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So, if you want an IRS in your fourth gen car, no qualifiers, there are a number of questions to answer.
Update: I can't speak to the quality, and it is still a cut weld affair, but Heidts has a kit to install IRS in a 4th gen ('93-'02) specifically.
Solid vs IRS
(Cars are both 4th gen F-bodies, but not the same car. Meant for comparison of 4th Gen SRA vs 4th Gen IRS)
What type of rear: The C5/C6 Rear Suspension
Someone is already doing this on a third gen Camaro.
For those less adventurous, just reusing the cradle and control arms could be the best option. A custom shop, 21st Century Street machines, had been making adapter plates with a custom pinion. However, they’ve since gone out of business, and with them went their adapter plate.
The adapter plate allowed the use of a standard drive shaft and transmission in place of the transaxle setup. 21St Century going under means you have to look at other options (Viper or Ford diffs), pricier if they don’t work with the existing Corvette CV shafts.
Recently, another rod shop, Hot Rod Jim, has stepped in with an adapter plate. However, it may not have yet reached production.
C4 rear suspension
The Dana 44 is a hot commodity in the rod builder community, and will cost around $1,500. The newest of them is now 14 years old, so it may need some TLC.
The wear items, like u-joints, wheel bearings, seals, pinion bearings and bushings probably haven’t been changed since these Corvettes rolled out of Kentucky new. Rubber bushings are available only with new camber, and spindle rods.
There are poly bushings, but poly is a poor choice in control arm applications (stiction, binding, metal fatigue, etc. covered ad nauseam elsewhere). There are a variety of bearing based (rod end) bushing solutions for more performance that can be used. The aforementioned items are definitely worth replacing, along with the usual bits (brake pads, and rotors if they are worn).
There is the Dana 36, used in automatic equipped Corvettes, which can be had for song. The Dana 36 is pretty resilient even under higher HP applications. As long these differentials are not exposed to drag racing, high rpm clutch dumps in first gear, or if you are using an automatic, then they will usually hold up for a long time. It ends up being cheaper and easier to begin with the Dana 36. If an owner opts to run the Dana 44 at some point, there are adapter plates to fit the 44 to the 36 batwing.
This post is going to focus on the C4 stuff since that’s the most “common” direction. There are some other options too, beyond the C4/C5/C6, but we won’t bother with them here.
Before going further, Who will be doing the work?
DIFM/DSFM (Do It For Me/Do Some For Me):
• A lot of shops quote hour time rates for an estimate. Even a very modestly priced shop at $40 an hour that takes 100 hours minimum to do this swap, not including wheel fitment, shock and spring matching, alignment, PCM reprogram, and exhaust is going to be expensive.
DIY (Do It Yourself):
• What is your skill set?
How much are you willing to spend? The elephant in the room is how much does this modification cost? For a conscientious do it yourselfer, this might be not be too costly an expenditure, but for a lights out, do it all for me, install you could be looking at around $15,000+. A member here, 99_orange_SS, was able to do the entire swap for about $1,100 total. He was able fabricate everything himself, and got a steal on the rear assembly.
The C4 uses four virtual swing arms created by drawing an imaginary line through each of the arms to their intersection points, starting from the bottom center of the wheel for the strut rods and half shafts.
(Courtesy of 1MeanZ)
As an engineer from Art Morrison pointed out, the operating range to keep the geometry on the C4 optimal is pretty tight. How you arrange those arms affects a lot, including anti-dive, anti-squat, camber, and toe just to name a few.
To extend his point, the C4 has a number of draw backs. Because it’s a trailing arm suspension those trailing arms force the wheels to travel in an arc from front to rear, as opposed to side to side with a solid axle located with a Panhard bar. The wheel base length changes slightly as the wheel travels through its motions. This is why trailing arm angle, length, and placement are so crucial! At the edges of this arc bind occurs with the strut rods which control camber. Add camber, and/or toe, and you can really exacerbate this issue, hence the reason poly really is a poor choice in this application and rod ends are optimal.
Again, because of these things, the window to keep the camber curve optimal is smaller than say a double wishbone setup, like the C5 or C6. Many points on the C4 are rubber mounted, the diff carrier (bat wing), and all the control arms. Extreme cornering will distort the center points on each of these bushings.
In addition, because of its placement not in line with the half shafts, the toe control causes some toe change in compression and rebound.
So why use the C4 IRS at all? The handling isn’t terrible, it’s ok, and from a fitment standpoint it’s easy to customize for a variety of chassis’s. It’s reasonably inexpensive ($1,500 for a Dana 44, even cheaper for the Dana 36, $500 or less), and can be narrowed if necessary, and lots of stuff is available for it.
Really too, there isn’t much suspension travel with an F-body to begin with. At a common height for lowered F-bodies, there will be somewhere between 2.5” and 3”, more if the car is at stock height, of compression travel before the wheel touches the inner fender well. Limiting the suspension travel really mitigates the C4’s short comings. Also with proper geometry and a little sacrifice in ride quality, the handling can be quite impressive and far exceed the solid axle setups available for the F-body.
How do you get it on the car?
Securing the pinion: Torque arm, C-beam, custom pinion mount. There is somewhat of a space issue, and this presents a challenge to mounting a traditional torque arm. The C4 rear has the mount on the passenger side unlike the 10 bolt and replacement axles.
(Courtesy of Jay Cutshaw/Zeus Performance)
The standard C4 geometry may not be optimal for the f-body, and depending on your requirements may not be desirable anyway. The C4 is lighter, has a 53/47 ( vs the f-bodies 56/44) weight distribution ratio, and therefore different center of gravity than the Corvette. The vette’s wheel base is also 4.9” shorter than the f-body.
Stock Geometry
(Courtesy of 1Meanz)
Modified Geometry
(Courtesy of Jay Cutshaw/Zeus Performance)
Corvette also doesn’t have a back seat. Keeping the backseat and the C4 geometry is possible, but comes with some sacrifice.
(Courtesy of FlyDoc)
Do you want a sway bar?
If you keep the leaf spring, it will provide some anti-roll. Currently 99_orange_SS is using a VBandP race monospring with no sway bar which allows for great riding characteristics while still digging hard in the corners. Regardless, mounting the swaybar can be accomplished a couple of different ways. Stock corvette style, in front of the rear assembly, or a custom sway bar. Mounting the sway bar from the rear using the C4’s stock mounts will require removing more material from the frame and more fabrication.
(Courtesy of Jay Cutshaw/Zeus Performance)
Braking, and Traction Control:
The rear brake discs are a little thinner than the f-body’s, but that won’t make a noticeable difference. Some road racers say the rears are a little too helpful on the f-body to begin with, so some reduction in rear braking maybe helpful.
You could run the c5 rears on a c4 , but you will need the C5 calipers also. Doing this will eliminate the stock parking brake. You can, however, buy an aftermarket parking brake system, from Precision Brakes, it’s called a “Mr. Parker”. According to 99_orange_SS, he has the CTS-V 4 piston Brembo's up front with the stock rear C4 setup and he says it performs flawlessly; brake bias is perfect, the rears don't lock prematurely, and there is little nose dive.
ABS and Traction Control (ASR/TCS): Is it 3 channel or 4 channel? If it is a 4 channel it’s a lot easier to retain. The 95/96 corvettes came with the Bosch 5.0 system, and the 98+ f-body’s came with the Bosch 5.3 system.
The sensors from the 95/96 vettes will plug in to the existing wiring harness on the 98+ fourth gen four channel. The sensors will need to be salvaged from a junk yard. Most parts companies, for whatever reason, only keep the ’96 right-hand sensor in stock, if they stock them at all. The sensor pictured below is the correct type.
The prior versions of the sensors (BOSCH 2/2S) yellow connectors, those won't work.
Then there’s the exciter ring. The 98+ ABS/ASR(TCS) cars came with 47 tooth exciter rings, good news here is so did the 92+ Corvettes. If the car has a 53 tooth four channel ABS unit (97 and earlier) then new rings will need to be made. Costs will vary, but expect to pay around $300 to do so.
Things are murkier for the 3 channel cars. The best approach maybe to mount a smaller ring on the pinion if possible. This might require a different yoke depending the year of the C4 IRS that is being used.
Then there’s the option of just flat out deleting ABS altogether. SJM makes a kit for doing just that.
Track width, Wheels, and Tires:
The fourth gen rear is 64.72” flange to flange. The 90 to 96 vette rears are 63.25”, the earlier rears are 62.5” wide.
The weights:
10 bolt (without brakes and with gear oil): 146.4 lbs
Dana 44 C4 IRS (without brakes and without gear oil): 165.4 lbs
Overall weight increases by roughly 20 lbs, but unsprung weight falls by more than half.
Wheels:
A spacer or adapter may work if it doesn’t push wheels out past the fender. If that is not an option then the backspacing on the wheels could be changed. This option is trickier and more costly. If the wheels can’t be rehooped the only option is to order a new set.
If wide wheels (315 or wider) are being used special attention must be given to how much static negative camber is set, how much will be gained during compression, and the amount of suspension travel allowed. There isn’t a lot of clearance with the stock 10 bolt running 17x11’s, and with a boxed steel frames now occupying the spring pockets, for the C4 IRS, options for clearance are further limited.
The drive shaft needs to be dealt with as well, as it is too long. The stock shaft can be shortened, or a new one made. The rear U-joint will have to be replaced as well to allow it to bolt into the Dana 36 yoke.
Shocks and Springs:
It must be decided how the weight of the car will be sprung and what kind of shock will control those springs. Coilovers, or reuse the leaf? Coilovers will require additional bracing of the knuckle and the frame mounts. Neither was designed to carry the weight of the car; however there are kits out there to do this (Vansteel, Qa1, AFCO, DRM, Guldstrand). The lighter unsprung weight of the IRS will also change the required dampening. Depending on the new rates and setup, new antiroll rates will have to be established and a custom bar will need to be fabricated; unless you setup your system around the factory bar which will create its own set of issues.
Last edited by lees02WS6; 11-29-2020 at 10:17 AM.
Reason: grammar
Then there’s the exhaust to deal with. The stocker is in the way of independence. Options: chop it off, dump it, run it underneath and out the back (like a C4 vette does), or go true duals? Y-pipe, x-pipe, h-pipe, 2.5” or 3”, mandrel bent, mild steel or stainless? Obviously the exhaust has to change, but what options chosen will determine the cost. Be mindful of how the pinion will be supported. If the pinion support will be some sort of c-beam or torque arm that will need to be taken in to account when planning the exhaust.
Conclusion:
If you're looking at this option and thinking it would be great for handling, then the assumption made is that you're intending to race it. If that's the case, and assuming it actually did provide some advantage, what class would this modification be legal in? You might end up racing against yourself.
This is a modification that has only one justification, you wanted a 4th gen with an IRS. This topic comes up often, it's a shame the threads aren't consolidated.
I'm not trying to be a n00b or anything, but what all benefits can be had by swapping in an IRS rearend? Is the ride improved, and is there more to it than that?
I'm not trying to be a n00b or anything, but what all benefits can be had by swapping in an IRS rearend? Is the ride improved, and is there more to it than that?
Great writeup.
At the same ride height the IRS will ride better. It has a lower unsprung weight, and each wheel follows the road independently.
To relate a real world experience: when I used to drive over train tracks in my lowered f-body it used to be "brace for impact", now the car just soaks it up.
As stated the handling can be improved as well depending on how you set the car up (springs, wheels, tires, sway bars, and shocks). You can set camber and toe, and gain both dynamically which you can't do with a solid axle.
Lee is dead on! One thing I noticed from your sig to is that you are more into drag racing. So it would seem that this would not be a mod for you Stippy. The solid axle will always perform better in a drag situation. The benefit of the IRS comes as ride quality and far better handling characteristics in course racing or AutoX; however straight line performance would be lost.
At the same ride height the IRS will ride better. It has a lower unsprung weight, and each wheel follows the road independently.
To relate a real world experience: when I used to drive over train tracks in my lowered f-body it used to be "brace for impact", now the car just soaks it up.
As stated the handling can be improved as well depending on how you set the car up (springs, wheels, tires, sway bars, and shocks). You can set camber and toe, and gain both dynamically which you can't do with a solid axle.
Originally Posted by 99_orange_SS
Lee is dead on! One thing I noticed from your sig to is that you are more into drag racing. So it would seem that this would not be a mod for you Stippy. The solid axle will always perform better in a drag situation. The benefit of the IRS comes as ride quality and far better handling characteristics in course racing or AutoX; however straight line performance would be lost.
Thanks for the replies guys. I already knew a SRA setup is optimal for drag racing. I just like to read up and reasearch 'different' mods that not everyone out there wants to do, or has the ability to do. I wouldnt swap an IRS into one of my Fbodys, but if I ever pick up a 2wd 73-87 square body an IRS setup would be something cool to look into. Especially since it would be a driver. I have enough track cars lol!
some people want what they want.............no matter how expensive ............or BAD it is..........and you can get a IRS in a camaro............its called a 5TH GEN !!!!!!
some people want what they want.............no matter how expensive ............or BAD it is..........and you can get a IRS in a camaro............its called a 5TH GEN !!!!!!
some people want what they want.............no matter how expensive ............or BAD it is..........and you can get a IRS in a camaro............its called a 5TH GEN !!!!!!
Really? If you wanted a tank you can buy them from the middle east. They sell them pretty cheap nowadays. A 5th gen is a pathetic Car let's leave them out of this thread
Since that allows you to run a dual exhaust that looks the same as a vette does it also sound like a vette instead of a f-body?
Mine sounds similar to other f-bodies with true duals. The sound has to do with the material and diameter of the metal used, it's length, shape, type of cross over if any, type of muffler, manifolds, catalytic converters, acoustics of the car, and of course the engine.
This is a video clip of the exhaust system on my car:
08-31-2011, 06:14 PM
Indepedent Rear Suspension (IRS) in a 4th Gen 
