How to spec a true motorsports-grade Ford 9-inch rear end | Project Vintage Race Mustang

Tim
Update by Tim Suddard to the Ford Mustang Fastback project car
Dec 12, 2021

Even though our 1965 Mustang vintage race car build is based on a base, six-cylinder car, we’re still working from the Shelby playbook.

Original Shelby Mustangs used a 9-inch Ford rear end fitted with a Detroit Locker–a ratchet-style, limited-slip differential. Rear brakes were enlarged, with the stock 10-inch drums replaced with 11-inch drums grabbed from a Ford Galaxie.

[Installing a limited slip differential | Project Ford F-250]

As a base model, our car came with those 10-inch brakes fitted on an 8-inch rear.

Our expert advisor, Curt Vogt of Cobra Automotive, insisted we not install a used 9-inch Ford, reasoning that anything pulled from a street car will be bent or worn-out–and when it starts acting up while hauling down Road Atlanta’s back straight, we’d wish that we followed his advice.

[Who's the Boss? Cobra Automotive's Boss 302 Mustang]

Start with a new 9-inch rear designed for motorsports, he said. As these rear ends are racing favorites across so many brands, they are readily available from a variety of specialists.

We contacted Spraker Racing Enterprises of Mooresville, North Carolina, as we had met them at a local swap meet. Jeff Spraker was familiar with our needs and told us that he was doing more and more rear ends for HSR competitors and other race car builders.

Here is what Curt told us to order:

  • 9-inch rear end housing with standard 1965 Mustang flange-to-flange length of 43 inches on center
  • OEM-style round Banjo 3-inch axle tubes with 3.150-inch tapered roller bearings
  • Vent hole relocated to the center of the pumpkin to reduce the chance of fluid starvation in turns
  • Ford Torino-style drum brake mounting and spring perch mounting flanges
  • Moser 31-spline axles with an access hole and 6-inch flange with 3-inch wheel studs (The rear end showed up with 1 ¾-inch wheel studs, but they should be fine with our wheels.)
  • Drain plug be mounted flush to lessen the chance that it gets damaged
  • Eaton Detroit Truetrac differential
  • 3.70:1 final drive
  • 2.77-inch brake drum locators

While HSR does allow Cobra Automotive rear disc brake kit that mostly contains Ford parts, we decided to stick with the tried and true 11-inch rear drums that came on original Shelbys. This move saved a bit of time and money (about $400). Mounting the competition rear drum kit from Cobra Automotive was very straightforward.

We did opt for some competition brake shoes from Cobra Automotive and larger 1-inch rear wheel cylinders to keep the brake bias closer to optimal for racing.

We spent about $2400 on the rear axle, while the competition rear brake kit retails for $1095.95. Check the Cobra Automotive site for various brake packages for the early Mustang.

Could we have saved money with salvage yard parts? Probably. But with some endurance contests in this car’s future, we figured we’d start with a bulletproof driveline that won’t let us down just as we’re getting into things. Everything also went together perfectly.

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stylngle2003
stylngle2003 GRM+ Memberand Reader
12/6/21 2:22 p.m.

as of 2:21pm,  link not working

 

stylngle2003
stylngle2003 GRM+ Memberand Reader
12/6/21 2:22 p.m.

link is broken, at the moment

Tom1200
Tom1200 UltraDork
12/6/21 2:56 p.m.

$2400 is pretty reasonable as these things go. 

The used axle for the Datsun cost me $1500 and that was only because I sold some of the parts I wasn't going to use. 

wawazat
wawazat SuperDork
12/6/21 3:46 p.m.

Links are working now.  
 

I did a 9" Ford build with a mixed bag of salvage/used and new parts.  The used pieces were axle housing and OEM 31 spline shafts for $200 plus powder coat at $75 and center section with Auburn diff and 3.70:1 gears for $700.  New bearings, seals, Daytona pinion support, and heavy duty spring perches plus assembly for $700.  I used a modified Wilwood rear disk brake kit for $1100 and a new 1350 spec rear yoke and straps for $200 and new 3" ARP studs so not much less then an entirely new unit as Tim noted above.   I did resell the stock rear ended for $300.  That also doesn't include my time and effort finding, transporting back and forth and nailing down multiple suppliers.   Plus mine is an OEM housing and axles-good or bad depending upon your perspective.  If I had been planning on racing I would have done exactly as Tim did.  

Tom1200
Tom1200 UltraDork
12/6/21 4:09 p.m.

In reply to wawazat :

My path was similar; bought another Datsun with larger Diff for $1000, sold the car (had a title) and other parts for $800, then spent $1300 on the LSD 3rd member. Net cost was $1500. It was just enough savings to be worth it. 

These days I would just buy a complete unit.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
12/6/21 5:49 p.m.

I probably have $2500 in mine as it sits.  Shoulda went with a Speedway (not Speedway Motors) full floater, which was $1500 for everything but the pumpkin.

APEowner
APEowner GRM+ Memberand SuperDork
12/6/21 6:23 p.m.

I'll be interested in seeing if you're happy with the rear drums long term.  I ran rear drums on my Camaro for a bit and at Sandia Speedway and Arroyo Seco Raceway they were fine although I could feel the brake balance change as they got hot and faded. however, at PPIR where I was hauling down from about 150 MPH to 30 MPH coming off the banking and into the infield, they faded dramatically to the point where I was backing up braking  points by several car lengths.  The pedal never got soft they just stopped working.  When I pulled them apart after they cooled off the drums were a really pretty blue color.

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