Installing a 99+ Cobra IRS into a Fox Body

Tom Beverly and Lewis Tanner

Mouthbreather Motorsports



The purpose of this project was to install an Independent Rear Suspension out of a 1999/2001 Mustang Cobra into Tom Beverly's 92 Mustang coupe for use on the street, drag racing, and in open track road race events. He chose to do this over the "regular" torque arm/panhard bar setup because he was looking for something a little different, and the opportunity arose for him to make the swap for little to no cost. Tom worked out a deal to trade his built solid rear axle assembly (Auburn Pro, 31 spline axles, 3:73's, 98 GT brakes, ect), for a modified 99 IRS, (Raxles, 3:73's and steeda bushings, as well as Kenny Brown sport springs for 99 Cobra convertible).

Tom's car had the full Global West treatment prior to this install and he felt like he had reached its limits and was ready for more (this car has been through many different stages since 1994). While the TA/PB has lots of potential, (probably more then the IRS, as it is tried and true), he feels that there is a lot to be learned and improved upon regarding the IRS. His future plans include rear coil overs, delrin rear lower control arm bushings and possibly custom upper and lower tubular control arms as well as modified control arm pickup locations.

Tom's front suspension currently consists of the stock k-member, stock sway bar with poly bushings, 1100LB linear front springs, GW camber caster plates, Koni DA's, GW del-lum front control arm bushing, 94/95 spndles and 13" Cobra front brakes. He plans on changing the front suspension in the future to either the Griggs Racing tubular k-member, tubular control arms and front coil overs, or to the Maximum Motorsports Double A-arm front suspension, that hopefully will be released sometime in the spring off 2002.

We will let you know how the rear works out in the real world behind his 380HP, 351W (and hopefully 500+HP 392 or 408 stroker soon, look for a story on that in the future too :). We will keep you updated on the front suspension as well, but now lets get onto the swap!!

Oh, and we really have to thank Jim at PPI for his help in finding all of the proper parts for attaching this thing to the car! Give him a call at (703) 742-6207 or visit his website for all of your Mustang parts!


This article is meant as an informative example and is not intended to be a full set of instructions for this project. Please use common sense and don't try to do this without knowing more than we tell you here!


This article details the installation of the Cobra IRS on a car with the stock suspensions already removed from the donor and recipient vehicles. There is a lot of information on removing and installing the stock stuff, and we aren't going to tell you anything you shouldn't already know if you are going to attempt something like a suspension swap!

The following items should be removed from the recipient vehicle before beginning this project:

  • Driveshaft
  • E-Brake Cables
  • Upper and Lower Control Arms (All Hardware and Springs)
  • Rear Shocks
  • Center Differential Snub and Bracket
  • Solid Rear Axle Assembly
  • Catback Exhaust Assembly
  • Fuel Tank

Also, the rear brake hose should be disconnected from the vehicle and left on the solid axle. It will not be used.





Step 1: With the rear of the car supported by jackstands, remove the brackets that hold the rubber axle snubs. These are spot welded to the frame rail and may require some coaxing. We found a combination of grinding, drilling, chiseling and very large prybars works well. You may want to use a spot weld removal tool, availible at body shop supply stores. Ignore the IRS in these photos, it is much easier to do this with it out of the car.

Step 2: Place the IRS on a dolly that allows the unit to freely be rolled under the vehicle. The one we used is about 5" total height.

Step 3: With the rear mounting arm brackets installed (marked in red), locate the dolly so that the IRS front lower mounting arms are positioned under the torque boxes/ LCA front mount position. These arms are highlighted in green.

Step 4: Using two jacks (one under each side of the car), raise rear of the vehicle off of the jackstands and remove them. SLOWLY lower the vehicle into position, moving the IRS assembly as needed to line up the mounting points. Make sure the mounting arms are lined up and the bushings are not binding on the mounting holes.

Step 5: Continue lowering the vehicle until the bracket can be bolted to the frame rail into the rear quad shock mounting location using the rear bolt hole on the bracket as seen in the picture on the left. The bolt is not in place in this photo, but you should get the idea. (It's the leftmost bolt location...)

Step 6: Raise the vehicle and support it using jackstands. Remove the dolly from the now suspended IRS assembly and place a jack underneath it, raising it to support the IRS assembly. Remove the bolts that were placed into the rear quad shock mounting point.


Step 7: Jack the IRS up until the rear mounting brackets come into solid contact with the framerail and inner wheelwell. Mark the interference points with a black marker or anything else you have laying around.

Step 8: Lower the IRS until there is room to use a hammer to massage the offending wheelwell bits out of the way. Massage it good, real good!

Step 9: Raise the IRS until the mounting brackets make contact with the framerails again. Locate, mark, and curse the interference point on the passenger side trunk floor/fuel tank. Lower the IRS and grind and hammer as necessary. We didn't have any interference on the driver's side, but you may want to have a look-see.


Step 10: Raise the IRS again(up, down, up, down... it's a bitch, huh?), until the rear brackets are completly flush with the frame rails. If you can't get them flush, go back a couple of steps and try again. Now, mark the position of all of the bracket holes with that marker you used in step 7. Lower the IRS.

Step 11: Using a 1/2 inch drill bit, drill out all of the holes you just marked. Make sure you make it through both sides of the frame rail and into the trunk. (You may want to move the trunk liner before doing this. Then again, you may not.....)

Step 12: Remove the bracket from the IRS upper mounting arm and attach it to the frame rail, but don't tighten the bolts down too much, as some slop is good here.

Step 12a: Raise the IRS again. Carefully position the upper arms into the brackets, make sure you don't catch the bushing on the bracket and bind it up. Bolt the Upper arms in and torque the bolts holding the brackets onto the car to the point that you are comfortable that the suspension is not going to fall out from under the car.

Step 14: Lower the jack holding the IRS in place. Look! It (hopefully) didn't lower the IRS with it! Get that jack out from under the car and admire your handiwork, as the hard part is done. Go ahead and put the gas tank back in now (not shown in picture). Right now is a good opportunity to call "Miller Time" for the day.



Part 2 - Shocks, Springs, Brakes, and Exhaust


Mouthbreather Motorsports: Doing it the hard way is the only way!

Everything on this page (c) Copyright 2001 Lewis Tanner + Tom Beverly.

All Rights Reserved, Copying this will bring on an ass kicking!