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V6 (1994-2004) Mustangs Technical discussions on the 3.8L and 3.9L V6 torque monsters

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Old 03-22-2011, 11:36 AM   #1
97StangEmUp
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Question IRS Swap

I'm considering doing an 03/04 Cobra IRS swap into my 97 V6.
I was wondering what all is involved in the swap, is it a direct bolt up, it says it is with 99-04 but idk about the 94-98 stangs. Also, will it interfere with my exhaust, I have true dual exhaust from a stock gt.
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Old 03-22-2011, 11:43 AM   #2
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I think there may be a slight issue with the input shift from the drive shaft to the differential housing but not sure exactly what spline it is, but that can be easily adapted. Yes your exhaust will have to be changed with IRS the exhaust runs under the rear instead of over like a solid rear axel. Other than that it's pretty much a direct swap. You will also need the shocks/springs and all the bushings so make sure the thing comes with that. One disadvantage is the IRS weighs more than a solid rear end which will reduce your 1/4 mile times and mpg on a stock sixer. In short you're really not making enough power to justify having one.
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Old 03-23-2011, 02:32 AM   #3
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You will need to have a GT pinion flange put on or change the U-Joint, the U-Joint being the easier and cheaper of the two.

2001 Cobra IRS's do no have this issue and are still a 31 spline IRS, very slight changes were made from 01 to 03 and 04. The biggest to name is the 3.27 gears in the 01' Cobra and the 3.55 gears on the 03/04.

The increase of the gear ratio from your stock 2.73 will help your in town gas slightly though dont expect it to be better than it is now, it will just be better than an IRS that had 2.73 gears (just an example.) but the higher gears may hurt your highway fuel economy depending on your speed.

But its not going to be such a huge deal that its going to really bother anyone in the modding world.

The 3.55 gears and LSD will also be far better in the 1/8 or 1/4 mile because you will put power to both wheels and have a lower gear ratio to get you out of the hole.

So while it was said that it could be bad because of fuel and 1/4 mile times, its not bad enough to not justify the mod. Just dont expect to be faster or have better fuel economy than you have now, but its not going to be as bad as if you had an open dif 2.73 gear IRS (Again example.)

Lets not forget that your general driving and handling experiance will grow from this mod as well.

If you plan to build more power down the road then this is a great place to start because its a high tier project thats already done. Not a lot of people would go for that right away but it doesnt hurt the progress of the car.

btw, lets not forget the 11.65" Rear vented brake upgrade you also get from this mod.
Matching the front with Cobra or Bullitt brakes is a Must after going that far.

Also it is imperative that you upgrade your front swaybar to ballance out the swaybars.
The 03/04 Cobra stock has a 29mm hallow swaybar. There are lots of great sway bar options out there such as a Saleen 35mm front sway bar. Be sure to change your end links too.

If you have gone this far you may as well match your front suspension with at least Stock Mach 1 Tokico struts or 03/04 Cobra Bilstein struts and some decent springs like Roush, Steeda, FRPP, H&R and so on. You can also use Bullitt or Mach 1 Springs which are similar to FRPP C-Springs.

Lastly a set of 03/04 Cobra or mach 1 Lower controll arms will help top off a mild OEM suspension build using top OEM parts.




I guess this is in some ways, what J was getting at but there is a lot that needs to be upgraded to make the IRS swap properly matched.

Last edited by NeoTokyo; 03-23-2011 at 02:43 AM.
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Old 03-24-2011, 10:39 AM   #4
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So my first question is, what is a good price to pay for the IRS. I saw 3 on ebay ranging from 1,299 to 1,799 depending on year and miles. That seems high.
And also I would need a 31 spline driveshaft correct? (I could just get the FRPP aluminum driveshaft?)
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Old 03-24-2011, 11:33 AM   #5
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That's just it, they will vary widely in price. Sometimes you can get a deal on one, other times not. If you find one on ebay see if they have a "best offer" on it. These things aren't exactly hot sellers because very few people actually go with the conversion. More often then not they're changed out in Cobras when people get in accidents.
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Old 03-24-2011, 12:24 PM   #6
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I'll just keep an eye out for one at a good price.
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Old 03-25-2011, 04:14 AM   #7
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$500-800 is a decent price depending on condition.

If there are aftermarket parts then that adds to the value.

As for the driveshaft, I covered that.
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Old 03-25-2011, 06:18 AM   #8
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here is a great tech writeup we have. pretty much what neo has already said just with pics. good luck with install.

https://mustangforums.com/forum/v6-1...ml#post6753903
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Old 03-25-2011, 09:34 AM   #9
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Thanks!
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Old 03-25-2011, 06:17 PM   #10
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I forgot that Ryan did a write up.

In your 97' you will need to drill a couple extra holes but its not a big deal, just mount the bracket and mark where the hole should be and drill away.

You could also weld the bracket to the body.
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1994, 38l, 9498, cobra, dual, exhaust, gt, independent, irs, mustang, rear, stock, suspension, swapping, true, v6

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