OK, so i like to be different. So there are several things i'm doing that you don't hear of very often. First off, i'm putting IRS in an FB RX7. Not many people have done it, and i haven't found anyone that's done it like i'm doing it.

Second, I'm using an IRS rear end from a 1986 Jaguar XJ6. These cars are all over junkyards (compared to FDs and Cobras), they're cheap ($136 for my entire rear end, from hub to hub) and people use them in kits cars a lot (Factory 5 Cobras mostly).



I have seen some people say things like "use FD IRS" and "solid axle > IRS" but i want IRS so that's what i'm doing.

Now on to the actual rear. The differential is a Salisbury/Dana 44. That means any Dana 44 carrier and gears can be installed in this rear end, given the right model is used. The main thing is that you get a 19 spline differential, as that's what the Jag uses. Here is an example of a usable LSD diff for this rear end and probably the one i'll be going with: Auburn Dana 44 LSD from Summit Racing. Note that this carrier is for 3.73 and numerically lower gears. The carrier for 3.92 and higher is the exact same price and available from the same location.

Moving outwards, the brakes are next in line line. Yes, this IRS has inboard brakes and yes, they will be staying there. The brakes on this rear end appear wimpy since they are not vented and are only about .5" thick. However, it does have 2 piece, 2 piston calipers and when you think that these brakes were on a 3900 lb car, it should do just fine in stopping a 2300 lb car. There is a kit available somewhere to widen the calipers and install cross drilled/slotted/vented rotors but i haven't been able to locate the kit and i fear they may no longer be available.



The actual suspension is handled by 2 coil-overs on each side with no sway bar. Again, this used to control 3900 lbs, so 2300 lbs should be nothing.





It uses a floating hub with a 5x120 (5x4.75) bolt pattern. That means Camaro and Corvette rims, as well as many BMW rims, will bolt right on. I'm sure there are other cars that use this bolt pattern, but I can't think of any right now.

All of this is suspended inside of a sub-frame that only weighs ~10 lbs and attached to the XJ6 via 4 bushings. Each bushing bolted to the sub-frame using 2 bolts and a nut, and to the car with 2 bolts/nuts that went all the way through the frame. This made the sub-frame almost "hang" under the car. The only other attach points were a trailing arm on each side and the driveshaft. I'm porbably not gonna use these mounts though, as i dont particularly like my rear end hanging from 4 pieces of rubber. I'm currently making some nice mounts and will bolt/weld them to the top of the subframe.



Obviously everything is gonna get cleaned up and painted, powdercoated or polished.

I set the subframe under the car just to see how it fits. Looks like it will fit under there nicely!!

 

Nice.

What is the difference between the rear track widths of the FB and the Jag?

 

hmm interesting. just based on my BMW experience you want more than 4 bolts holding the rear in

 

I don't have an exact measurement but it's approximately 4-5" wider

The subframe was held into the Jag by 4 bolts on each side (2 per mount, 4 mounts). The way i'm attaching is similar, and will probably use at least the same amount of bolts on each side. The actual diff is held into the subframe by several different attach points.

I actually might modify the subframe so the coilovers bolt to the actual car, not the subframe. I could then narrow the subframe, shaving some weight off and raising it up into the car a little more.

As you can see in this picture, the "frame rails" prevent it from going up into the car any more, and if i mounted it at this height, the center would only have about 3-4" ground clearance, depending on ride height.

 

Did you continue this after running into the 3-4" clearance issue? Any updates?