Live axles are great for drag racing. The suspension is easy to tune, the parts are strong, plentiful, and inexpensive. For straight-line racing, the live axle is hard to beat. Put the same rear end in an uphill right-hand hairpin, however, and your suspension is going to cringe, which can leave you in the gravel. For road racing, there is nothing better than an IRS or Independent Rear Suspension. Giving the drive wheels the ability to articulate to the changing road surface, independent of either side, gives you an advantage over the traditional live axle. The problem comes in converting a straight axle car to IRS.
Part of what changed the Mustang from a “secretary” car into the most successful performance platform in the world was its success on both straight and twisty tracks. Vintage road racing is full of early Mustangs, making the aftermarket full of great race parts. The proven performance of an IRS swap can be found for the first pony cars, but they definitely are on the pricey side, starting at around $5000 for a basic kit. For the budget enthusiast that doesn’t want to drive a Miata, that tends to push the IRS swap out of reach. When we decided to add an IRS to our 1962 Mercury Comet Wagon project, we knew this would be a challenge, especially trying to make it happen on a budget of less than $3k.
The early Falcon/Comet platform is what the Mustang was developed on, but there are a few key dimensions that changed, specifically the width. The Mustang is 1.5” wider than the 60-63 Falcon, so even the aftermarket IRS won’t be an exact fit. After about 8 months of researching, we found an answer- Factory Five Racing. FFR builds some of the coolest reproduction kits on the market, the MK-series being the Cobra roadster platform, currently in the 4th generation, the MKIV. When you order an MKIV, you have the option of live axle or IRS. The FFR IRS uses a few factory components from the 89-97 V8 Thunderbird installed into FFR’s tubular IRS cradle that is designed to match up to the MKIV chassis. The Thunderbird IRS is often used for IRS swaps for trucks, but the issue with using it as is in the Comet is the width. The track width on the Comet is a scant 54.5 inches, where the T-bird is rocking 60.2, which would put the hubs past the rear quarter panel. The Factory Five IRS cradle also uses the 60.2 inch width, unless you opt for the pin-drive set up, which is designed for using knock-off adapters. This narrows the overall track width to 54.25 inches, and that we can deal with.
Once we had the basic plan, we called FFR and discussed it with them. While they thought we were off our rockers, they provided us with the necessary dimensions that we would need to ensure the system maintained the proper geometry. The upper mount is separate from the lower cradle, it has to be installed a specific distance from the lower cradle, centered above it and at a specific distance to the front of the cradle, this keeps everything from binding up at full articulation. We ordered the pin-drive kit, which comes with new axle-shafts, tubular upper and lower control arms, parking brake cables and all the bolts. We also ordered a set of Koni coil-over shocks from Factory Five.
The installation required some planning, and this is not a bolt-in by any means. The cradle is a close fit to the Comet’s subframe, but not close enough, so each mount was fabricated under the car. We did not need a tubing bender; all of the mounts are made from thick-wall square tubing that we sourced at the local metal supply shop. As with any fabrication project, cut once, measure, cuss like a sailor and then cut it again….
There were a few unexpected issues we found during the install; one in particular is the upper control arm to subframe clearance, there isn’t any. With the upper arm installed, it comes into contact with the subframe before ride height. To fix this, we simply notched the subframe using a piece of 4×5 square tubing. Another issue is factory tolerances. 1960s manufacturing processes were notorious for low tolerance; we found several areas that were not the same side to side. In order to keep the wagon tracking straight, we needed verifiable reference points on each side. In the end, we measured from several points on the car.
Depending on what vehicle the donor parts come from, you could end up with an open differential. We didn’t know the specs of the 8.8 that we had, so we chose to rebuild it with parts from Randy’s Ring and Pinion, adding a Yukon Sure-Grip LSD and a set of 3.73 gears. Those parts are not included in the project total as they are not required for every swap.
Aside from the basic hand tools, you will need a MIG welder, chop saw, sawzall, and a plasma torch if you can get one. The plasma torch is really nice when working with some of the tight spots and thick metal, but you can do it without one. A lift is really helpful as well, we put the Comet on our Quality Lifts 4-post lift and the added head room under the car made this project much easier than lying on the ground, dropping slag on your stomach.
One other note- With the cradle installed, fitting the center section is a tough task. There is a sweet spot on the passenger rear section of the cradle where the pumpkin can be rolled into the cradle. Keep that in mind for your swap. The 8.8 weighs about 80 pounds, so it takes 4 hands to move it around in this tight spot. I
For the running gear, we picked up a set of Weld Racing S71 18s and 20s, wrapped in BF Goodrich G-Force Radial TA KDW rubber. The lightweight forged wheels look ridiculously good against the Comet body and the large open spokes provide ample cooling area for the brakes.
This swap will work on any early Mustang or Falcon platform, and the total price is less than any custom-fit IRS kit, ringing the register at $2600 for the cradle and T-Bird IRS components, we replaced the hub bearings and rotors for an additional $250, bringing the total to $2850, under our $3k goal.
We spent about 4 days installing the cradle into the Comet, and the results could not be more impressive, on the ground, you can’t tell the wagon has an IRS, and who would even think it would?
01. The entire process was done on a 4-post lift to make access easier, but it could be done on the ground just the same. The first step was to mark the centerline location of the stock rear. We used a carpenters square, a level, and a measuring tape to mark out the exact vertical and horizontal centerlines, as well as took some measurements and noted those on the tape. We used masking tape on the body so we could read the measurements, and this also ensures we don’t lose them. Next, we removed the entire stock rear suspension.
02. The Factory Five cradle comes with most of what you need, including the bolts to put it together.
03. The parts the kit does not come with are the donor parts from the 87-97 Thunderbird (V8 8.8 only). We scored an entire car (96 model) for less than $500, yanked the 4.6 and scrapped the rest. In the end, we broke even and got the parts we needed. The spindles, center section, brakes, and the associated hardware are all that is needed, the rest is junk.
04. We had the factory parts blasted locally and then disassembled them. Our favorite shop tool is this cordless power ratchet from Ingersoll-Rand, it made this job go much faster.
05. To keep the parts looking good, we sprayed them with Eastwood’s Rust Encapuslator and followed that up with some of their Extreme Chassis black.
06. The Koni shocks require assembly, which is simple enough. You do need to pay attention to the direction of the threaded sleeve; it can be installed upside down.
07. The tubular control arms come in a nice gloss powder coat, the only problem is that sometimes the internal threads get some coating too. We used our ratcheting tap and die set from Gearwrench to clean them out.
08. Before doing anything on the Comet, the entire assembly was pre-assembled on the bench.
09. Using the precise measurements from Factory Five, the upper control arm crossmember was positioned over the main cradle and tack welded to the temporary support beams (white slotted bar in the upper right)
10. Then the cradle components were positioned under the Comet. We used an assortment of beams and shims to the get the cradle centered correctly. During this time, the hubs and new rotors were sent to the machine shop to be drilled for the correct 5 on 4.5” bolt pattern and new bearings were pressed in.
11. The wheel centerlines on the body were used to transfer the centerline to the subframe for better reference. Then we made some measurements under the car to make sure it was square. Use more than one point to take into build tolerance from the factory.
12. At this point, the upper control arm was bolted in to check the alignment. It was here that we discovered a big problem, the arm hit the subframe before it would be at ride height.
13. We marked the offending area on both the inside and outside of the subframe. The using our Miller Xtreme 380 plasma torch, the subframe was cut out. We used a recip saw for the vertical sections.
14. The subframe was boxed in using a section of rectangular tubing to regain the loss in strength. This will be sturdier than the original.
15. The upper crossmember is not wide enough to span the width of the subframe, so we made a pair of adapters. These are made from 2”x.25” strap and 3/16” wall 1.5” square tubing.
16. These adapters increase the width of the upper crossmember and match the angle of the subframe. We bolted them to the upper crossmember and welded them in place.
17. Next, the upper crossmember was repositioned under the car and the subframe was drilled to match the holes in the upper crossmember.
18. Then we bolted the upper crossmember to the subframe. It will be welded in after the rest of the cradle is tacked in place.
19. The lower cradle is designed to match up to the MKIV chassis, but the Comet does not need the extra material. We marked the excess material on the front lower portion of the cradle.
20. Then cut it off using the plasma torch. We made some end caps and welded them to the ends to box it in. This also provided a nice base for welding the forward cradle-to-frame supports.
21. The lower cradle was repositioned under the car and the rear upper crossbars were welded to the bottom of the subframe. We cut a triangle gusset and added that to the end of the upper mount. This is similar to the MKIV process.
22. The front frame supports were made from ¼” 2×2 square tubing, running at a 45-degree angle from the cradle to the corner, where the subframe meets the floor, in front of the stock shock mounts.
23. The Ford 8.8 center section was in good shape, but it had been sitting around with the bearings exposed to dirt in the shop for about 6 months, so we rebuilt it. Using new 3.73 gears and a new posi carrier from Randy’s Ring and Pinion. The 8.8 shouldn’t have any problem transferring power from the 347 stroker, and TCI 6X transmission, to the ground.
24. Before installing the control arms, we sprayed the raw steel tubing with more Eastwood Rust Encapsulator and Extreme Chassis Black.
25. The control arms have both adjustable rod ends and shims. This allows for more adjustability in the caster, camber, and toe. We stacked the shims equal on both sides for the baseline.
26. The shocks are kind of a pain to bolt down. Both the nut and the bolt have limited access, so you have to use an open-end wrench. The aluminum spacers are specific to the front/rear placement; otherwise the body will hit the control arm.
27. The axles are next. These are also new with the kit, since this is the narrowed pin drive setup. The non-pin drive reuses the stock axles. The spindles (also referred to as knuckles) bolt to the control arms using the original hardware.
28. We filled the center section with Royal Purple Max-Gear gear oil. There is actually plenty of room to put the bottle, surprisingly.
29. We slipped the rotors and brakes onto the spindles and everything fits. This is about as cool as it gets.
30. The last step was bolting on the new 18s and 20s, from Weld Racing, with 30-series BFG tires. We were concerned about everything fitting, but we measured correctly and had no fitment issues. There is even room for proper caster/camber adjustments. These forged wheels are deceptively lightweight, the tires weigh more than the wheels.
A life-long gearhead, Street Tech Magazine founder and editor Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 5 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced. You can follow Jefferson on Facebook (Jefferson Bryant), Twitter (71Buickfreak), and YouTube (RedDirtRodz).