Removing irs unit -best way ?

[Melb-Mike]

I have a MK4 with the older T-Bird style IRS unit. I didn't do the build so I'm a bit handicapped. My IRS has a 2.73 ratio and with the base 5.0 engine, it's a little lackluster in acceleration. I would like to change the gear ratio on this 8.8" rear end somI need to remove it. I did a search on this forum and didn't get much info. It sure looks like a tight squeeze in the frame with the cover plate bolted to the frame. It looks to me like I need to remove the cover from the diff in order to be able to drop it down from its perch. Of course I will have remove the rear hubs, half-shafts and anything else attached to the diff.

Question I have is there an easier way to get this done or is it going to be as I envisioned it ?

[Gordon Levy]

Get help, you will need another pr of hands. Preferable strong hands. The unit weighs 85lbs and you need to manipulate it to drop it out nose first. First unbolt it and turn it on it's side and drain the fluid then clean and duct tape the stub axle holes. After manipulating it around for 10 or 15 minutes spouting expletives you should be able to get it out.

[Joecobr]

I have a set of brand new FFR half axles for this setup for sale cheap on this forum. You may as well replace these if yours are rebuilt donor ones.

[Railroad]

I have 2 of the complete rear gears and housings. They are stock removals from the 99 Mustang Cobras. I think the ratio is 3.27 or 3.31. It has been a long time since I looked. They are like new, low mileage with 28 spline side gears. I will sell one for $250 if that is a better route than changing the gears out. Right now, shipping is not an option. I live in central Alabama. Good luck,

[Melb-Mike]

I have 2 of the complete rear gears and housings. They are stock removals from the 99 Mustang Cobras. I think the ratio is 3.27 or 3.31. It has been a long time since I looked. They are like new, low mileage with 28 spline side gears. I will sell one for $250 if that is a better route than changing the gears out. Right now, shipping is not an option. I live in central Alabama. Good luck,

Your offer sounds pretty good. However is it the old style of IRS ? Mine is. I'm not sure which year since I didn't build the car but from parts listings I've seen it looks like a mid 90's 8.8" rear end. The other issue is getting since you said shipping is not an option. I will be racing at Barber Motorsports Park in late August. Barber is in Leeds, Alabama, about 25 miles East of Birmingham. Are you in that area ? Also we need to know what ratio your differential is. I don't want guess only to find out it's what I have already.

[Bob Cowan]

Like Gordon said, it's kind of a pain. Remove the axles, and cover the holes with duct tape. Remove the rear cover, and cover that with duct tape, too. That helps keep some of the gear oil inside and makes less of a mess.

Rotate it around so the axle holes are pointed fore and aft, instead of side to side, and the nose is pointed down. Be careful. Once the diff is pointed in that direction, it almost falls out. It is heavy, so you need a couple of pairs of strong hands. Or, I used a motorcycle/ATV jack, which was a lot easier. Especially getting it back in. The jack makes it a relatively easy one man job.

[Railroad]

Your offer sounds pretty good. However is it the old style of IRS ? Mine is. I'm not sure which year since I didn't build the car but from parts listings I've seen it looks like a mid 90's 8.8" rear end. The other issue is getting since you said shipping is not an option. I will be racing at Barber Motorsports Park in late August. Barber is in Leeds, Alabama, about 25 miles East of Birmingham. Are you in that area ? Also we need to know what ratio your differential is. I don't want guess only to find out it's what I have already.

While working on the roadster today, I looked at the housings. They are 3.27 gears with trac loc clutches and 28 spline.
I think they are the same as the T birds, but would need someone to confirm for sure. I am about 25-30 miles from Barbers.
I shoot skeet about 10 miles from the track on weekends. I will PM my contact information to you and let you do the research.
The housing from my personal car had less than 3000 miles on it and the other guessing about the same.
To give you full disclosure on these items. They were replaced by Ford in several of the 99 Mustang Cobras. This was Ford's first year for IRS and the drive lines were to a higher spec and tolerance. Guess what, Ford's assembly line did not get the right trans, and driveshaft in the Cobras. Many received the Mustang GT equipment, same ratios, but looser tolerances. The consequences were a mild buzz at 2000 rpm, felt in the drivers seat. Ford put a complete driveline in a test car in Detroit, verified no vibration, pulled it out, put it on a pallet and shipped it to my dealer. They pulled and swapped the driveline, which included a new rear end. Ford did not want to pay shipping on the pulled parts, the dealer was scrapping the trans, ds and rear end. I was able to keep my gear housing and they found another that did not go to the scrap yard. I missed all the transmissions. Anyway, the gear housing were not defective, it was run out on the driveshaft flanges and yoke and the tolerances in the transmissions. I met with Fords NVH engineer. He said, the buzz was the result of a stacking effect, a little run out on the ds, a little in the trans, input and output and they coincided creating the buzz. Some owners got a correction with rotating the driveshaft to a sweet spot. The engineer brought a monitor we zip tied to the rear end. He knew the harmonic frequency of each part of the drive line. Mine showed up as the ds, but rotation would not bring it into spec. He was a super guy and did everything to my satisfaction. While I am doing the history, I might as well recognize the engineer. His name was Bill Franco, again super, super person.
I thought I could feel a little buzz after the fix and bought an alum ds. Smooth

[Melb-Mike]

After looking at the way the IRS is bolted in, it looks to me that the cover plate support attachment likely gets in the way. I'm thinking of making it a detachable support by welding a plate to where it is now welded and making a matching plate to which the cover plate is bolted to. This would add clearance to the opening that the IRS drops down from. I can see there is enough room for me to TIG weld the plate to the frame. I'm wondering if anyone else has done this. Being a newbie to the FFR scene, I hate making stupid assumptions.

[rich grsc]

In the time you spend making and welding brackets you can have it out and back in. Turn the pinion up, rotate it so the axle opening are parallel to frame and it'll come right out.

[Bob Cowan]

And, it's not like you're going to be doing this job very often. If you're lucky, you'll never have to do it again, ever.