IRS curious.....

[blueboss]

I'm looking into adding IRS to the car for DD duties. The 7.5 in the car is tired, starting to make noise and I'm not interested in rebuilding it. I don't have the required equipment to perform this task anyway so I'd have to pay someone to do it regardless. Which aint cheap. I've thought about the 8.8 sra but I'd rather have the cool factor of the IRS. I see these things going anywhere from around $600 to $1300 in various states of repair. I know you get you pay for but what should I be looking for or what questions should I be asking sellers for tipoffs as to the history of the rear end? Thanks gentlemen.

[Raven855]

The 99 Had 28 spline and all the rest had 31 spline diff and half shafts which was no real improvement as the spline count was not the problem. The 99 & 01 shafts taper down at the CV boot and create a weak point. The 03 Shafts are a constant diameter and are by comparison much stronger. The outside shaft is 28spline on all models. The 03's also had stiffer bushings along with better shocks, stiffer springs, the addition of a forward differential pinion brace, and lower inner toe control mounting points. These improvements made it handle a little better, improved durability and reduced bump steer. The 99-01s will have 327 gears and the 03/04s will have 3.55s. The first IRS were subject to a recall
"Number Involved: 8,100


Models Involved: Mustang Cobra, Mustang Cobra R


Dates of Manufacture: November 1998 - August 2000


[NHTSA Recall No. 01V121/Ford Recall No. 01S15]


Defect: If the ball joint assembly where the lower control arm attaches in the rear knuckle on certainof these cars moves forward or backward from its installed position, greater stress will be placed on the knuckle casting. If this occurs, the casting could fracture, allowing the corner of the vehicle to drop, and the lower control arm could contact the inside of the rear wheel. In some cases, steering of the vehicle could be reduced.


Remedy: Dealers will replace both rear suspension knuckles. The manufacturer has reported that owner notification began on April 19, 2001. Owners who do not receive the free remedy within a reasonable time should contact Ford at 1-800-392-3673."


The big issue is the up rights, if the torque values were not followed while being assembled they cracked, specifically do not over tighten the tapered shaft on the rod end or bump steer shaft. This will distroy an upright. Follow the factory torque setting exactly.
From William Mathis:
"Let me see if I can help clear up a few things about the IRS of the Cobra. There are three basic IRS systems used in the Cobra Mustang, each with its own set of problems. The 99-01 IRS was designed for a smooth ride and low NVH. To achieve this, soft bushing were used in the subframe and the control arms. Ford mandated that the car ride smoothly and so soft springs, dampers and bushings were used. One of the main issues with this system was the location of the inner tie-rods pivot, which produced a small amount of roll steer. I know the designer of this setup pretty well and saw the original prototype in action. He believed in the possibility and designed this in his spare time. It is amazing how close this came to not going into production. When he submitted the original design, the higher ups demanded the usual understeer be designed into the system. It actually started out as a modified Lincoln system.


The second generation IRS was installed in the 2000 Cobra R. It had very high durameter bushings in the control arms as well as a pinion brace, amoung several other durability and road racing modifications. The half-shafts were stronger and unique. Additionally, it was equipped with 750 lb/in Eibach springs and road racing valved shocks. This was a significant improvement over the "smooth ride" version that was in the 99-01 Cobras. Unfortunately, the wing demanded the very high spring rate. The more correct spring rate of about 675 lb/in was inadequate to offset the downforce of the wing. The high rear spring rate created a frequency ratio greater than 1.2 and a pretty choppy ride.


The latest rendition found in the 03 Cobra was designed with handling in mind. The control arm bushings are higher durameter than the previous Cobras, although less than the Cobra R. It was equipped with 600 lb/in springs and semi road racing valving. Additionally, Ford lowered the inner tie-rod pivot to match the Cobra R setup to improve the roll steer. Unfortunately, the subframe bushings are pretty soft. This was actually done to provide some rear steer to the car. Hope this helps."


The springs are different: Front/Rear
2000 SVT Cobra R 800/750
2001/99 SVT Cobra OEM 500/475
2003 SVT Cobra (Conv.) OEM 500/470
2003/04 SVT Cobra OEM 600/600
2003/04 SVT Cobra Convertible 500/470

Several people have installed the 99 units, w/470# springs and kepted the front SVO Springs(410#)and liked them. Ideally you should beef up the front springs to match the rears or be within 5% spring rates for ride and handling. See M&M for rear shocks with the 470s.

Oh, the sway bar was 1mm bigger on the 03/04s too. I didn't address the brake issues and balance. I bought a 01 unit off of Craig's list last year for $200. Also check svtperformance.com. You will have to register though

[Raven855]

Boss, I updated some info in this thread. The big item is that the differential is 28 spline on the 99. That is why you can usually find them pretty cheap. I have an IRS unit at the lake house that I will be putting in the 2R. I still need to get the 3.73 gears, a hop brace, shocks and the SS brake lines before I start rebuilding it.

The other item I updated was that the 99/01 coup rear springs appear to be 475 lb springs and the All vert ones are 470s.

[MikeFleming]

... that the 99/01 coup rear springs appear to be 475 lb springs and the All vert ones are 470s

So the difference between 475 and 470 is maybe they used a slightly harder paint on the 475 Lb/In springs?

[Bob Holmes]

So the difference between 475 and 470 is maybe they used a slightly harder paint on the 475 Lb/In springs?

LMAO!

[Raven855]

So the difference between 475 and 470 is maybe they used a slightly harder paint on the 475 Lb/In springs?

Are you mocking me?

I have have differing p/n and differing specs, but I suspect it's a Ford, SVT, FMS thing like the Cobra control arms. The 03 Cobra arms are no longer available, but the replacement control arms from Ford are the Cobra spec arms. Just different p/ns. So in Ford speak "they are the same, but they are different, So we can charge you more. It's a win win for everyone!"

[MikeFleming]

Well, dayum. Seems like the 475s are too harsh for the little lady and it bottoms in the large dips with the 470s. I wonder if i can get 473s or just shim the 470s?? Does anyone have IRS spring shims?

Yeah - mocking you.

[Bob Holmes]

I keep looking at this swap as an alternative to building my own torque arm for the 7.5.

It'd be great drive one before making the decision.

[Raven855]

For an everyday driver the IRS wins hands down!

[caroth]

Nice reply Raven, that is more information than I have found anywhere else.

I have a 99 IRS but it has the Motorsport center section with 3.73 gears. One spindle was busted so I got it cheap. I picked up another spindle but have not done anything else with the parts. I recently found a Magnaflow cat back without mufflers for cheap that will be the exhaust under the IRS.

I have stock Cobra shocks and springs but I think I want to go with coilovers. Any feedback on coilovers?

Also, fulltiltboogieracing has replacement bushings for the IRS, will the delrin bushings transfer lots of NVH into the car making it no fun to drive?
http://www.fulltiltboogieracing.com/mystery_of_irs.htm

If upgrading the bushings and such, where should a person start? Subframe? Control arms? I mostly just drive my SVO on the street and don't drag or road race it. I do drive it like I stole it.

[Robertosvo]

I have replaced all the bushings on my IRS with MM stuff. I had everything apart for detailing so I did it all subframe, control arms, and Differential.
I dont have before the bushing comparisons, but the ride is not harsh at all, I am using the stock IRS springs cut down 3/4 coil iirc

[Raven855]

From what I have found the kit from FTBR is actually as quiet as stock. The noise will come from the slop in the subframe assembly. You can get rid of the dreaded Cobra clunk by replacing the front 12mm bolt in the subframe with a 9/16 bolt or the low profile 14mm from M&M. The subassembly was designed to be installed on the assembly line with the regular mustangs. It uses the same pickup points as the standard differential. Because the line had to slow down to install a 14mm bolt, they subbed in a 12mm to make it easier to install. If you install coil overs you will have the same issues that exist with any coil over in that the body was never intended to handle the stress and you WILL have to beef up the shock towers, just like the torque arm guys. I intend to install the FTBR kit this weekend, if my halfshafts come in. I have the FTBR tools when you need to install those bushings by the way.

I will add that I talked to several people in evaluating the kit from FTBR and M&M. The consensus seems to be that the poly bushings in the M&M kit make it noisier than the delrin!

[Raven855]

Are you using the 470ish or the 600# springs?

[caroth]

Raven,

Do you have the IRS in the car now to compare before and after on the FTBR setup? What type of shocks and springs are you going to be running?

[Raven855]

It is currently out for rebuilding. I am waiting for half shafts. Since I had the unit out for cleaning and rebuilding I decided to grease and put new boots on the half shafts. Well boot prices were 20 to $25 and the price for the whole shaft with boots, bearings and the shaft was $70! Hopefully the shafts will be available tomorrow.

Here is what my finished IRS set up will be:
99 Cobra IRS with 470# rear springs(stock 99) 94-04 v6 front springs(450#)
Bilstein shocks and struts from M&M
3.73 gears 28 spline with torsen T2R
Complete FTBR bushing kit with competition type tierods and adjustable sway bar end links
Inner tie rod mounts modified and lowered
Outer half shafts boots modified for breathers
Billet differential cover brace
9/16 bolt in front subframe
03 anti hop brace
rear sway bar bushings from M&M
rear Mathis subframe brace with interior bracing as well
lizard skin and sound padding on interior
3 " dual Cobra tailpipes
SS brake lines
new rotors and pads

According to Bill Mathis most of the Noise will come the rear subframe bushing and mount. You can also eliminate alot of the compliance of the rear end by beefing this bushing and mount up. When you take out that flex it is a noticeable improvement in handling and more noise.

[caroth]

Does anyone know the different in length between the IRS shocks and the SVO rear shocks? Coil overs sound like a good goal, but I would rather get moving on the IRS so running the stock Cobra stuff seems doable. I am anxious to find out the results of the FTBR bushing kit, so keep us informed.

Thanks,
Chris

[Bob Holmes]

Tony, where'd you get the information from Mathis?

[Raven855]

Tony, where'd you get the information from Mathis?

Corner-carvers. He posted there as MustangTaz. He has not posted for about 6 years. He address the IRS, handling, NVH, spring rates, modifications and some cheap shocks. Less than 40 posts total. Very good info.

[Becools]

I went with the full tilt boogie delrin bushing kit in my 03 irs. Its one of the more affordable ones.. sorta

[Raven855]

I went with the full tilt boogie delrin bushing kit in my 03 irs. Its one of the more affordable ones.. sorta

How do you like it? Would you do it again?