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The C4 Corvette IRS Swap |
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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
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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:
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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:
Click to see the shortened axles and toe rods and the custom camber arms
Update 10/26/01 - I decided to forego the leaf spring/shocks and go for a coilover shock setup. More details below. |
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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. |
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Trial Fitting
On the left, the Nissan IRS is out and some reference points marked. On the right is the initial trial fit. Nothing is connected and the IRS is just resting on the jack, but it already looks at home.
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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!
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|
The Biggest Challenge The next step was the big challenge, connecting the suspension links.
On the left, here is what the link looks like from inside the car and the hole does not look so bad from this angle, LOL. On the right 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. |
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Shocks and Springs
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 you see the shock with a very simple top mount bracket to bolt it up to the Z car strut tower.
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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. Update 05/03/02 - I have been dealing with a nagging problem of how the IRS deals with "square edges" in the road. I suspect I am dealing with the problem of suspension binding as a result of the polyurethane bushings in the trailing arms. This is apparently a known problem (found out too late) and need to decide how to correct it. If you are considering this swap, DO NOT use polyurethane bushings in the trailing arms if the car is driven a lot on the street. Works great on the track but gets old on the street. |
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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. |
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
you see
how the shocks is attached to the hub and positioned to fit up into the stock strut
tower. The
shock does not have a bump stop so I made one using the Vette bumpstop. Here you
also see a shot of the custom toe rod mentioned below. 
Mounted up the Z-28
"Salad-Shooter" wheels with 255/50-16 BFG
drag radials. BTW, these wheels weigh at about 17lbs each. 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 
