cobra IRS in fox swap

393windsor · Nov 17, 2007

how well does it perform in a 3,000lb coupe

85ragtop · Nov 17, 2007

There is a guy on here that did it. It looked real good. He seemed to like it.

fox racer · Nov 18, 2007

Well? What are you looking to do with the car?

393windsor · Nov 18, 2007

fox racer said:
Well? What are you looking to do with the car?

I do plan to drag the car but not on a weekend to weekend basis. It will be a street car for the most part so handling and ride quality are most important. I know there are inherent benifits and downfalls of the irs. My question, is how will it perform for what I plan on using it for in a much lighter fox coupe?

fox racer · Nov 18, 2007

393windsor said:
I do plan to drag the car but not on a weekend to weekend basis. It will be a street car for the most part so handling and ride quality are most important. I know there are inherent benifits and downfalls of the irs. My question, is how will it perform for what I plan on using it for in a much lighter fox coupe?

I've heard nothing but good things about it, that's why I'm doing the swap this week
. It IS heavier than a similarly loaded Solid though. Anywhere from 80-125lbs will be added to the car, BUT the rear will help with unsprung weight, by about 125Lbs. Anything deeper than 12.5s (being generous) with a stock IRS (03/04 ONLY) would not be to reliable. Look into a BF brace & Poly/Delrin/Aluminum bushings. Try to ONLY look into an 03/04 unit, as they are much stronger than the earlier models.

I'm doing the swap for the same reasons you are, but I probably won't ever hit the strip with this car. Ride quality is my biggest concern, and I know I could have accomplished it with the solid, but I decided to be different. Performance in a fox? Like I said, I've heard good things, but it depends on what you're looking for. Going from a stock solid, or even a drag setup rear, to a stock 03/04 unit (With brakes shocks etc) will be a NIGHT AND DAY difference in handling, and even in ride comfort. Have you researched the swap? It's very straightforward, but there are a few little nuances that can cause headaches.

393windsor · Nov 18, 2007

fox racer said:
I've heard nothing but good things about it, that's why I'm doing the swap this week
. It IS heavier than a similarly loaded Solid though. Anywhere from 80-125lbs will be added to the car, BUT the rear will help with unsprung weight, by about 125Lbs. Anything deeper than 12.5s (being generous) with a stock IRS (03/04 ONLY) would not be to reliable. Look into a BF brace & Poly/Delrin/Aluminum bushings. Try to ONLY look into an 03/04 unit, as they are much stronger than the earlier models.

I'm doing the swap for the same reasons you are, but I probably won't ever hit the strip with this car. Ride quality is my biggest concern, and I know I could have accomplished it with the solid, but I decided to be different. Performance in a fox? Like I said, I've heard good things, but it depends on what you're looking for. Going from a stock solid, or even a drag setup rear, to a stock 03/04 unit (With brakes shocks etc) will be a NIGHT AND DAY difference in handling, and even in ride comfort. Have you researched the swap? It's very straightforward, but there are a few little nuances that can cause headaches.

I have read an article someone posted and it does seem pretty straight forward. I am a fabricator so any modifications would be no problem. I am having a hard time finding one. Someone I know just sold his before I made the decision to do the swap. My coupe is just sitting there waiting for me to tear it down. I'm in the process of building a carbureted 393w. I will be stripping it completely. I basically have everything new for the interior besides the dash and major back panels. Also have all moldings and weatherstripping. So basically I have some time to find one just impatient. Thanks for your reply and just luck with the new handling and ride quality.

fox racer · Nov 18, 2007

If you're a fabricator, the swap is a cinch. All you need to do is drill 6 holes, weld 2 (if you want 2 smaller ones for a total of 4) plates and grind some 2 spot welded brackets off. Everything else is bolting it straight in, literally. You should have no problem with this, seeing how I'm just a wrencher and can do it. If you have any questions about the swap let me know.

Sicarius428 · Nov 19, 2007

I am in the process and finally have all the parts involved... much more than I though. Do get a 03/04 IRS unit. They are much stronger and have 3.55 gears. It already sounds like you did your research though. You will need the driveshaft flange off the cobra doner and to adapt the 1350 flange to your current 1330 DS you will need a Precision 448( i believe) u joint. Bilstein shocks and the rear cradle brackets if you can get them with the IRS unit is a plus. Used shocks are easy to come by... I got mine for 65 shipped at svtp's marketplace. Brackets and driveshaft flange were a bit hard to come by. All the 94+ rear disk brake calipers will work... Only difference is the bracket between the cobra vs. GT/V6. You can use the GT/V6 brackets but you might have to dremel some of the bracket away. The GT/V6 does not use a vented disk so they are a bit thinner and use thicker pads... Not too hard.
Kevin

393windsor · Nov 20, 2007

thanks for your help guys, i look forward to the swap

85_SS_302_Coupe · Nov 20, 2007

I would love to do this on my car....then beef it up real nice. I think the added weight is a good thing, it'll push the weight distribution closer to 50/50.

fox racer · Nov 20, 2007

85_SS_302_Coupe said:
I would love to do this on my car....then beef it up real nice. I think the added weight is a good thing, it'll push the weight distribution closer to 50/50.

Plus, the extra weight is over the rear wheels, so it helps with traction at little.

85_SS_302_Coupe · Nov 20, 2007

Yeah, then if you've got a tubular K member/A arms, aluminum heads and fiberglass hood, trunk mount the battery...you might really be close to 50/50.

393windsor · Nov 20, 2007

all things to do in the near future

Mustang78 · Nov 20, 2007

i dont see how people are finding the 03-04 cobra IRS units, i've never even seen one advertised...i can get ahold of the lincoln mark VIII and thunderbird IRS though, but i hear it's worse for some reason. And a guy at the 4I BBQ in Arlington had supercoupe IRS under his 86GT

Sicarius428 · Nov 20, 2007

I found one on my local mustang forums. They pop up on ebay a bit but the shipping is pretty bad. Out of the box the Cobra IRS is pretty floppy. They used rubber at every joint which can give it some pretty bad wheel hop.... especially with the power that some of these terminators are running. For strength and simplicity they swap it out for a solid axle. I have weighed out both options of either going with the IRS or building up my solid with gears, 5 lug and brakes. Even so the IRS came out cheaper for me with the added perk of better handling and better weight distribution. I can always address the wheel hop issues later with replacing all the rubber but I never go to the drag strip anyways.
Kevin

2000xp8 · Nov 20, 2007

They are so expensive to mess with, level 5 axles are like 2 grand if you need them.
High coolness factor, but i'd rather put the money into the rest of the handling setup.

And even beefed up with bushings and braces, they can still wheel hop, watch them run on drag radials at the track, looks like the wing is going to break off the trunk lid when wheel hopping.

fox racer · Nov 20, 2007

2000xp8 said:
They are so expensive to mess with, level 5 axles are like 2 grand if you need them.
High coolness factor, but i'd rather put the money into the rest of the handling setup.

And even beefed up with bushings and braces, they can still wheel hop, watch them run on drag radials at the track, looks like the wing is going to break off the trunk lid when wheel hopping.

The IRS swap isn't something you should be looking at if you like the strip. And unless you're running a professional track car (auto x, road racing, American Iron type stuff) the IRS can be pretty helpful with its added handling help. The main thing people are looking for with the IRS (Myself included) is improved street handling and ride quality. Some guys use them for weight distribution and help in the handling dept. for the track, but not many. The coolness factor is what lures most in, plus being unique from the pack is just another perk. The 03/04 units are pretty strong (outside the bushings, and need for a Billet flow brace) for what MOST people put them through. You can hear a few *lucky* guys that run 10s-11s with the IRS and no serious (Level 5 halfshafts) mods. I think you're looking at the IRS with a certain mindset (such as serious drag racing), which is perfectly fine as it might be your perspective. But to each is own, and it's understandable why you don't want it.

The IRSs aren't too bad to find. The trick is to save up for one, and when one pops up jump on it. That's what I needed to do. I had no intentions to do this swap a month ago, but I had some extra cash and found a well price one so I bought it. Just be careful, ONLY buy the 03/04 unit. There should be a yellow sticker on the halfshaft that says "2R3V" to indicate the 03/04 unit. Check out SVTPerformance.com, there are always guys looking to swap out on that site. Also keep an eye out on Ebay, as they do pop up on there every couple weeks.

Sicarius428 · Nov 20, 2007

Yeah... the Level 5 axles are pretty steep. Typically the stock axles are prone to fail because of all the slop from the factory bushings causing binding. Please understand aftermarket bushings aren't the answer for everything in the Cobra IRS... it does aleviate a lot of the issues with it though. If you check out svtp's marketplace, Bruce with the screen name of AC427cobra has built a full custom kit for the IRS using Delrin and AL. This kit also makes the diff solidly mounted in the cradle. I highly suggest reading up on the feedback and results to show the full potential of the IRS. Again.. for strait line you can't beat the old stick but the IRS has a lot more potential than its given credit for. Either way I'll find out soon.
Kevin

393windsor · Nov 22, 2007

Sicarius428 said:
Yeah... the Level 5 axles are pretty steep. Typically the stock axles are prone to fail because of all the slop from the factory bushings causing binding. Please understand aftermarket bushings aren't the answer for everything in the Cobra IRS... it does aleviate a lot of the issues with it though. If you check out svtp's marketplace, Bruce with the screen name of AC427cobra has built a full custom kit for the IRS using Delrin and AL. This kit also makes the diff solidly mounted in the cradle. I highly suggest reading up on the feedback and results to show the full potential of the IRS. Again.. for strait line you can't beat the old stick but the IRS has a lot more potential than its given credit for. Either way I'll find out soon.
Kevin

Sounds great. Thanks for all the helpfull info. You'll probably do the swap before me(still looking for irs), so if you can let us know how well it handles and how sweet the ride is on crapy pavement. also let us know on the backspacing you'll have to use. thanks

393windsor · Nov 24, 2007

I just bought one from ebay. It has the bilstein shocks, calipers and rotors. I can't wait for it to come in so I can install it.

cobra IRS in fox swap

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