gasket maker for the seal.

One step, which is easy to miss, is the use of aluminum or copper crush washers under the differential nuts on reinstallation. As some of the aluminum ones originally supplied became offerings to the missing parts gods, we decided to just use all new copper pieces under the studs.

While opinions differ wildly when it comes to how to get a leak-free seal, we opted for the way Currie did it originally. A Fel-Pro gasket with a bead of Right Stuff gasket maker on each side did the trick.

Once the third member was bolted in and the axles reinstalled it was time to fill the rearend with the required four quarts of fluid – a full quart more than a standard 9-inch housing thanks to the additional volume from larger axle tubes and fabricated center housing.

While everyone seems to have their own ideas on what the magic gear oil is, we turned to Torco USA to get their input. There are a couple caveats for our rearend which Torco took into consideration when making a gear oil recommendation. Eaton recommends conventional gear oil for use with the Truetrac, and Currie recommends a heavier 85W-140 oil for use in their rearends.

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“The rule has always been break-in with mineral in nearly every application. It’s not 100-percent — some applications will break-in with a synthetic oil, especially now with the different groupings of synthetic bases offered — it may just take longer,” says Ernie Soliz of Torco USA.

“So why use a true Group IV or Group V synthetic for break-in, if it takes longer and it is sometimes two to three times as expensive? Break-in should be considered break-in, cleansing and flushing. All this in a short interval. So expense wise, a mineral oil it is for break-in. As far as performance, mineral base gear oil has always been the leader in high shock load protection. Synthetic gear oils outperform in the area of extreme heat, longer intervals, endurance type use.”

Another note that Soliz added is that they have seen the Truetrac run well with synthetic, if the application demands it. “The way I normally approach this is I gather information on what the car will be mainly used for and what the owner is expecting out of it,” Soliz says. “On this specific deal, I would go with Currie’s weight recommendation because their equipment/housing is used.” With that, Torco’s mineral oil-based RGO (Racing Gear Oil) in 85W-140 was chosen.

After some discussion with Torco, we decided on RGO 85W-140 non-synthetic gear oil, which meets the specifications of both Eaton and Currie for use with their parts. Unlike a factory Ford 9-inch, the fabricated unit takes an additional quart, bringing the total gear oil capacity up to four quarts.

A Few More Street Touches

Besides wanting to pull the entire rearend out to put a fresh coat of paint on it, we also wanted to replace the solid-bushing housing mounts and control arms with much more street-friendly urethane bushing pieces. While the spherical bearings might be the most efficient at transferring power and controlling the motion of the driveline, on the street, they are loud, harsh, and snappy.

The Maximum Motorsports bushing tool makes bushings removal and installation a snap. It worked perfectly for removal, and the installation cup just needed to be clearanced slightly to work with the shape of the fabricated housing.

Since we were going to be swapping control arms anyway, we decided we needed to fix the bit of rake that was in the rear of the car after pulling all the weight out of the back when prepping it to be a racecar. For that, we went with a set of Maximum Motorsports height-adjustable Sport-Series lower control arms.

Made from beefy two-inch diameter steel tubing, each end of the control arm features a three-piece polyurethane bushing. The adjustable spring perch design has its own grease fitting on the screw adjuster and bearing on the spring perch, which allows easy adjustment with the car on the ground. The sway bar plates accept both OEM and aftermarket sway bars, and the black powdercoated finish should last for a long time.

Since we wanted to replace all the solid bushings in the system with urethane pieces, and we have a little bit of rake to dial out, we decided to use a set of Maximum Motorsports sport-series adjustable control arms with the height-adjustable spring-perch. The design allows quite a bit of adjustment both above and below the OEM perch height, by simply cranking on the adjuster screw with a 1/2-inch ratchet.

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Getting Back on the Road

With all of the new parts installed, all of the bolts torqued, and bushings greased with the proper urethane grease, it was almost ready to see the road. First, though, there was a very slight break-in process required for the new gears. Even with Eaton’s Super Finish applied, there’s still a recommended process which is extremely quick and simple.

The process consists of simply putting the car on jackstands, and running it in First gear for three minutes, and then in Reverse gear for three more minutes, followed by a 10-mile drive, with mild acceleration and deceleration while in gear. Once the drive is complete, the rearend needs to be allowed to cool, and then you’re off to the races.

With everything bolted back together, there was only one step left to complete before we could take it on the 10-mile break-in drive. With the car on jackstands, we let the driveline spin in First gear for three minutes, then Reverse for three minutes.

Road Manners

The first test of the car’s new manners was on the 10-mile break-in drive, and to be frank, 10 miles wasn’t enough time to enjoy the transformation. Gone were the harsh bangs during shifts, the random tail wanderings as the locker engaged and disengaged during seemingly normal driving, and the car’s desire to swap ends whenever making tight 90-degree turns (like at the intersection in front of the local police station) while giving it throttle.

In its place is a car which drives much like we remember the stock car driving (which was about eight years ago, at this point) even though there is an insanely beefy rearend under the car. Now, approaching a roundabout is no longer a process fraught with anxiety, but rather an opportunity to romp the car and pull some lateral-Gs.

The 3.70:1 gear ratio throws off the factory speedometer by about 15 miles per hour at highway speed (we’ll have to recalibrate it). The indicated 80 mph showed closer to 65 on a free Android GPS speedometer app. Also of note are the conditions in which the car was driving. Previously, even the slightest hint of inclement weather immobilized the car.

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As if Mother Nature wanted this article to be even more complete, the weekend following the completion of the installation, we got caught in the first monsoon of the season while driving the car. Previously, a dark cloud anywhere in the sky meant the Mustang was flat out off the road. With the new Truetrac and a set of fresh ultra-high-performance all-season tires on the car, it was a different ballgame.

Turning onto a main road, some healthy throttle was applied and, as expected, the back end of the car did what any rear-wheel drive car does when you stab the gas in a turn in the rain. However, it amounted to a non-event, as there was little more than a shimmy as the car continued on its way like nothing had happened.

The ensuing smile lasted the entirety of the remaining 15 miles home, rain and all. Now rather than being a car that begrudgingly tolerated street driving with a bad attitude, the car behaves like it just came off the showroom floor, albeit with a little more traction on tap when it’s needed, thanks to the Eaton Truetrac and a little more grunt, thanks to Eaton’s 3.70:1 Super Finish gears.

Over the course of this swap, the car has undergone a transformation — both literally and figuratively. With that, it’s time to retire the old keytag and replace it with the more appropriate version.

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