Help on Cobra IRS conversion

VIRstang · Dec 27, 2007

I'm am looking at buying a complete 03' Cobra IRS rear to install in my 99' GT. Will it bolt in or is there extensive fabrication required? I'm not scared of welding or fabricating, I just have a track event to go to March 1st and I don't won't to get in over my head. Has anyone done this conversion?
Thanks!
Ben

hotmustang331 · Dec 27, 2007

Bennyblown2V has done it several times. He should chime in soon. May I ask how much you payed for it...or a rough guess of how much they are worth?

VIRstang · Dec 27, 2007

I haven't bought it yet. I want to find out how hard it is to install first. I'm curious to what everyone thinks it's worth myself since I'm considering buying one. It supposedly comes with everything needed to install.

hotmustang331 · Dec 27, 2007

Well whatever it costs it probbaly worth it. I came from a TT 04 GT with ofcourse a solid axle. I LOVE the IRS in my 03 cobra so im not considering getting rid of mine LOL. I was just curious as to what they run. You will absolutly love it

BennyBlown2v · Dec 27, 2007

It's a simple swap. On a 99+ chassis the holes are already where you need them on the frame rails. The biggest thing holding you back is going to be how complete the stuff you buy is. Basically you need the entire rear end, e-brake cables (you can use stock ones but they don't have provision for attaching to the LCA), ABS sensors, brake soft lines (you can use GT lines but you have to bend them JUST right to clear the suspension), and drive shaft centering yoke from the cobra (if 03/04 IRS is used, the pinion flange on the rear end is different so you need the larger yoke, also if you go this route you'll need a hybrid ujoint to hook it up to your GT driveshaft model #443 if I remember right). Of course for springs you'll need an internal spring compressor, cut springs (stupid), or coilovers.

I would highly recommend that before you put the IRS into the car you get a Maximum Motorsports "IRS grip box" or at least subframe bushings, diff bushings, oh and an IRS differential brace so you don't crack the diff cover.

If you have any more specific questions just PM me and I'll be more than happy to answer them!

BennyBlown2v · Dec 27, 2007

hotmustang331 said:
Bennyblown2V has done it several times. He should chime in soon. May I ask how much you payed for it...or a rough guess of how much they are worth?

$800-900 is a fair price for a COMPLETE STOCK setup with all brake lines, cables, abs, bolts, brackets, etc... IMO I payed $1000 for my current IRS which already had a few mods (diff brace, subframe bushings), and $850 for my first (stock) one a few years ago. If you're lucky sometimes guys who are more motivated to get rid of their stuff will throw in exhausts, etc.... But I really think those guys are dumb, IRS is great for a street car

VIRstang · Dec 28, 2007

Great Info! Thanks! It is supposed to be complete ready to bolt in. I'm going to try and get it. If I do I'm sure I will be PM'ing you! The price is right in line with what you quoted.

tomustang · Dec 28, 2007

VIRstang said:
I'm am looking at buying a complete 03' Cobra IRS rear to install in my 99' GT. Will it bolt in or is there extensive fabrication required? I'm not scared of welding or fabricating, I just have a track event to go to March 1st and I don't won't to get in over my head. Has anyone done this conversion?
Thanks!
Ben

You don't need an IRS to run great, just so you know, but go for the IRS if you can get it

BCB00GT · Dec 28, 2007

I just bolted an IRS into my 00GT. I used my GT e-brake cable. I got some "P" brackets that hold the e-brake cable to the lower arm perfect.

BennyBlown2v · Dec 29, 2007

BCB00GT said:
I just bolted an IRS into my 00GT. I used my GT e-brake cable. I got some "P" brackets that hold the e-brake cable to the lower arm perfect.

P-bracket as in adel clamp?

BCB00GT · Dec 29, 2007

Yes. Called one of the supporting Ford dealers on another forum. Bought the two clips/brackets and lower isolators. It amazes me why so many IRS that are for sale do not have the rear springs or isolators. It's not like the previous IRS owners can use them on a solid axle. Any ideas why this stuff is missing (and add ABS sensors also into that group).

BennyBlown2v · Dec 29, 2007

BCB00GT said:
Yes. Called one of the supporting Ford dealers on another forum. Bought the two clips/brackets and lower isolators. It amazes me why so many IRS that are for sale do not have the rear springs or isolators. It's not like the previous IRS owners can use them on a solid axle. Any ideas why this stuff is missing (and add ABS sensors also into that group).

They are worth a decent chunk of change. Coupe springs can bring a couple bucks for vert guys wanting better handling - and ABS sensors are just expensive to begin with. Plus, alot of people are stupid and don't realize they aren't the same

BCB00GT · Dec 29, 2007

No isolators though? I could see selling the ABS sensors since they are 2x the cost of GT. But honostly, the oldest IRS is a 99 and I am sure that not that many ABS sensors are failing.

Side question, the rear isolator/spring should have the pigtail by the hole in the lower arm (from what I remember this is the outside position of the arm closest to the tire).

fobra559 · Dec 29, 2007

Funny this thread was mentioned. I was offered by a buddy of mine if I wanted to trade my solid axle for his IRS on his 03 Terminator. I passed on the offer because the wheel hop sucks for drag(which I don't want).

VIRstang · Dec 31, 2007

Well I just bought the IRS unit. $800.00. It appears complete with everything. ABS sensors, etc. It is missing the springs.

My 99' GT did great at the track (road course) I just had the wrong rear gear. I was getting ready to swap in a 3:73. This rear end already has a 3:73 so it will save that money and time.

Bennyblown2v- I'm sure I will have questions. Thanks for your help so far!

secondchance · Dec 31, 2007

IRS install

Hi,

I just posted a note on the "talk" page trying to get a list of like minded folks who have done or are contemplating making this conversion. Let me know how yours turns out. It is pretty straightforward and, I think, well worth it.

BCB00GT · Dec 31, 2007

I finished mine about 2 weeks ago. Very much worth it.

LaserSVT · Dec 31, 2007

Couple questions, is it from an 03/04 or 99/01? Does anyone know if the 99/01 have the brace infront of the pumpkin? Do the solid axle gears fit the IRS pumpkin? Also I drove two Termis this weekend and on rough roads the ride is so much better its worth the price of admition alone IMO. Also you can get a live axle to be just as good on a track, but on the road its a different story. Off camber corners with potholes the IRS dont even care, same corner with my live axle I have to take 10 mph slower because of the a$$ wanting to bounce all over the place. I hop to do the same mod if I can find one, although on my Aero I think I have to do some custom work through the trunk floor for brackets Benny has been a great help with the info for me too, thanks buddy

secondchance · Jan 1, 2008

VIRstang,
I have a set of stock rear IRS springs left over from my conversion. I swapped out the stock GT components for H&R and Bilsteins. Will you be retaining the front end setup? The Cobra rear will be much stiffer.

BennyBlown2v · Jan 1, 2008

LaserSVT said:
Couple questions, is it from an 03/04 or 99/01? Does anyone know if the 99/01 have the brace infront of the pumpkin? Do the solid axle gears fit the IRS pumpkin? Also I drove two Termis this weekend and on rough roads the ride is so much better its worth the price of admition alone IMO. Also you can get a live axle to be just as good on a track, but on the road its a different story. Off camber corners with potholes the IRS dont even care, same corner with my live axle I have to take 10 mph slower because of the a$$ wanting to bounce all over the place. I hop to do the same mod if I can find one, although on my Aero I think I have to do some custom work through the trunk floor for brackets Benny has been a great help with the info for me too, thanks buddy

99/01 no brace (or a real whimpy one, I've never dealt with stock 99/01 stuff). Kenny Brown made an IRS brace which was quite beefy, but also the OEM ford 03/04 brace is actually sourced from the Cobra R - so that's what most everyone runs. Also some braces, and some bushing sets, require that you take off the big vibration counter weight often called a "dog bone" for some reason; that's not a bad thing, however. So don't be scurred to ditch the brick

The trunk floor brackets are NOT a bit deal, heres a set my friend made when putting a modded 99/01 rig in his 97 cobra.

You can use some tubing and run it in the holes also so you don't crush the frame rails...but really that's sort of pointless cuz the bolts are more or less letting the IRS "hang" from the frame rail, rather then squeezing the rail in that direction. plus, you'll pull the floor of the trunk down WAY before you crush the frame rails, hah. Brackets are just two mild steel plates welded together, you could use a single plate but that puts more stress on the mounting holes instead of allowing the smaller narrower top plate to more evenly distribute the stress along the bottom plate.

Help on Cobra IRS conversion

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