Back by popular demand and now that the swap is 90% done, I'll post a condensed and more instructional thread on how to swap a Dana 44 C4 IRS into a thirdgen. Also as I move along finishing the swap I will update as I finish my rear sway bar (once the parts come in) and begin to dial the car back in for neutral handling.

From my research this swap has been done on 5 cars before me.

The first car that was done was a GM concept car called the Fire Fox. No details or pix given, but it is interesting that GM seems to have considered it for the 4th gen. Here is the article. http://www.gtasourcepage.com/91FirefoxTest.html

Rapid Rods, a user here has a car with an IRS swapped thirdgen. His pictures don't reveal anything and he is not forthcoming with any information nor does he reply to my PMs. I'm not sure if he is even the guy that did the swap or if someone else did it. Here is a link to the only pix of his car he has produced. https://www.thirdgen.org/forums/album.php?albumid=2253

A guy named Jay Cutshaw has a metallic orange 4th gen Formula that has a Dana 36 swapped in. (dana 36 is the lighter duty IRS found in automatic trans equipped C4s) The pix of this setup are easily found online and the swap was done by a shop Zeus Performance. On this particular swap they did not use the stock corvette trailing arms or arm locations which vastly changes the suspension geometry. They also cut away more of the car than I feel was necissary and they did not reinforce the differential mounts or "batwing mounts" as they are called. There are also no pictures of how they supported the front of the differential where the C-beam attaches (looks similar to the F-body torque arm). The front of the differential must be supported and there is no info on how they did it. The pictures also do not show a rear sway bar. They claim the car now has a rear sway bar but there are no pix of it. This is a very clean looking swap on a beautiful car even though there are a few things I would have changed. It could also be that the owner of the car didnt not want to cut the wheelwells to mount the trailing arms which would explain the use of non-stock parts and geometry. There is always more to the story. Here are the pix from Zeus' website http://zeuscatalog.com/jay_cutshaw_s_irs_swap

There are 2 other thirdgens sneaking around with this swap done. One is a yellow convertible that was done 10 years ago and most of the pix are lost. There is also another car that someone's brother or something did on here but the pix are taken from the ground and you can't see much, but it does look like an OK job.

And that I think is the proper history lesson on these swaps and a little background on who has done it. I'm not the only one who has done the swap, although I am probably the only one that will be abusing this setup on road courses and autocross.

I'll begin posting my pictures with explanations with descriptions in the following posts. It will be done in steps as I have time. There is a lot of work to outline so stay tuned.

 

Part 1: Why am I doing this swap and why have I chosen the C4 Dana 44.

My 383 stroker makes a little over 500ftlbs torque. After a few years of abuse on the street, autocross, and a hard track day, my stock rear axle was starting to leak from the pinion seal. The Zexel Torsen posi I had installed years ago didn't have enough bias ratio to spin both rear tires under a hard corner with my road race slicks on. I was looking to upgrade my rear suspension and upgrade to a stronger rear gear set so I didnt have to worry about grenading my stock diff. Bolt-in 9" and 12 bolt rear axles are very expensive and much heavier than the stock rear axle. I didn't want to spend tons of money on an axle that would damage my handling. I could fab in an 8.8 axle like some have done but again, that was a large amount of fab work for 0 performance gain. If this was a drag car the choice would be very different.

I chose a Dana 44 Corvette IRS from a C4 corvette. Mostly because it can be mounted in the stock locations as the stock coil springs and re-use the stock shock absorber locations. I would be able to install the axle and not disturb any of the original loading points on the chassis. The only other attachment point is the trailing arms and I was planning to tie them into the roll bar that is being installed in the car at the same time.

There is another C4 IRS that looks nearly identical to the Dana 44 I'm using. It is the Dana 36. It uses the same suspension pieces, but the differential is smaller. The ring and pinion are basically the same size as the stock 7.5 axle so there would be no gain in strength over the stock axle. The Dana 36 is much cheaper and easier to find, and if power levels would have allowed, I would have used the Dana 36 instead.

Below are pictures of the unit when I bought it. You can see the 5 attachment points in the picture. 2 diff mount ears, 2 trailing arm pads, and the pinion support that still has the C-beam attached. In a corvette the C-beam attaches to the tail housing of the transmission. Corvettes don't have a transmission crossmember, the whole driveline hangs on the motor mounts and the diff mounts. In my application I'm going to mount the pinion to the roll bar in the car. Others made a "torque arm" like device that attaches to the floor or the rear of the trans. As long as the differential housing can't rotate under power it will work fine.

 

Subscribed!!!

I see a very educational & interesting thread!!

( X2 Mongoose )

 

+1

 

Part 2: The beginning.

To start the project off I decided I needed a datum surface to measure from to get the new rear suspension square in the car, and properly positioned front to rear. I attached a piece of aluminum angle across the back of the car. It was perfectly paralell to the rear axle in the car. This what I used to make sure all pieces were correctly positioned front to rear in the car. Below you can see the aluminum angle across the back of the car.





Once the rear axle, exhaust heat shields and panhard bar reinforcing strut was removed as shown, the cutting began. I marked and cut the panhard bar mount and the journey began.

 

Part 3: coil spring perch mods

The next task was to flatten the rear coil spring perches. I needed to flatten the perches so I could attach the diff mount brackets. There are a few layers of metal here so I cut the spring perch 'bubble' off as best I could. I then cut slits in what was left and hammered it flat and welded the edges down. I then cut reinforcement panels and welded them in where the spring perches used to be so I would have a flat clean place to weld the diff mount brackets. Notice the hole in the middle of the reinforcing panels is welded to the frame, this was to prevent the panel from "oil canning" in the middle. The welds look pretty crappy because I was welding over my head with is far from ideal. I also couldn't grind the metal as clean as I wanted because I couldn't fit the grinder in all the nooks and crannies, nor could I get a good torch angle with the welder since I was basically welding in a corner. The pics make the welds look worse than they really are. Once the welding was done I was comfortable that they weren't going anywhere.

 

Part 4a: placement of differential

Once the coil spring perches were finished it was time to begin locating the differential in the car. This is one of the places that I made a mistake on this swap. I mistakenly assumed that the center of the hubs liked up directly with the center of the output yokes on the differential. In other words, I measured from my datum bar forward and placed the output yokes on the diff at the exact same location as the hubs on the stock axle. The problem with this is that the vette suspension is made to locate the center of the knuckle approximately 1/2" BEHIND the center of the output yokes. So later on when I went to install the knuckles the wheels were too far back in the wheelwell. Lucky for me, I was still able to force the knuckle forward so that it was correctly located in the wheelwell. The bushings in the lower control arm (just below the half shafts) allow enough deflection to allow me to move the knuckle forward where I wanted it. This winter I'll change the stock lower arms out for adjustable heim joints and everything will work out just fine. The only other ill affect from locating my diff too far rearward is that at full suspension droop the toe link ball joints come close to hitting the rear leaf spring. Again, a very minor issue that will not affect the performance or handling of the unit as far as I can tell. This is just one of those things you run into on a project like this, nothing is ever perfect and this is the one spot where I goofed up.

Here I was using a home made jack to start rough locating the diff in the car.

 

WOHOOOOO HE BROUGHT IT BACK!!!

 

yes I am bringing it back. I have taken a lot from this forum over the years, I can contribute back for once. Please bear with me as I'm trying to really do an in-depth write-up on this and it's time consuming. I'm in the middle of moving nearly 2 hours away, changing my job, and finishing my IROC and my Trans Am. My goal is to make this write-up complete and a resource that someone could use to do this and not make the same mistakes I made.

 

Subscribed. I remember starting to read your thread on frrax but forgot about it until now. Can't wait to see it done.

 

Keep up the good work. A bunch of us are following your progress and want to see you finish this trick mod. Good luck on the move and job.

 

Looks good! Thanks for tuning us all in.

 

A lot of good info.

 

This is a mod I have dreamt about doing to my cars.

 

Subscribed!
That's what I've been planning to do for a long time now.
I can't wait to see the whole progress together.
Best regards from Hungary, Europe!

 

this is really sweet. Subscribing!

 

Hey guys just get the back half of a vette, I paid $250, then just weld it in,make your tubs and your done.You will loose your rear seat and fuel tank.Then roll the IRS under and bolt in I used chevy gas tank on the inside and then made a false floor with vette gas filler and door to finish it off. No fumes in car either. I will post pics soon.I also did the conversion 12 years ago ,and only finished in 09. Notice the old Morris Minor in the garage it has a 2002 Camaro stuffed into it.Some old pics www.majormotorshotrods.com

 

rapid rods--
There may be good tips in your closed-for-construction site.Thanks for the link.

I believe the point 1MeanZ is showing us is some one with fab & suspension ideas, along with some personal drive, can do this, & with out molesting the car. IDK, maybe he will have to do tubbing & no back seat.

No, I don't have experience in fab or suspension--this stuff IS VERY INTERESTING.

In the future, I will check out your site, but for now I am watching the OP's work.You folks with fab skills make it look easy!

Meanwhile... at the 1MeanZ hideout...

 

Part 4b: placement of differential

I used my datum bar of aluminum to center the rear front to back, and to make sure it was perpindicular to the center line of the car. Then I adjusted it side to side by measuring off the outside of the fender lips. I wanted to locate the diff up as far inside the car as possible. This allows me to run the car at a low ride height, but still keep good suspension geometry. I moved the diff up about 1/2" below the fuel tank straps.

Once I was happy with the squareness and position of the diff I made the first brackets to weld to the car. I used 1/8" material. I added gussets later, but for now I just wanted to get the diff in the car in the correct location.

 

Part 5: Knuckle and trailing arm placement.

Once the diff was in the correct position I began cutting small amounts away to get the knuckles and trailing arms to fit. This took forever because I was a wimp and didnt want to cut too much out.

Eventually I got the cutting done and mounted the plate for the trailing arms. I started out by knowing what ride height I wanted, in other words, how far from the fender lip did I want the knuckle. Then with the knuckle at the correct distance away from the fender I set the correct angles for the trailing arms. I re-used all the factory corvette angles. I set the center of the knuckle at 17" below the fender lip, set the angles on the control arms, bolted my 1/4" mounting plate on and welded it to the car. Again, more gussets to come, just trying to get things roughed in.

You can also see in the first pic I cut off the part of the inner fender where the stock bump stop is mounted. I needed to do this to make room for the knuckle and for tire clearance. More on that later.

In the second pic, look along the bottom of the big hole I cut. That is where the rear subframe stamping attaches to the floor pan. On this and every other place I cut on the car, I welded the metal back together. In other words, any place I cut that had more than one piece of metal thickness, I welded the metal back together on the cut edge. This adds a tremendous amount of strength and gives me a robust place to weld panels back in.

 

Part 6: Additional wheel clearance and gussets.

The next task was to open the wheelwells up and box in the trailing arm mounts. The pix of this are pretty self explanatory. I cut the bottoms of the wheelwells off, cut the floor all the way to the frame rail, cut slits in the fenders to allow for some expansion, and welded new metal in the gaps. This gave me about 1" of additional fender clearance.

Again in the pix of the driver's side cutting you can see more seam welding...








Since I already knew how much to cut, I just made all the right cuts the first time over on the driver's side.

 

Very cool. You have us on the edge of our chairs.

 

dying for more!

 

sorry I'm so slow on this guys. My life is upside down right now trying to finish building our house and move in when it's 2 hours away. I'm adding pics and detail as I get time.

 

I want to do the same mod to my RS, other than the Drive-in Cool Factor. Are there signifigant handling gains to make this mod worth the work over just a watts link? Does the trailing arms locating prevent installation of the rear seat? I know that you are not going to use a rear seat.
NOT everyone needs to chime in with their opinions. I just want to hear from 1MeanZ.

 

I'm in for the updates. AWESOME DIY work here. I'm digging it.

-Tom

 

How did you set the pinion angle? Are the trailing arms adjustable? Do you need to set it straight with an IRS? Diff. = directly behind the tranny?

 

Pinion angle should never be straight. U-joints need to be run at a slight angle or the needle bearings won't travel and the joint will get flat spots, vibrate and wear out.

I set my pinion angle with a regular craftsman gravity type angle finder. The pinion is 2* nose up and the trans is 2* tail down.

I have adjustable length trailing arms, but when I put the car on the alignment rack it was so close I didnt have to adjust them.

FYI guys, I just moved into a new house and don't have internet access there yet. I'll have that ironed out in a few days and will be back to finish this build thread. thanks for being patient.

 

i cant wait to see more!

 

So the tranny points right at the rear? +2* up at the rear -2* at the tranny, that would be 0* right? I must be confused on how to accualy set pinion angle. +2* up and -2* down from level and they are at different heights off the ground from each other? More than the 4* difference Im imagining?

 

Yeah. If the tranny was 2* up & the rear 2* down that would give a 4* difference. Maybe he got them switched when typing it out here?

Otherwise, you'd get a direction shot, even at different levels, right? Unless they were offset side to side as well.

 

He has it right. COming out the trans the angle goes 2*, the the shaft is somewhat level lets just reference it at 0* then the pinion is -2*.

Basically at the ends of the shaft one anlge is up 2* and the opther goes down 2* so the "cancel" each other in harmonics.

You need angle on the u-joints. The tailshaft can not come out and be straight zero degree with the drivesahft because the needle bearings on the u-joints need some angle resting on them to spin properly as well as self lubricate.

 

But 1meanZ said "The pinion is 2* nose up and the trans is 2* tail down". So since the trans is down & the pinion up.....Are they at the same height/level?

I guess I was thinking the trans higher than the rear end.

 

I do not know why this is so hard to for you to understand. Its quite simple and clear to me what he wrote, and then I even gave you a further example explination and we both explained why the u-joints can't be at 0* on either end.

Maybe you should just wait and look at what pictures he will later post.

Stephen, it does not matter if the drivesaft is 168* yaw or 279* pitch.. The end of the trans must misalign with the drive shaft by a certain degree for needle bearing movement, then that same degree should also be at the other end in OPPOSITE form to cancel harmonics.

 

Thats an unknown, to us anyway. Could be higher, could be lower. The relation of the two lines we know though, they're both parallel. Thats about as far as it goes as to what we can determine from that little bit of info.

Hey meanZ, are you going to add a plate to where the ears mount for the C4 diff? I mean the two spots where you have a steel plate on either side of the ears, it would help (resist bending of those two plates) if you closed in that one side. It looks like there is space to do so, without interfering with anything you've done so far. May not do much, but would help stiffen things up if need be.

 

It isn't difficult at, to understand what I said.

If the tranny is higher & pointing down 2* & the rear end is pointing up 2*, that would equal 0* combined.

But if they are level, then yes. The 2* of each would correctly cause the needed rotation in the needle bearings.


That is what I was getting at. You were so quick & eager to find fault in what I was saying, you misread what I said. If the tranny was pointing down & the rear pointing up, both 2*, they would have resulted in 0*. Resulting in zero needle rotation.

Look at pics? Can you see 2 degrees in an internet posted pic?


I know the two of us rarely, if ever, see eye to eye. But damn dude.....For once I am AGREEING with you! Be happy.

 

looks good meanZ.

stephen, i understood what you asked because i was a little confused too. If the trans is pointing down 2* and the rear end is pointing up 2* that would mean it is a straight driveline and would get no needle movement. Like you said, if they are at the same height i guess it would make a little bit of an angle and that would be enough to keep the u-joints happy.

 

meanZ this a very interesting swap i am looking forward to your updates.

now about the pinion angle needing to be offset to spin needle bearings in
u-joints.
correct me if i am wrong but aren't the u-joints there so the drive shaft can spin without binding as the rear axle goes up and down.
and with the independent rear suspension the differential stays in place it don't go up and down and the transmission stays in place. so if they are both staying in the same spot are u-joints necessary. couldn't you just get the differential and the transmission shaft lined straight up with each other and run a drive shaft that don't have u-joints. i hope they makes sense i know i don't know much about this stuff but that makes sense to me. am i an idiot or would that work.

 

thanks forthe awesome thread. i know what im doin now

 

any updates?

i am getting the stock 454 from an 82 chevy truck and a t56 from fourth gen. and the rear end is the only thing i haven't figured out yet i was thinking a dana 60 or ford 9 inch cuz they are known to be strong axles. but this wont be a drag only car i also want to do autocross racing so i want it to handle great. so i was wondering if one of these rear ends would be able to handle a 454?

 

looking good so far. i was really interested in this the first time around... great project
oh, and to clarify the pinion angle debate...
99% of all passenger cars will look like the upper pic
in fact, most trucks as well.
its not till you start lifting your vehicle that you have to worry about the second pic

 

alright i am confused first you say "the vette suspension is made to locate the center of the knuckle approximately 1/2" BEHIND the center of the output yokes."and you put yours in line with the center of the knuckle. now if i read that rite you got it a 1/2 inch to far forward. but then you say "locating my diff too far rearward " which is it to far back or to far forward? can you please clear this up.


also i thought vettes had coil overs but you said "the rear leaf spring."was that just a typo?

 

coil-overs are one option, but the preferred suspension type is indeed a leaf spring suspension.
dont think for a second however that it shares anything at all with the leaf spring suspensions of old.
the corvette IRS uses a transverse mounted leaf spring, ie, the single spring connects to both control arms and is mounted to the "frame" of the rear end in the middle.

it acts a lot like a sway bar, except that instead of controlling body roll, it is the suspension.
as a bonus, it also does a great job of controlling body roll, allowing you to get away with no rear sway bar if you dont want one.

i say that it is preferred because coil-overs give you very little benefit for the cost involved, and you have to add a larger sway bar, which adds to the cost even more.

 

alright i see a coil over but i can't find a leaf spring going any direction? does he just not have one.

also do the trailing arms eliminate your backseat?

 

The diagram should help. I mistakenly thought that the output yokes on the diff (green circle) were directly in line with the center of the knuckles (red circle) I was wrong, as you can see on the right side of the diagram the set back is about .5". So I put the diff in with the green circle in the center of the wheelwell, then when I went to hang the knuckles the wheels were too far back in the wheelwells. So the diff is too far rearward in the car, the knuckles are where they are supposed to be. make sense now? This has had no poor effects other than that I lay in bed at night knowing it's not right...

No C4 vettes came with coil overs, but they are a popular add-on. The unit I bought came with them and I was planning to use them, but they wouldn't fit when I tried to re-use my stock upper shock mounts. I wanted to use the stock upper shock mounts (shock loads can be extremely high so I wanted to re-use that part of the chassis) so I stuck with my Koni yellows on the back, they are fantastic shocks. They are now controlling a much lighter unsprung load and a different spring rate than they were valved for, but so far they seem to be working nicely on full soft.

 

The half shafts are basically like 2 more driveshafts. They cannot be directly in line. They need that 2 to 3 degrees (or so) offset to cause the needle bearings of the u-joints to rotate & live. If they were directly in line, they would burn up, just like a driveshaft would.

 

The rear leaf spring wants to bend in a U shape, either up or down. In order for one knuckle to move up and the other knuckle to move down (like they would during body roll) it would force the spring to bend into an S shape which it does not want to do. So a transverse leaf spring does add to roll stiffness.

It could be because of the low ride height or stiff spring rate, but my car does not need a sway bar in the rear so far with street tires. The tires simply don't grip enough on the street to force the car to roll over. I fabricated a rear sway bar and put it on the car and I can't tell it's there. Now once slicks are mounted on the car, they will probably grip enough to force the car to roll a little bit, in which case the rear sway bar will be needed.

 

true statement, but I thought they would be in line fore and aft, and the up and down movement would keep the needle bearings moving. At factory ride height, the halfshaft runs downhill away from the diff as well.

 

In the photos there is no leaf spring present, it is pictured later on. Yes the trailing arms are where the back seats should be, again more pictures on that as I continue with the write up. If you go to the first post and review the pics of the swap by Jay Cutshaw at Zeuss Performance, you will see how he moved his trailing arms so he didnt have to cut the wheelwells.

 

If that were the case, then the up/down movement of the rear axle would suffice.

Since the tranny & the pumpkin are both centered the angle helps to move the needle bearings & the only sideways offset comes from the flex of the rear springs allowing sideways movement in most cars, although 3rd gens get the "benefit" of the panhard bar swinging the axle side to side.

Since it is relatively simple to offset the half shafts forward/backwards as well as up/down.....That essentially makes it as "better" on the needle bearing than what the driveshaft sees.