IRS: MN12 vs MarkVII vs 99-02Cobra vs 03-04Cobra... - TCCoA Forums
 
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post #1 of 17 (permalink) Old 10-10-2009, 05:02 PM Thread Starter
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IRS: MN12 vs MarkVII vs 99-02Cobra vs 03-04Cobra...

I need to verify the information I have thus far.

To my knowledge the Explorer rear diff. does not work so I did not include it in the thread title.

I have a 1995 T-bird and my dad has a 95 Mustang GT.
I parted a 95 Mark VIII 2 years ago and kept the rear diff and LCA.

I recently bought a 2003 Cobra IRS complete assy.

Here is the plan thus far. I want to take the 03 Cobra rear diff and axles out of the 03 Cobra IRS assy. and use them for my 95 T-bird. This way I can upgrade to the 31 spline set up.

I put the 95 Mark VIII rear diff back in the 03 Cobra IRS assy and install it into my dad's GT since his ride is only being built as a street car.

I have heard the 99-02 Cobra rears are still the smaller spline count. First, is this true? If so, can I use a set of 99-02 Cobra axles with the 95 Mark VIII rear diff in the 03 Cobra IRS assy for my dad's car?

Thanks in advance for any help, correction, or verification.

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post #2 of 17 (permalink) Old 10-10-2009, 06:25 PM
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03/04 Cobra trac loc/carrier is 31 spline, but aren't the axles the wrong length for use on an MN12? The spline count at the hub end is indeed 28 for both Cobra and MN12s.

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post #3 of 17 (permalink) Old 10-10-2009, 06:36 PM
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The halfshafts are too short.

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post #4 of 17 (permalink) Old 10-10-2009, 06:57 PM
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"Here is the plan thus far. I want to take the 03 Cobra rear diff and axles out of the 03 Cobra IRS assy. and use them for my 95 T-bird. This way I can upgrade to the 31 spline set up."

Nope, The axles are over an inch too short. Why do you think you need a 31 spline diff? Got over 300 RWHP? My SC with 305 RWHP is fine with 28 spline axles.

"I put the 95 Mark VIII rear diff back in the 03 Cobra IRS assy and install it into my dad's GT since his ride is only being built as a street car.

I have heard the 99-02 Cobra rears are still the smaller spline count. First, is this true? If so, can I use a set of 99-02 Cobra axles with the 95 Mark VIII rear diff in the 03 Cobra IRS assy for my dad's car?"

99 Cobra axles will work. I hope you're planning to change out the open diff in the Mark VIII rear so Dad has some traction.

Just install the Cobra IRS in Dad's car, and get yourself a Tru-Trac or Torsen diff and put it in the Mark VIII case.

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post #5 of 17 (permalink) Old 10-10-2009, 07:36 PM Thread Starter
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Quote:
Originally Posted by DLF View Post
"Here is the plan thus far. I want to take the 03 Cobra rear diff and axles out of the 03 Cobra IRS assy. and use them for my 95 T-bird. This way I can upgrade to the 31 spline set up."

Nope, The axles are over an inch too short. Why do you think you need a 31 spline diff? Got over 300 RWHP? My SC with 305 RWHP is fine with 28 spline axles.

"I put the 95 Mark VIII rear diff back in the 03 Cobra IRS assy and install it into my dad's GT since his ride is only being built as a street car.

I have heard the 99-02 Cobra rears are still the smaller spline count. First, is this true? If so, can I use a set of 99-02 Cobra axles with the 95 Mark VIII rear diff in the 03 Cobra IRS assy for my dad's car?"

99 Cobra axles will work. I hope you're planning to change out the open diff in the Mark VIII rear so Dad has some traction.

Just install the Cobra IRS in Dad's car, and get yourself a Tru-Trac or Torsen diff and put it in the Mark VIII case.
I am modding the Cobra axles to get the length correct. There are adjustments that can be made to them. If memory serves me correctly each side needs to be lengthened 5/16". This I am fine with.

As far as the "why" I need them... I am not actually 100% however, I would rather be safe than sorry. I am building a 2004 DOHC engine w/ supercharger. The goal is a high 11 sec ET in a full trim car. So we are probably looking at numbers hovering around the 500HP range. This being said, I will not be relying on the trac loc from the original cobra center, or the stock axles by themselves. However, I need a set of the stock axles to build the race axles.

My only concern was with the 99-02 mustang axles and if they would fit the Mark VIII center. I know the 03-04 Cobra uses the 31 spline axles, but haven't verified that the earlier cobra's used the same spline count as the MN12 and mark VIII.

I have a trac loc set up from a 94 SC that would be rebuilt for the Mark VIII center for use in my dad's mustang.

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post #6 of 17 (permalink) Old 10-12-2009, 09:11 PM
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Quote:
Originally Posted by mercury267 View Post
I am modding the Cobra axles to get the length correct. There are adjustments that can be made to them. If memory serves me correctly each side needs to be lengthened 5/16". This I am fine with.
IIRC, they are about an inch too short. I'm sure its more than 5/16. I've seen them both on a bench, but a beer bottle is not a good measuring device, lol.

AFAIK, everyone making large power is using Raxles halfshafts. I could be wrong, and I'm sure someone will chime in.

You can beef up the shaft of the stock ones, but the outer bearing shell itself may be the limiting factor.

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post #7 of 17 (permalink) Old 10-13-2009, 10:35 AM Thread Starter
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Quote:
Originally Posted by Grog6 View Post
IIRC, they are about an inch too short. I'm sure its more than 5/16. I've seen them both on a bench, but a beer bottle is not a good measuring device, lol.

AFAIK, everyone making large power is using Raxles halfshafts. I could be wrong, and I'm sure someone will chime in.

You can beef up the shaft of the stock ones, but the outer bearing shell itself may be the limiting factor.
I've got limited info on this subject thus far, but Mike S. uses a modified Cobra axle. I am probably wrong on the difference in length, but according to him this is the set up that he has been running. When I really get into it I'd like to document it so I can post a thread here on what was done.

I am not really excited about Raxle products as I have been hearing some poor feedback on their axle shafts. Obviously you can't believe everything ya hear, but I trust Mike's word enough to basicially copy his set up if I can get the info on it.

The rear end for my T-bird is not the main focus right now. I am more concerned about getting a set up for my dad's car. The goal is to build the engine for my car now. Get the rear end in his car next spring (the 03 IRS with the Mark VIII center and trac-loc, and the 99-02 cobra axles). I just want to collect the rear components for my car now so I am ready to get moving on it when I tear into the rear on mine later down the road.

Beer bottle?!?!? The "hops" system of measure is fine!

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post #8 of 17 (permalink) Old 10-13-2009, 12:06 PM
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http://www.sccoa.com/forums/showthre...ra+half+shafts

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post #9 of 17 (permalink) Old 10-13-2009, 11:20 PM
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I stand corrected. Nice post!

So they Will set up to fit, with diff changes...

Me, I know where a set of Mark axles are, lol.

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post #10 of 17 (permalink) Old 10-14-2009, 11:14 AM
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Converting to 31 spline is really a waste. The only people who are having issues with breaking axles are those running a manual trans with high hp engines, and launching on slicks at the track. Other than that, the axle strenght simply isn't an issue. Dave Neibert runs STOCK SC axles in his turbo'd 347 laying down over 800hp at the rear wheels! Also, for those people who have run into issues with breaking axles, they all break at the hub side, not the diff side, so converting to 31 spline on the diff side won't help at all since if they are going to break, they will still break at the outside, which you haven't improved. I would say run the cobra setup in the stang, and the MarkVIII setup in the bird, saving yourself a bunch of un-necessary complication. Also, for axles, get either 5-speed SC axles, or 97/98 MarkVIII axles. Those are the strongest factory axles that will be a direct fit for our cars.

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post #11 of 17 (permalink) Old 10-14-2009, 03:05 PM Thread Starter
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Agreed... the upgrade is not needed unless there are heavy launches. However, the SCCOA thread linked above refers to the outer hub also. The level 5 axles come with a custom made 33 spline outer hub. That same thread also verifies that mustang guys running stock set ups experience the same break by the outer hub. The 31 spline center is just half of the upgrade conversion... the second half being to upgrade the 31 spline axle with the custom 33 spline outer hub.

I myself will not be anywhere near 700 to 800HP. The block I am building now will certainly max out at around 500 give or take a few. In addition I do not plan on running at the track except several times per season just for fun. However, when I do I want to run to the fullest extent of what the car will deliver WITHOUT having to worry to much about busting an axle. Broken parts rank pretty low on my fun-o-meter so for me, this is a worthwhile upgrade. I'd rather over build the known weak links so my car is dependable no matter what.

Aside from that, I agree with you that the upgrade is not needed until some higher HP and TQ numbers are established and the center swap with stock cobra axles would be useless without committing to the full conversion that would include the custom 33 spline outer hub, and the choice to run with slicks would greatly impact the need to do any of it.

I am just glad my original question (the actual purpose for starting this thread) was answered. I now know I need to get a set of 99-02 Cobra stock axles so I can use them with the Mark VIII center I have to set the Cobra IRS set up I have for my dad's mustang.

Thanks for everyone's help and references!

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post #12 of 17 (permalink) Old 10-14-2009, 03:16 PM
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Oh also, just a minor thing, but it is 99/01 axles. There was no cobra in 02, and in 00 the only cobra was the 5.4L CobraR.

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post #13 of 17 (permalink) Old 10-14-2009, 03:59 PM
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IIRC the 33 spline level 5 outers are in the price range of two grand, not including the halfshafts.

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post #14 of 17 (permalink) Old 10-14-2009, 11:16 PM
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What do you do about the bolt pattern from the cobra not fitting your t-bird wheels? Or you use the t-bird hub? Also is there a post on here showing how to convert a tbird to mustang bolt pattern? I don't wanna hijack thread, just wandering if he needs to worry about his wheels fitting?
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post #15 of 17 (permalink) Old 10-15-2009, 11:28 AM
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Quote:
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What do you do about the bolt pattern from the cobra not fitting your t-bird wheels? Or you use the t-bird hub? Also is there a post on here showing how to convert a tbird to mustang bolt pattern? I don't wanna hijack thread, just wandering if he needs to worry about his wheels fitting?
If he is going to all this trouble, I simply assumed he would be converting over to the mustang bolt pattern. You wouldn't be able to use the Tbird's 28 spline hubs with an axle that has a 33 spline outer joint. Basically the mustang hub conversion would be the only way to do that.

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post #16 of 17 (permalink) Old 10-16-2009, 09:30 AM Thread Starter
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If he is going to all this trouble, I simply assumed he would be converting over to the mustang bolt pattern. You wouldn't be able to use the Tbird's 28 spline hubs with an axle that has a 33 spline outer joint. Basically the mustang hub conversion would be the only way to do that.
This is correct. Also, the car is currently converted to the SN94 hub swap. I am running a 93 Cobra "fanblade" wheel (originally a 4 lug wheel) that was sent to Wheel Concepts in CA to be filled, re-drilled and balanced for the SN95 circle and the center bore was drileld out also, so the 33 spline outer hub would allow me to retain the current wheels I am running.

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post #17 of 17 (permalink) Old 10-16-2009, 09:31 AM Thread Starter
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IIRC the 33 spline level 5 outers are in the price range of two grand, not including the halfshafts.
Ok.

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