One of the trickiest things to do in physics is change the axis of rotational motion 90 degrees without failing or giving up unreasonable parasitic losses. While it may seem fairly simple, just run a set of bevel gears and voila! In a motorsport application, getting everything just right is just as much of an art as a science. First, consider the stress- es that a rearend undergoes throughout a pass at the dragstrip, and then consider the fact that factory rearends were never designed to handle the rigors of our sport. Now, all things considered it no surprise that you see so many bro- ken stock rearend components at the track. Thankfully, the standard rearend configuration has been around long enough that companies like Currie Enterprises have had time to perfect the art and science of rearend design and setup, which makes it extremely easy for the average racer to equip their racecar with equipment that will not only handle elevated power levels and abuse, but will do so reliably for many seasons.
To attach the bearing end to the axle tube, they machine a recess into the tube for a perfect fit before they weld the end on.
On the other end of the axle tube, they machine a flat on the end of the tube, so that it “keys” into the center housing’s bulkhead, ensuring perfect orientation of the tube, as well as extra strength.
Before any of the parts are welded together, they are all aligned and checked numerous times to ensure they are all exactly where they are supposed to be in relation to one another.
Here, Currie drills the fill hole in the top of the housing center. They also add a drain plug on the bottom, which makes changing the fluid a ton easier, and a lot less messy than loosening the center section.
When it comes to wanting spool-like traction that can still be driven on the street without chirping the inside tire every time you make a turn, then the Detroit Locker is your answer. We’ve used the Detroit Locker in a number of builds, and we continue to use them. There is a very simple reason for that—they work. The Detroit Locker differential keeps both drive wheels moving at the same speed when you are going in a straight line, meaning that the power is evenly distributed 50/50 to the left and right drive wheels. That is exactly what you want on the strip for maximum traction on the hit and going down track. However, when you turn, your wheels need to move at different speeds in order to properly execute the turn. To handle this, the Detroit Locker unlocks and allows the outside wheel to speed up as it needs to during a turn. Once the turn is completed, and the outside wheel matches the inside wheel’s speed, the differential locks again. The best of both worlds.
When we were considering what to do with the rearend for Nutjob, we considered just rebuilding the stock 8.8 for the time being, before upgrading to a 9-inch down the road. We contacted our friends at Currie Enterprises, and they said they would be happy to build either one for us. After some consideration, we realized that it made more sense to just do it right the first time, and not have to worry about dumping the clutch (or letting go of the transbrake button) on slicks and having something let go—or worse, having a stock component fail at the top end. Project Nutjob is being built with safety as a high priority, so taking that into consideration among all the other factors, it just made sense to do it once, and do it right. With our decision made, we called Currie back and told them we wanted to go the 9-inch route. Brian Shephard quickly went to work spec’ing out the rearend, asking us questions along the way and within minutes, he had put together the build sheet for what would become our rearend. See, Currie has been doing this for a long time…over 50 years, in fact. So not only do they know what you need, but chances are, they can have it together for you, quick. Whether it’s one of their “crate” rearends which bolt right in for 27 common vehicle applications, or a completely custom application, is all the same to them. So a stock-width 9-inch rearend for a 1994 Mustang GT was no problem at all.
The center housing is welded together from three sections of 3/16-inch Hi-Form 50 steel and a third- member mounting flange made from 3/8-inch Hi-Form 50 steel, before having internal bulkheads welded in to tie all the sections together.
These are the parts that will go together to create our third member. Pictured are a Currie 9-Plus Race case, 9-Plus big-bearing Daytona pinion support, 9-Plus nodular iron 1350 pinion yoke, 35-spline Detroit Locker and 3.70 ring and pinion gears, along with Currie’s shim and bearing assembly kit.
Once all the welding has been completed, they straighten the housing out in a press, if anything has shifted or warped from the heat of welding. This ensures a perfectly true housing.
Currie began our rearend by fabricating one of their F9 rearend housings. While they manufacture a round-style rearend housings, they spec an F9 for all 1979-2004 Mustang applications, because of the Fox platform’s suspension geometry and the design of the F9 housing. Specifically, they start with a CE-4303A housing, which is a bare housing, with 3-inch, .250-inch-wall axle tubes and large-bearing housing ends. The main housing is made from only three separate pieces of 3/16-inch Hi-Form 50 steel, before a piece of 3/8-inch Hi-Form 50 steel is added for the third-member mounting flange. Then inner bulkheads are welded in place to strengthen the housing by tying the face, body, and inner ends of the axle tubes (which have had flats milled into them to “key” into the bulkheads) together for unparalleled strength. From there, Currie adds a set of their 1979-2004 suspension brackets (which can also be added to a customer-supplied housing as well) to mount the upper and lower control arms, along with the rear lower shock mounts. Then, because we will be pushing this rearend hard, Currie added a back brace to further strengthen the rearend, completing the housing. At this point, you can opt to have the housing powdercoated, which would be a nice touch, but we decided on a little different approach. The welds were so pretty, and since we’re fans of the industrial look, we opted not to powder coat the rearend. Instead, we clear-coated it, so that it would still be sealed from rust, but still show off the awe- some craftsmanship of the welding.
Once the basic rearend is welded up, it’s time to officially make it a Mustang rear end. To do that, they bust out the Fox suspension jig, and weld on the upper control arm bushings and then the lower control arm mounts.
The core of our Currie third member is their 9-Plus Race gear case. It is a 100-percent new unit, cast from 65,000psi nodular iron, and has been designed with 3.250-inch carrier bearing bores to accept 35- and 40- spline carriers. This heavy-duty piece should handle anything we throw at it.
However you refer to it—pumpkin, third-member, center-sec- tion—the third-member is a critical part of the rearend. For our build, Currie started off with one of their 9-plus Race gear cases (P/N: CE-4027D) which is a new unit cast from nodular iron for strength, with 3.250-inch carrier bearing bores to allow it to accept 35- and 40-spline carriers for high-power applications. Then, Currie added one of their 9-Plus nodular iron big-bearing Daytona-style pinion supports (P/N: CE- 94031). Then, they used a 9-Plus 1350-series yoke (P/N: CE- 94044) cast out of nodular iron for high-strength. Then, with the Currie 3.70 gears (P/N: CE-4005) bolted to the Detroit Locker, Currie’s master assembler pressed, shimmed and aligned everything together in a blur. The design of the Ford 9-inch makes it so that you can easily set up the gears on a work bench as opposed to in the rearend housing itself. While that makes it easier to setup yourself, it’s still a task we’d opt to let the pros handle.
Once the third-member was assembled and bolted in place, next came the axles. For our purposes, Currie spec’d out a set of 35-spline forged alloy axles. Of course the flanges were drilled in the 5×4.5-inch bolt pattern, but while Currie recommended we think about upgrading to 5/8-inch studs, Nutjob’s Weld RT-S wheels are drilled for 1⁄2-inch studs with conical lugs, so we opted to stick with a set of standard 1/2×3-inch screw-in wheel studs for now. Currie pressed on the bearing and retainer plate and then popped the axles into the rearend, almost completing the build.
Once the third member was assembled and back- lash set, white grease was applied to the ring gear, and the pinion gear was spun up with a drill to check the tooth-engagement profile. In our case we had to add a shim after the first try, before getting the wear pattern seen here.
For our application, we went with 35-spline, forged alloy axles. Currie machines all of their axles in house, and because of that can come up with almost any combination of length, spline-count, and bolt pattern that you can dream up.
We went with a set of 3-inch long 1/2-inch studs, only because our Weld RT-S wheels are a conical-seat design. For our application, Currie recommended a 5/8-inch stud upgrade, but we’re going to hold off on that for now.
It only took our installer a few minutes to press on all the bearings and shim everything up. The pros make this process look deceptively easy—even with the ability to set up a third member on a work bench, we’ll still leave it to the pros.
Once the rearend was all assembled, all that was left was to mount up our Baer backing plates, rotors, and then shim the calipers to the appropriate location.
Once the axles were in, the only thing left to do was to install the rear brakes. For that, we grabbed our Baer SS4 kit that we wrote about previously. However, those previous brakes were designed for an 8.8-inch rearend, which uses a floating caliper design. Since the 9-inch design uses a fixed caliper, we sent Baer back our calipers and backing plates, and they were able to swap them out for a pair designed specifically for the large-bearing 9-inch housing end. It was simply a matter of bolting everything on at that point, and then measuring the caliper position, and using the included shims to center it up on the rotor. Then we bolted the new stainless Baer brake lines onto the calipers, bent up some hardline to meet the center body-mount rear brake line, and our brand new kick- ass fabricated Currie F9 was ready for installation into the car. Watch out for next month’s issue when we install the rearend, along with an all-new rear suspension in the car.
Next month, you’ll see us install the rearend along with an entirely new suspension, including the Currietrac® control arms you see here.
Currie
714-367-2676
Eaton/Detroit Locker
Baer
602-233-1411
