Cobra IRS swap for 93 notch

MTV593 · May 7, 2018

Anyone know where I could source a cobra IRS in the SC/GA/northern FL area that isn't gonna make me broke. I wanna swap it into my 93 coupe and yes I know it requires some fabrication.

90sickfox · May 7, 2018

They are getting very hard to find..... especially cheap. The sought after 03 04 use even harder to get lately.

It doesn't really need fabrication. Just a brake line adapter, master cylinder, 93 cobra booster, e brake short cable, remove axle bump stop brackets, and drill 8 holes. I chose to shorten my aluminum driveshaft. Just about bolt in. I dont call drilling fabrication.

Stay on all the normal sales spots....CL... FB...EB...

MTV593 · Apr 4, 2019

Alright so have been on in a while but... finally got a hold of an 03 cobra IRS. A couple questions for anyone out there with experience with the swap. Am I going to need a new driveshaft? Rear mounting point advice, I was planning on drilling the bottom bolt holes through to the trunk with a 1/4 mounting plate in the trunk, but I’m unsure what I want to do about the hole I need to relocate, I was thinking extending the bracket and drilling a new hole. Any issues I can’t predict or I am not seeing?

eighty5fox · Apr 4, 2019

i just finished up my irs swap i did not need to do anything with the driveshaft but i also have a coyote swap so not sure how much my motor is moved compared to a small block. Also i believe the pinion flange needs to be swapped on 03-04 units not 100% positive on that i have a 01 irs and did not need to change anything. And for the mount i drilled and tapped plates and slid them into the frame rail after cutting a hole in the trunks floor. And you probably might want to upgrade your bushings while you have it out. Good luck

FoxMustangLvr · Apr 5, 2019

I have a 90' hatchback and I did NOT have to shorten my stock driveshaft. You do need to remove your pinion flange from your SRA and install it on the IRS. 03' IRS has 3.55 gears from the factory, you may want to very it's still that way and wasn't regeared at some point.

All finished up along with the brackets that will drop in from the holes I cut in the hatchback floor. I used 3/16" steel, 1.5"x6" with 1.5" grade 8 bolts.

Brackets installed

FoxMustangLvr · Apr 5, 2019

I detailed my install pretty well in my build thread if you want to check it out. It starts around page 43. An alternative to cutting the trunk floor and bolting through the bottom frame is that some people just weld the bracket to the frame. I see nothing wrong with that so long as you feel this is a permanent install.

https://www.stangnet.com/mustang-fo...ra-irs_-kenne-bell-twin-screwd.878656/page-43

90sickfox · Apr 5, 2019

My IRS went in without a hitch. You shouldn't need a new driveshaft. I had my aluminum driveshaft shortened but plenty run without modifying driveshaft.

I opted to make ( about I inch ) holes in the trunk floor to drop bolts and washers through. I then used rubber plugs to cover the holes.

It's not an overly complicated process. The axle flange has to be swapped.

Looks like a really nice IRS. Very clean.

When you install the driveshaft there has to be some room for play at the transmission seal. I shortened my driveshaft because I wanted the seal to ride in the same place it did with the SRA. My driveshaft also had some surface rust on the yoke. Didn't feel confident that there would be enough room for movement...and that the seal would seal properly in a different location. ( I did this almost 13 years ago ). Was one of the first to do it.

Since the stone age people found that its perfectly fine to use a stock length driveshaft.

The relocation of the IRS into my notch is in my build thread, too.

MTV593 · Apr 5, 2019

-Dope write-up makes me far more comfortable doing mine, and I think I might do the hole in the trunk method. I'm waiting on MM bushing and sleeve kit for the IRS not a big fan of taking things in and out a bunch of times. Any advice on the pinion flange swap? I'm grabbing a beam type inch pound tq wrench today. I am going to measure the drag before, pull the cobra flange, install the smaller diameter flange, and than tight until the drag matches the before measurement. Sound right?

-it is a 3.55 the junkyard verified and I verified once I got it.

FoxMustangLvr · Apr 5, 2019

MTV593 said:
-Dope write-up makes me far more comfortable doing mine, and I think I might do the hole in the trunk method. I'm waiting on MM bushing and sleeve kit for the IRS not a big fan of taking things in and out a bunch of times. Any advice on the pinion flange swap? I'm grabbing a beam type inch pound tq wrench today. I am going to measure the drag before, pull the cobra flange, install the smaller diameter flange, and than tight until the drag matches the before measurement. Sound right?

-it is a 3.55 the junkyard verified and I verified once I got it.

I'll let Scott (90sickfox) give advice on the pinion flange install. I dropped my center section off to a local reputable driveline builder and he installed FRPP 3.73's and new clutches and seals for me along with the pinion flange swap.

I did everything else myself and I'll tell you that even with the MM bushing removal tool the bushings are a PITA!!!! I removed and installed all of my subframe bushings and I threw in the towel afterward. I still had all of my upper/lower control arms to do so I called up my local shop and they R&I all of my control arm bushings for $50 and the turn around was really quick and it was totally worth it.

FoxMustangLvr · Apr 5, 2019

Here's some IRS motivation for ya

90sickfox · Apr 5, 2019

I'll be honest. I " ugha dugha'd " this flange on years ago. I didn't even change the crush washer. Went by feel. That had to be....40- 50k miles ago.

Take about 3ft piece of angle iron. Get it to go across two bolt holes.....angle should be facing away from flange bolt so you can grind the flat edge to fit flush on flange with a couple bolts.

It takes around 400ft.lbs. to crush a new crush washer to get to proper preload.

When I set up rears for customers I put the pinion in before anything else and properly set it up. With the gears already installed you want everything to go back to where it was. About 150lbs will get you there using the old crush washer.

The last couple performance rears I built had shims in place of the crush washer. Worked out really well...but I don't advise that in your case. If you were setting up the rear from scratch it would be an option.

Warhorse Racing · Apr 6, 2019

I would recommend going with the FTBR IRS upgrades. I autocross with a few people who have done IRS-swaps, or run 99-04 Cobras, and they all swear by the FTBR parts. I have personally used FTBR upgrades on 2 of my New Edge autocross cars and I can attest to the quality and the benefits of the parts.

I'm including a couple of links to videos on my channel. They detail the parts, offer some install tips (not for a Fox conversion, unfortunately) and review the mods made to my IRS cars. Hope the info helps!

View: https://www.youtube.com/watch?v=9T46tdvDnt4

View: https://www.youtube.com/watch?v=g4Ezcsopi7M&t=32s

MTV593 · Apr 7, 2019

Welp... I'm already committed to the MM street kit. Making some progress this afternoon so when the bushings come in the process will be expedited.

MTV593 · Apr 7, 2019

"Grinder and paint make me the welder I ain't" rear brackets are modified and I have settled on a threw bolt solution for the bottom mounting holes. I'm using 2" 3/16 steel as a mounting plate on top to distribute the load on the sheet metal.

MTV593 · Apr 10, 2019

So I broke down the IRS. And swapped the pinion flanges. The SRA pinion flange pretty much fell off... I'm not an expert but I'm pretty sure thats not supposed to happen. Fit nicely on the IRS pinion so I feel good about it. Also finished the mounting plates in the trunk... Now to wait for the bushings to come in.

MTV593 · Apr 10, 2019

90sickfox · Apr 10, 2019

Looks like ya know what you're doin'.

Just for folks that don't know....a drop of blue Loctite helps keep those axle bolts from loosening over time. Tried red Loctite....bad idea.

The IRS in my car is all stock except the addition of a girdle. This turbo car was the first time I ever experienced wheel hop. I think it was due to the alignment and a little more power.

Guess it's time for me to put together an upgrade list for myself.

I've installed a bunch of bump steer kits for IRS but never got around to doing my own. They do help the cars feel more stable....especially exiting turns on the throttle.

Davedacarpainter · Apr 10, 2019

Mine's sitting in a storage unit waiting for someone to care enough to finish him up.

So.......I got nothing, just feeling talkative..........

FoxMustangLvr · Apr 11, 2019

90sickfox said:
Looks like ya know what you're doin'.

Just for folks that don't know....a drop of blue Loctite helps keep those axle bolts from loosening over time. Tried red Loctite....bad idea.

The IRS in my car is all stock except the addition of a girdle. This turbo car was the first time I ever experienced wheel hop. I think it was due to the alignment and a little more power.

Guess it's time for me to put together an upgrade list for myself.

I've installed a bunch of bump steer kits for IRS but never got around to doing my own. They do help the cars feel more stable....especially exiting turns on the throttle.

I haven't had any wheel hop the past couple years up until a couple weeks ago. I put my kids in my car, it was a cold morning (about 36* ambient), and my summer only tires were hard as rocks. I pulled up to a stop sign and rolled through it because I saw a small opening in traffic so in first gear I gave it 1/4 throttle and just lit up the tires. The rear end shook violently and freaked my kids out. Last week I let the twin screw eat in second gear and roasted the tires when it was about 60*+ out and zero hop. My BFG G-force comp tires do not like the cold!

bird_dog0347 · Apr 11, 2019

I did not do any research before swapping the pinion flange on my IRS.... I ruined my gear set and eventually just ordered a whole new gear change kit from LMR and had a reputable guy redo them. The rest of it is cake, and I too have a fairly detailed writeup in my build thread.

Cobra IRS swap for 93 notch

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