Despite a lot of work setting up the IRS in the Datsun 240Z to get it to launch hard at the drag strip, I still felt I had a need to make a modification. Before deciding to make the switch to the C4 IRS, the Datsun IRS was made up of the following components:
IMO, to date, this is the strongest setup that can be installed in a 1st-gen Z with stock Nissan components. The "weak points" in this setup are the CVs and the stub axles. However, when I refer to it being weak, I am thinking of a Hybrid Z with a stout V-8 or in my case, a Buick V-6 turbo with about 500 lb/ft of torque, launching hard off the line at a drag strip. Anything less or one not driven that hard should be fine with this setup. Custom-made CVs for this setup (~$500) and the R-230 swap could prove to be better. However, the R-230 swap using the same CVs and stub axles has no advantage other than an LSD because there is no issue with the strength of the R-200. It is important to remember that in most all cases, such a setup is being put together with used parts and you have to deal with the luck of the draw. E.g., stub axles and a R-200 pulled from a 75 280Z is 26 yrs old and the diff might still have the original lube in it.
So, why did I need to switch (other than wanting an engineering exercise. LOL)?
Alternatives
The Corvette IRS
I am sure there are other Z cars out there with a Corvette IRS, but to my knowledge, there are only 2 and I want to say thanks to both for taking the time to help me understand the swap and finalize the decision. You know who you are. I will jump ahead and say after getting the IRS and sizing it up, I wonder why others have not done this. What drove the final decision was an opportunity to purchase the IRS at a very reasonable price AND someone purchasing my entire Z IRS to help fund this project. The particular unit I purchased is a DANA36 with a 3.07:1 ratio from an '84. So, what are the pros and cons of doing this swap? The obvious con is that there is no kit and you have to be willing to pay someone to do a lot of fabrication if you are unable to do it yourself. For me, the pros as I see it so far are:
Here is a frontal view of the C4 IRS. The thing that jumps out at you are the beefy 3.5" axles. Below the axles are 2 arms that go from the bottom of the diff to the bottom of the hub and using a cam on the diff end, adjusts camber. The 2 items with the 3 holes are referred to as "dog bones" but are equivalent to suspension links you see on hot rods.
From the rear you see a massive triangulated bar that is used to hang the IRS, similar to the "moustache bar" on a Z. However, that bar also servers as the diff rear cover and the mount point for a couple of pieces. From the top you see a piece that looks like a large wheel cylinder with 2 rods that attach to the top of the hub. The ends are similar to a tie-rod but it is passive and serves to adjust the toe. Below that you see the transverse, monoleaf, composite spring which weighs about 5lbs!
The measured distance from the outside edges of the tires with the Nissan IRS was 66". That was with 275/50-15 tires on 15x8 Weld wheels with a 4.5" backspace. Keep in mind my Z has fender flares. My objective was to end up with the same basic dimensions. The C4 IRS, measured the same way, was 70" with 245/50-16s on 16x8.5 wheels with a 32mm offset. That means there is approximately 3" from the inside mount point of the wheel to the outside edge. Add about .5" for sidewall bulge to make it 3.5" on each side and a total of 7" for both sides, making the hub to hub width of the IRS 63". Way too wide for a stock-fendered Z and leaving me only 1.5" to play with on each side. I decided to narrow the entire unit 3" which would give me 3" on each side and with late Z-28 or Trans-Am wheels that should solve that problem. Those wheels are 16x8 with about a 6+" backspace. Will consider something "fancier" at a later time.
Narrowing the C4 IRS at first would seem daunting, but looking carefully at the unit, it ended up being fairly simple. The following items need to narrowed:
Update 10/26/01 - I decided to forego the leaf spring/shocks and go for a coilover shock setup. More details below.
Installation
Here then is a summary of the installation steps I have laid out. As the project progresses, I will keep updating this page and since I have installed the unit 100 times already in my head, I do not expect any deviations. Yeah, RIGHT! Knock, Knock. "Who's there?" " A Mr. Murphy to see you".
Obviously not a project for the timid or mechanically-challenged. Asked myself a couple of times just what in the hell did I get into, BUT, it would not be a challenge if I did not ask that.
The Nissan IRS is out and some reference points marked.
Trial fit. Hubs not connected so everything is hanging low.
Trail fit. Here you see the suspension link hanging. Final position will have the 2 bars about parallel and the mount point in the lower inside corner of the fender well. That part of the fender well will need to be trimmed and a solid mount point fabbed and welded in. Also note that the mounting eyelet of the triangulated bar ends up at the bottom of the strut tower and does not protrude into the wheel well so it will not have to be modified. The hanger will be fabbed and welded to the frame rail at the bottom of the strut tower well.
Here is one of those points when I questioned what the hell I got myself into. Here you can see the fabbed hanger in position on the bottom-right.
Here you see some minor sheet metal trimmed from the base of the strut tower well and the hanger welded in place on the frame rail. A little sanding, rounding off the lower edges and paint should pretty it up nicely. The entire unit was made of 3/16" flat. The horizantal piece welded to the frame rail is 7"x2". The 2 side pieces are 6"x3" spaced 2 5/8" apart. The other piece adds rigidity. Actual cost - $70/pr
Update 10/26/01 - The hanger location did not work and was relocated. With the halfshafts and hubs connected, there was some minor interference with the top of the hub touching the bottom of the hanger when the hub was jacked up simulating extreme squat, which a Z-car does quite nicely, thank you. Hi, Mr Murphy. Nice of you to drop in.
This picture shows the relocated hanger. If you can imagine the hub coming up, you can see it would touch the hanger in the original location which was more outboard. The hanger was cut off, modified and relocated directly under the big bolt that the original Z moustache bar was bolted up to. This necessitated that the triangulated bar be narrowed 3" on each side. The aluminum welder thanked me for the business and took my $120. I was glad to pay it as the triangulated bar did not have solid surfaces to work with and is angled from the diff out.
This picture also shows the custom-made, shortened and adjustable camber arms at the bottom of the IRS. Look at the picture above of the IRS from the rear to compare the difference. The picture also clearly illustrates how the suspension adjustments will be made. Loosening the lock nuts on the camber arm and turning the rod causes the bottom of the hub to move in or out, changing the camber. Loosening the locknut on the toe adjustment bar and turning the rod causes the rear of the hub to move left or right, changing the toe. You can also see that the leaf spring and its mounting brackets are eliminated.
To support the front of the differential, I used the original differential cross-member and fabricated a bracket to connect the differential to it. With this design, if the pinion angle needs to be tweaked, a spacer between the cross-member and the frame lowers it or a spacer between the bracket and the differential raises it.
The next step was the big challenge, connecting the suspension links.
YIKES!! Up to this point, the only mod to the car was to cut off the big bolts for the moustache bar to make room for the hanger picture here. At first, that hole was frightening but after seeing what others have done to their cars to make similar modifications, I realize I just "nicked" mine. I did have to cut into the frame rail at the bottom of the hole but it was a very short piece where it ended. I will replaced it with a much beefier piece. This is the extent of "cutting up" the car to make this IRS fit and that I think is quite amazing.
This picture also gives a good point of reference for where the coilovers will be installed, so I will discuss that now. If you refer back to the picture that shows the entire unit from the front, you see the original mount point for the shocks. In this picture you will notice the shiny portion of the hub at 11:00 behind the rotor lines up perfectly with the center of the strut tower. I have decided to use a coilover shock that will have its bottom mounted on top of the hub and the top will be mounted in the original mount location at the top of the strut tower. More details on this when I get the shocks and do the installation.
Here is what the link looks like from inside the car and the hole does not look so bad from this angle, LOL. Next was the fabrication of a mounting bracket with substantial bracing to take the expected forces from the suspension, especially under hard launches.
I will add some more bracing but have not decided how/where at this point. Once I have the first test session at the track and check for stress in the welds, the area will be dressed up.
As previously stated, I decided to forego the C-5 monoleaf spring and standard shock and go with coilovers. This is not uncommon, but I decided to mount the top of the shock in the Z-car strut tower and the bottom of the shock on top of the IRS hub. By my measurement, this required a shock with an extended length of 15.5". Most of the well-known shock companies have a limited choice in custom shocks and a 15.5" shock usually has ~3.5" shock travel and a 10" spring. I chose the Aldan Shock Absorber Co because they are a custom shock manufacturer. Aldans shock are made from an aluminum billet and has a good reputation in the custom hotrod and drag racing worlds.
After listening to my requirements, they made me a pair of shocks with a 15.5" extended length, 4.5" shock travel, 2.5"/160# spring with a 12" length and adjustable rebound, $380/pr/shipped. The key to getting a 12" on a 15.5" shock was extending the threads as far down on the body as possible.
Here are the 92-96 Z-28 wheels mounted up for a trial fit with 255/50-16 BFG drag radials.
Took the car for its first test drive, made it one block and panicked. Felt like the rear wheels were about to fall off. Turns out the inner ends of the toe adjustment rods were so badly worn, I could move them in/out about 1/16". That translated to the rear wheels wobbling badly. Mad with myself for missing that. VBP sells a custom replacement with heim-jointed ends for $260/pr. I decided to make my own and here is the results.
I had a difficult time finding a tapered rod to fit the IRS. GM, in its wisdom, makes all the front-ends with the same taper but the C4 IRS is different and unique. Instead of a taper, I drilled out the tapered hole in the hub to 7/8" and inserted a 5/8" id bushing (7/8" od). I then used a 4" 5/8" bolt with a stack of spacers on both sides of the heim joint. Repositioning the spacers that sandwiches the heim joint changes the angle of the rod and therefore the bumpsteer. The center bracket is make out of 1/4" flat with spacers to equal the thickness of the original. The rods are 5/8" aluminum tubes with threaded ends and the heim joints are Aurora 5/8" with LH and RH threads. The rods and joints were purchased from Coleman Racing.
Here is a shot of the custom toe rod installed in the hub. You can also see how the shocks is attached to the hub and fits in the stock strut housing. The shock does not have a bump stop so I made one using the Vette bumpstop.
Driving Impressions
With the IRS aligned, the setup really feels impressive. It is going to take a little while to get used to it. With the front of the diff mounted solid (my choice) and every rubber replaced with polyurethane, this thing is incredibly tight compared to the stock Nissan IRS. It is also a little noisier, as expected. Initial tests launching the car lights up the grin-o-meter. A 2psi launch on the transbrake with 15psi boost, lights up the 255/50-16 ZR tires for 250' !!! Of course, that is totally useless, but the important point is that the car launches straight, with no vibrations or dramatics. Will find out how well it really works at the strip with the drag radials, 7psi launch and 24psi boost.
For now, it was definitely worth the time, effort and expense.
If You Are Considering This Swap
Having worked on this swap for a little while now, I will make a statement for others considering it. I do not imagine there is going to be many Z-cars looking to make this swap. A high HP V8 Z that does not drag race or even a high-HP L28 turbo that does drag race does not need this. The Nissan R-200 or R-230 with CV axles will do the job. If you still have a desire to do it but also want to retain the stock fenders, my advice is to not consider this swap. The unit, even when narrowed, is still too wide for a stock-fendered Z and I would not advise narrowing it any more. If you plan to use flares wider than mine, then you are in luck. Nothing would have to be modified. What this means is you save the cost for all the modifications I did. The only thing you would need to do is fabricate a mount for the top of the shocks (left in the original location). The original hangers I designed could be used without interference and the links would be a little more to the center of the fenderwell requiring very little cutting, but a well-braced mount point would still be required.
If you just absolutely have to do this swap and you have or plan to get fender flares, then heed this warning. These Corvette IRS can be expensive. I previously stated this was the least expensive alternative for me because I got the IRS at a good price and sold my entire Nissan IRS to help fund the projects. If you are mechanically-challenged, do not have some fabrication skills and cannot weld or have access to a good low-cost welding, then this could be a very expensive swap over and above the cost of the unit. I feel confident that this document will be a good guideline but I have no intentions of providing any specifications for the cutting that I did and brackets that I made. Since this swap was never done before (at least the way I am doing it), I am "making it up" as I go along and I am amazed to find minor differences as I fabricate things from one side of the car to the other. Considering this and the differences between the model years, there is no way I could confidently give specifications from my swap and be assured it will fit all.
Stay Tuned....
Feel free to 
if you have any questions or comments.
Click to go to my Home Page.