Irs Swap Into Fox Questions

[FoxMustangLvr]

Will a 1999 Cobra IRS swap into a Fox? All I see are the 03/04 IRS rear ends for a swap. There is a 99' IRS complete rear end for sale locally and I'll never exceed the HP of an IRS but would love the ride quality and handling that goes with it.

Here is the CL post

Mustang Cobra Independent Rear Suspension

@RacEoHolic330 @bird_dog0347

 

[stykthyn]

That's a heck of a deal. I'd jump all over that.

 

[elarm1]

Do it!

 

[FoxMustangLvr]

So the 99' Cobra IRS is the same as the 03/04 but it's probably just a little weaker?

 

[2000xp8]

read all the way through

03/04 IRS vs 99/01 IRS - Forums at Modded Mustangs

That price isn't anything special. Not ripping you off, but you aren't stealing it either.

 

[FoxMustangLvr]

read all the way through

03/04 IRS vs 99/01 IRS - Forums at Modded Mustangs

That price isn't anything special. Not ripping you off, but you aren't stealing it either.

Thanks for that link. My car is just a street car. No drag strip and no road course or Auto X racing. I think this rear end would work ok for my intended use and I'd upgrade it with some MM bushings and other recommended upgrades. Do you agree?

 

[2000xp8]

As long as you do what it takes to eliminate wheel hop, which is what destroys the irs, you should be fine.

Just take note that if somehow you land up with sticky tires and more power, the level 5 half shafts are enough to break anyone's budget at $1600.

And make sure you have the wheels ready, not many sets fit right with an irs. Which is why I didn't do it.

 

[FoxMustangLvr]

As long as you do what it takes to eliminate wheel hop, which is what destroys the irs, you should be fine.

Just take note that if somehow you land up with sticky tires and more power, the level 5 half shafts are enough to break anyone's budget at $1600.

And make sure you have the wheels ready, not many sets fit right with an irs. Which is why I didn't do it.

Good food for thought, thanks. I'll keep doing my homework on this.

 

[FoxMustangLvr]

As long as you do what it takes to eliminate wheel hop, which is what destroys the irs, you should be fine.

Just take note that if somehow you land up with sticky tires and more power, the level 5 half shafts are enough to break anyone's budget at $1600.

And make sure you have the wheels ready, not many sets fit right with an irs. Which is why I didn't do it.

Good food for thought, thanks. I'll keep doing my homework on this.

 

[smokin joe]

Just for reference the last IRS I saw for sale on CL in Dallas they were asking $1200.

 

[FoxMustangLvr]

Just for reference the last IRS I saw for sale on CL in Dallas they were asking $1200.

I've never seen an IRS for sale but always heard they go for about $1500 for the Termi 03/04 versions that are a little more beefed up. I've never seen a 99 for sale nor did I know that 99+ Cobra came equipped with IRS. I'll probably pick it up tomorrow after our weather settles down today.

 

[90sickfox]

The 2003 to 04 IRS has 31 spine axles and a brace near the front. That's why you don't see a lot of people running the 99- 02. I run an IRS in a fox and haven't run into wheel hop problems yet in 7yrs. Guess I'm lucky.[emoji12]

 

[FoxMustangLvr]

The 2003 to 04 IRS has 31 spine axles and a brace near the front. That's why you don't see a lot of people running the 99- 02. I run an IRS in a fox and haven't run into wheel hop problems yet in 7yrs. Guess I'm lucky.[emoji12]

I'm not worried about having a 28 spline axle, that's what the straight axles have and they hold just fine for the HP level that I'll be staying in. The extra brace that's missing, is that something that can be purchased and added to the 99' IRS?

 

[stykthyn]

I believe so. MM carries parts for those. I'm sure they have all the goodies.

 

[Black1987]

Personally, I hadn't heard much about the capability of the 99 IRS except it was the weakest. I came across an 01 IRS which 31 spline and personally can say they can take a beating. Over on the SVT forums, IRS Informational Article is the go to for information and bushing kits. Hope that site can help answer some of your questions.

 

[FoxMustangLvr]

I tried about an hour ago to meet up with the guy and he just sold it. O'well, I'll keep an eye out for another.

 

[bird_dog0347]

Sorry I'm late chiming in here, but yes, it fits the same. The axles are 28 spline instead of 31, it didn't come with Bilstiens, and it doesn't come with the front diff cross brace like the 03/04 does. Also the 03/04 comes with 3.55 gears stock. IF you see another one at that price, snag it. I got my 04 unit for $500, the same day it was posted for sale.

When you install it, (after you get one of course) make sure you don't ruin the gears or pinion bearing swapping the flange. Also, at a bare minimum, install all poly bushings for control arms, diff mount, and subframe mounts. I didn't have the money to go FTBR or I would have, and I'm into the full poly bushings under a couple of hundred installed myself. Wheel hop isn't TOTALLY gone with the poly bushings but it's MUCH MUCH better and there's no noticeable difference in NVH.

Don't forget about the other expenses involved, things like exhaust as the mufflers are different, shocks and springs (I went full MM coilover), brake lines, and possibly wheels as fox width 5 lug wheels won't have the proper offset and about a 9" wide wheel is all you'll fit back there. I've got 275's on mine with no rubbing on a 9" wheel and the MM low profile subframe mounting bolts. The 9" wheel will require about a 6.72-6.75" backspace (+45ish offset) which is pretty tough to find sets with proper sizes for front and rear.

 

[90sickfox]

Just FYI to anyone swapping a fox. I had to shorten my aluminum driveshaft for the swap. I found that installed the yoke was all the way in the trans...almost riding the output seal. I had mine shortened and since haven't had any problems with vibrations or wheel hop. Its the 04 model with stock cobra springs, bushings, and (hate to say it) Monroe blue fox shocks with spacers on the bolts that go through the control arm. The only reason those high priced yellow and blues are needed is because the factory bolts are thicker. I opted for different bolts that fit the factory fox shocks and made plastic sleeves for them. I don't know why the driveshafts work for every other swap I've seen accept 1 other car. Under load the pumpkin moved just enough to move the driveshaft further in the trans ( t5 ) to cause a vibration. I did drive it for a couple years with a built 347.... Pushing about 400hp. Not one problem. Aligned it to cobra spec plus just a little negative camber...to help tuck the tires.

 

[bird_dog0347]

Just FYI to anyone swapping a fox. I had to shorten my aluminum driveshaft for the swap. I found that installed the yoke was all the way in the trans...almost riding the output seal. I had mine shortened and since haven't had any problems with vibrations or wheel hop. Its the 04 model with stick cobra springs and (hate to say it) Monroe blue fox shocks with spacers on the bolts that go through the control arm. The only reason those high priced yellow and buds are needed is because the factory bolts are thicker. I opted for different bolts that fit the factory fox shocks and made plastic sleeves for them. I don't know why the driveshafts work for every other swap I've seen accept 1 other car. Under load the pumpkin moved just enough to move the driveshaft further in the trans ( t5 ) to cause a vibration.

Did you replace the bushings in the rear with poly or Delrin? Also, with these cars being 20+ years old and not the most solid platform to begin with each one will be a little different and could be twisted in different ways to cause what you noticed.

 

[90sickfox]

Stock bushings. When I pulled it out of my shop I felt a weird series of bumps when acting a donkey. Lol I put it back on the lift and reinspected everything...all the bushings were good. Noticed the driveshaft and the fact that there wasn't hardly any play. The driveshaft was shortened about 1 inch...reinstalled and not a single problem.