Mustang IRS? kinda long and rambling

[Chaparral2f]

I need to change the rear suspension in the Z31. It needs shocks, springs, and a full set of bushings. I don't want to spend that much on a diff that isn't limited slip, and has a ratio that is way too low for me to run on the street. (th350 no od) I want to go with the 8.8" Ford out of a Cobra. What I have in mind is to build a differential subframe (much like a Jag) and double A-arm suspension. I can do the fabrication, but have been putting this project off for a long time. I'm getting tired of the stupid car squatting under acceleratrion. If the car is launched hard, I swear I hear the exhaust hit the ground. Here is what I need to find out. Will the mustang half shafts handle the horsepower and torque? (392 hp/ 420 ft lb torque) and will the double frame suspension deep the car from dragging its tail?

[Clifton]

8.8's are cheap and alot lighter than R200's. It would be alot of work though. There was atleast one guy that used a Z32 rear subframe under a Z31. They are a little wider but it worked. You could use a Q45 3.54 with it if ti's low enough. Just a thought.

[tube80z]

I race wtih a guy that uses this setup in a pickup truck, an extremely fast pickup truck I might add. He's running similar levels of power and using sticky tires. He's not had any problems with axles but has blown up a number of the LSD units.

 

As far as dragging the tail that will have a lot to do with the geometry of the suspension. You'll probably need to build in some anti-squat.

 

Cary

[boodlefoof]

Unfortunately, the terms anti-squat and IRS don't go together very well.

 

You can increase anti-squat by angling the front mount of the lower arm upward from the rear. This will raise your instant center (the imaginary intersect of the angle of the two control arms in side view) and in turn your anti-squat figure. However, by the time you've gained any appreciable anti-squat you will have reduced your side view swing arm length (distance from rear axle to instant center horizontally) to the point that you will experience brake hop. The solution is then to instead just add weight over the rear (like the rear mount transmission on the C5 'Vette).

 

Otherwise, if you're already going to go through all of the fabrication trouble, you will probably be better off with a solid axle system. It is easier to fabricate, typically will handle more abuse, and can be designed to give greater than 100% anti-squat.

[blueovalz]

I remember seeing some kind of issue with the rear differential mount on this IRS set-up. Something about the failure in the aluminum cover (on the back) breaking when a lot of torque is applied. I did see a fix for it in that the new part (aftermarket) used a bracket the distributes the torque load over the many bolts that hold the cover on rather than just the two that are positioned in the middle of the cover. This may be my imagination, but I'm pretty sure I saw this.

[jbc3]

The stock Cobra rears are 355 gears... not that far off from the stock R230. I think the R230 would be a good choice, it seems to be holding up to my abuse.

 

The Cobra IRS has a bracket on the rear cover that supports the rear to the frame. It is the weak point with the center section. There is a fix, it is a girdle that bolts directly to the perimeter bolts of the cover.

http://www.billetflow.com/irs_brace.htm http://www.steeda.com/store/-catalog/555-8118.htm

The axles are also an issue. They have level 5 axles (~$800.00 I believe) but I have seen the high HP Cobras break them also. It has alot to do with the wheel hop on launches. It seems that many of the the high HP Cobras are routinely swapping to solid axle rears because they are tired of dealing with the breakage.

 

I know that I have really tightened up my rear squatting issue by using double bump stops on my coil-overs in the rear. One set hard and the second set soft. I used to feel the same as you describe about the exhaust dragging on launches. No more.

 

Jody