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A new, affordable approach to Bullet Proofing the IRS
Well we all know how weak our C3's IRS is when given power and traction. I have raced my 79 vette with the costly 12 bolt conversion, 3 1/2 inch halfsafts, 31 spline outer stubs. this was all around 2009 with my 496 bbc with a bunch of nitrous. Later on I built a ford nine inch center for my 540 turbo combo. Today that car is a full tube parallel four link car set up for Radial Vs the World. IRS is long gone.
Now I acquired a New project corvette and wish to make it a solid 8 second, possibly 7 second street car with Stock Suspension to compete in a few classes around my area. I could do the tried and true way of the 12 bolt conversion, but after racing with my c6 vette against some of the newer 2015 and up mustangs, I realized those guys are running Stock untouched IRSs and doing 8 second quarter miles.
Surely it wouldnt be hard to use what they got in our C3's. they are ford 8.8's thay they dub the super 8.8 because they now are 34 spline carrier trac loc differentials. they are proven to be very strong.
then the next part of the equation is the weak outer stubs and wheel bearings. I know C7 corvette rear wheel hubs are cheap, about 75 bucks each on rock auto. They are 33 splined.
so you would no longer need the cast wheel bearing housing, welding a plate to adapt the hub to the trailing arm, you would have to fab up lower strut mounts attached to the trailing arm and you would have to make tabs on the trailing arm to bolt the calipers to.
then its the fab work to make the ford 8.8 bolt into the c3 which is a matter of brackets.
I already called The Drive Shaft Shop, the people who make all the 1000-1700 hp rated CV axles for C 5,6,7 corvettes,along with 1700 hp axles for 2015 mustangs. they said they can make 1000 hp axles for around 1000 bucks for this application and 1700 hp axles for $1699.
mustang differentials sell used for around 400-500 bucks on ebay, or a brand new mustang iron 3.55 gear 8.8 is 720 bucks. you would need to swap out the front pinion yoke to a normal 1330 yoke or up grade to 1350. they are about 100 bucks.
so all said and done, $720 for a diff, $150 for C7 rear hubs, $1000 for CV axles rated for 1000 hp and $100 for the 1350 yoke. $1970 in parts and then the metal fab work to mount everything.
Sorry, haven’t been getting notifications for this.
Moser shortened the axles for me. I’ve put some miles on the setup and made many hits on the street with 275 pro radials with no hiccups. Even with a pretty aggressive 1-2 shift on nitrous nothing has shaken loose yet. Haven’t made it to the track for the ultimate test of a dead hook on nitrous. The setup has taken the abuse I’ve given it on the street and i have no reason to suspect it won’t live at the track too.
Sounds great to me. We have upgraded our steering (Borgeson), headlights (electric conversion), suspension (coilovers), etc., why not new type rear end? I'm all in for that conversion kit!
You could be on to something. I figured a sheetmetal housing 9" could be made easy enough...but this looks easier with mounting points already on the diff.
One small issue I notice with the ford 8.8 is the two mounting wings on the forward part. those look like they will go right into the battery and storage boxes. you could either cut the batt/store boxes for them or trim off the ears on the diff and make a collar clamp over the snout and use that for the attach point to the forward part of the C3. I dont have one in front of me yet nor have i removed the stock one yet to test fit it up.
This looks like a great idea, my only concern would be that modern IRS systems do not use the half shafts (or CV joints) as a stressed member like the pre-C5 corvette suspensions do. Will this differential take the thrust loads of our suspension design?
I always had the six link design in my drag vettes. I plan on setting this one up with the upper struts also. This swap would use CV axles instead of half shafts, which has a few advantages in my book. lower diameter, strong joints, smaller safety loops, quicker removal/install.
Not sure what you mean by thrust loads and how its any different from any other IRS design. do you mean the transverse spring perch? I plan on using coil overs for my design, but a spring perch can be easily added, just make it integral to the upper crossmember rather then part of the 8.8.
I always had the six link design in my drag vettes. I plan on setting this one up with the upper struts also. This swap would use CV axles instead of half shafts, which has a few advantages in my book. lower diameter, strong joints, smaller safety loops, quicker removal/install.
Not sure what you mean by thrust loads and how its any different from any other IRS design. do you mean the transverse spring perch? I plan on using coil overs for my design, but a spring perch can be easily added, just make it integral to the upper crossmember rather then part of the 8.8.
The C5, C6 and C7 suspensions use a double wishbone setup that transfers the lateral forces from the tire to the frame. The 2015 Mustang uses an upper lateral link to transfer these forces. The C2, C3 and C4 suspensions use the half shaft as this link requiring thrust bearings for each shaft yoke. Let me see if I can get some pics to illustrate.
Well thats exactly why the six link is done on these cars, it removes the load on the half shafts and transfers it to the upper bracket installed on the stock differential. But in this case the upper link can be a tab off the new upper crossmember to be fabricated.
Well thats exactly why the six link is done on these cars, it removes the load on the half shafts and transfers it to the upper bracket installed on the stock differential. But in this case the upper link can be a tab off the new upper crossmember to be fabricated.
Wow. That is nice! And no more lateral loads on the stub axles or diff parts. Very nice idea without the high cost. Actually, seems like a better idea than the high cost one.
You could be on to something. I figured a sheetmetal housing 9" could be made easy enough...but this looks easier with mounting points already on the diff.
JIM
this project actually started with the intent to fab a sheet metal 9” IRS, but a lot of the one off axle stubs and such ran the cost up quickly. Using the 15 mustang diff and C7 hubs/bearings makes it a lot more “off the shelf” parts wise.
When I built my Factory Five car a lot of the guys building the Cobras were using the IRS differentials out of the 90"s Ford Thunderbird's.
Factory Five had three rear suspension setups to choose from, an IRS, a three link and four link.
The three link was actually used more in their road race car versions than the IRS.
Here are some photos from their website gallery.
IRS
Three Link
Four Link
The street rod guys have had 9" IRS setups for years...but they are pricy. Heidt's etc?
I've got a Dana 60 housing from a Hemi car made into an IRS under mine. Took a little fab work to install but it's been there for many years without touching it.
11-27-2017, 08:01 PM
A new, affordable approach to Bullet Proofing the IRS
