View Full Version : IRS at the track....

07-30-2003, 10:10 PM
Sorry for all the questions, but just trying to learn a thing or two...
Anybody have a 600+ RWHP car that has IRS and take it to the track on a regular basis? I'm just wondering if I should be concerned about doing damage to the rearend or half shafts on hard launchs while running slicks. I have 4.30 FMS gear in my '01 Cobra with '03 half shafts. '03 half shafts are supposed to be better, but are they strong enough for that much power?


07-31-2003, 01:29 AM
Nitrous Pete pushed the IRS to it's limit I'm sure with over 700 rwhp in an '03 cobra. I believe he ran slicks. I think the IRS will hold up, but the tranny won't. Your best bet is to apply smooth, steady power, or ease into the throttle. A high rpm launch is going to put a lot of stress on the driveline, and you'll break something. A smooth lauch and steadily rolling into the throttle, I believe you'll be fine. I'd discourage a powershift at the power level. Nitrous Pete's IRS held up with some insane power levels, but the use of an automatic trans with a loose converter, and smooth driving kept everything together, and rewarded him with a 9 second Et.

07-31-2003, 01:35 AM
Not to burst your bubble, but most heavy duty quarter mile guys spent a lot of money beefing up the IRS just to throw it out and go solid axle. To many weak points in the IRS, plus I beleive the IRS has more parasitic loss than a solid axle.

I would be concerned about putting that much power through the IRS and subjecting it to the heavy launching required for quick 1/4 mile times. If you do it, beef it up first and be prepared to yank it apart and inspect the parts frequently.

The new half shafts will help, but then other parts become the weak link (See this thread from FordPower.net (http://www.fordpower.net/forums/showthread.php?t=28719)).

07-31-2003, 08:10 AM
Joe Lynch's website is an excellent resource for IRS drag racing. He's had a lot of parts fail that I'd never even heard of. IIRC, he switched to live axle recently.


Lightning Bob
07-31-2003, 09:56 AM

I have a 99 Cobra with 4:10's, Nitto DR's and a Paxton Novi 100 putting down about 460 to the wheels. I broke one halfshaft prior to installation of the blower, and have been lucky since. I do not drag race this car regularly though, only a few times a year.

Your IRS may hold up for a while but is not preferred for drag racing. If you look at Joe Lynch's website, you will see why.

Several of the 03 Cobra's broke halfshafts at last weekends Mod Motor Shootout. Another actually broke his driveshaft. Never saw a driveshaft go before the halfshafts on an IRS Mustang before.

In spite of the fact that numerous people half already broken 2003 halfshafts, I still plan to upgrade my Cobra to them shortly, as they are much better than the 99's I currently have and I'm not ready to ditch the IRS just yet.


Nitrous Pete sold the car, and I heard that the new owner has since blown the engine.

07-31-2003, 11:05 AM
Swap to the straight axle with the power you plan to put down.


07-31-2003, 03:01 PM
Originally posted by Lightning Bob


Nitrous Pete sold the car, and I heard that the new owner has since blown the engine.

Yeah, I know Pete sold the cobra, but the new owner did not blow the engine. I read something about the KB Blower going down though. I guess the blower gave out and gave him a scare, but the engine compression checked good as well as the leakdown test. That's just what I've heard. I saw the video on SVTPERFORMANCE of the 12.9 @91mph run where supposedly the failure took place. BTW, I hope this doesn't sound harsh, because it's not meant to. :thumbsup:

Anyway, I wasn't suggesting earlier that the IRS will be ideal for the strip, but I think it would hold up to smooth power applications and low rpm launches. The slicks should help prevent wheel hop. But, there is a lot of other stuff that would need to be done to maximize the IRS's capability. If you drag race frequently, I'd ditch the IRS right away if I had that kind of power.

07-31-2003, 03:58 PM
That kind of power and the IRS are great for open track or road racing, but risky for drags.


08-06-2003, 01:13 AM
Has anyone here gone through the IRS to straight axle rear end swap? If so, who did the swap for you and how much did it cost?


08-06-2003, 10:41 AM
I did not do it for my car, but I was an anti-mechanic helping a friend with his. :D Luckily his future wife had a 2002 GT and we just swapped the IRS unit with the solid from that car so any cost he wanted to incur went into upgrades for the new stick. You can check around the boards and might find some GT owner wanting an IRS and you guys can swap plus some cash to you.

As for paying someone to do it, it may get expensive labor-wise. It took us a weekend + plus a few nights (not full days, maybe 8-9 hours total) to get it all swapped in. This is without a lift and having to take out the odds and ends with it (ABS cables, disconn. brake lines, etc.) as well as learning and reading everything as we went along. Its fairly simple if you're mechanicaly inclined. 6 bolts to remove the IRS from the body, and maybe 4-8 for a solid and all the odds and ends. :)

08-10-2003, 11:34 PM
Does anybody know if a '96-'98 cobra rearend will work in my '01 Cobra?


08-10-2003, 11:45 PM
96-98 rear ends are 3/4" shorter in length than 99+ housings. You'd have to use the dreaded "spacer" to keep from rubbing inside (?), or a rim with a different backspacing, but you can make it work. :thumbsup:

Edit: Oh yeah, your drive shaft will need to be longer (1") for the solid vs. the IRS.

08-10-2003, 11:59 PM
Like Scott said, your best bet is to find a 99+ Solid Axle car to swap with.. This will give you all the parts you need for the swap and you won't have to buy wheel spacers to make up for the difference in the length of the 96-98 Cars..