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Thread: IRS Center Section - Iron vs. Aluminum Center Section

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    Senior Member cv2065's Avatar
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    IRS Center Section - Iron vs. Aluminum Center Section

    Can someone school me on the difference between the iron and aluminium covered IRS center sections? Is one preferred over the other and why are there two different versions?

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    cv2065,

    If you're speaking of the 2015 IRS, I can offer some information.

    First is weight. The Iron, 3.55, Traction-Loc center weighs 100#. Iron, 3.73, Torsen weighs 105# and Aluminum, 3.31, Torsen weighs 82#. And yes, I have weighed these versions. Note: I believe that both options use an aluminum back cover, or at least that's what the three I've seen have.

    Then strength. I don't have the engineering on this but from various reports some Mustangs broke pieces of the aluminum case. I believe that it was the front mounts that are part of the main case, these extend a ways out from the case and appear to be the weak part. I'm not sure how these cars were being driven at the time of failure but I'd guess drag racing w/ power adders.

    Points to ponder. There's a substantial weight difference between the Mustang and a FFR Cobra, this helps the aluminum case. The Mustangs may have been running slicks, but I have no first-hand knowledge of this. If you consider power adders the damage may have occurred in cars with 500++ HP. So if you are considering aluminum v. iron you may wish to consider usage, power, tires and weight.

    From what I understand Ford now uses the aluminum case in the low power automatic cars and iron in the manual trans higher power. Please check this as again I don't have first-hand knowledge.

    The unassembled Challenge Car that I bought from a prior owner came with an iron case, 3.55, Traction-Loc center. I had Mike Forte build an aluminum case, 3.31, Torsen center as this is what I wanted for my build. Right for me, maybe not right for you.

    Jim

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    Senior Member edwardb's Avatar
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    X2 on everything Jim said. The difference is weight (the approx 20 lbs as described) and strength. When first released, Ford was using the iron case in manual transmission Mustangs, and the aluminum case in automatics. I suspect that might be different now since they're pushing the 2018 10-speed automatic as a performance option. But the concern is breaking off the front mounting ears. When Factory Five first released the 2015+ Mustang based IRS, they said to use only the iron case version. Again, out of concern for strength. I know because I was one of the early adapters with my 20th Anniversary Roadster where that option was first released. They later changed to say the aluminum version was fine. Given the weight difference between these cars and a Mustang, hard for me as well to imagine that we're going to break the ears off the aluminum case. Plus FF mounts them a little differently. Ford has them in pretty soft mounts. FF uses poly mounts that are quite stiff. I haven't heard a single case of anyone breaking one of these diffs in our builds.

    The version FF sells is an iron case. The only ones I've seen NIB in the aftermarket (e.g. Ford Performance) are also the iron case version (M-4001-88355). If you want the aluminum case version, pretty certain you would need to chase one down in salvage. Lots of options out there to do that. Or Ford sells the bare aluminum case (FR3Z-4310B) and then you can roll your own.
    Last edited by edwardb; 04-09-2018 at 06:05 AM.
    Build 1: Mk3 Roadster #5125. Sold 11/08/2014. Build 2: Mk4 Roadster #7750. Sold 04/10/2017. Build Thread
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    Senior Member cv2065's Avatar
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    Thanks for the clarification guys. So if you could buy a complete 2015 Mustang GT donor unit that was iron with 24K miles or another that is aluminum with 15K miles, which would you select?

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    Senior Member rich grsc's Avatar
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    Aluminum.

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    Senior Member cv2065's Avatar
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    Was reading about some kind of IRS brace for the aluminum version. Is that needed for street driving or just racing?

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    Senior Member edwardb's Avatar
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    Haven't seen anything like that. For these or Mustangs. Sure it's for this version of the IRS and not the previous generation? Pictures or a link? Have you seen one of these diffs in person? They're really stout. You're not going to break it.
    Build 1: Mk3 Roadster #5125. Sold 11/08/2014. Build 2: Mk4 Roadster #7750. Sold 04/10/2017. Build Thread
    Build 3: Mk4 Roadster 20th Anniversary #8674. Sold 09/07/2020. Build Thread and Video. Build 4: Gen 3 Type 65 Coupe #59. Gen 3 Coyote. Legal 03/04/2020. Build Thread and Video
    Build 5: 35 Hot Rod Truck #138. LS3 and 4L65E auto. Rcvd 01/05/2021. Legal 04/20/2023. Build Thread. Sold 11/9/2023.

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    Senior Member cv2065's Avatar
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    Quote Originally Posted by edwardb View Post
    Haven't seen anything like that. For these or Mustangs. Sure it's for this version of the IRS and not the previous generation? Pictures or a link? Have you seen one of these diffs in person? They're really stout. You're not going to break it.
    Ah, you are right Ed. I remember reading about the problems with the previous versions of the IRS and am getting them confused. Do you agree with Rich that the aluminum version would be preferred in my previous post scenario?

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    I bought aluminum because the iron case diff that I have wasn't configured the way I wanted, if it would have been I wouldn't have changed. If you can find what you want in aluminum I'd say to go that way but it all depends on what you want. Haven't checked but I'd guess that Ford only offered a few gear ratios in the aluminum case.

    In the big scheme of things saving 20-25# may not be worth the trouble for you.

    Jim

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    rrrose's Avatar
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    My thought is all things equal in the donor what are you going to use your roadster for. If competing in racing/rally/etc where power to weight ratio needs to be maximized or at least highly optimized and assuming I had say 550HP headed to the rear wheels I would probably go aluminum. If, as in my case I am building a driver/cruiser/touring roadster I would and did go iron because I wanted the extra weight in the back end and wasn't concerned about 20 pounds or so impacting how quickly I will accelerate. I was thinking I would like those 20 pounds in my 'axle' to help with as much down force as I could build into the rear. Same reason I put sand bags in my pickup truck bed in the winter. In my case it wasn't a strength issue even with 540HP headed into that diff. Now if I were crazy and competent enough to be putting 7,8,9 hundred HP headed towards the diff that would have been the more determining factor.

    Just my logic I am no auto engineer but maybe something to consider?
    Ron
    Last edited by rrrose; 04-10-2018 at 08:32 PM.
    MK4 # 9181 - Complete Kit - 408 EFI, TKO600, IRS - Delivered 2017-10-06
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  11. #11
    Senior Member edwardb's Avatar
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    Quote Originally Posted by cv2065 View Post
    Do you agree with Rich that the aluminum version would be preferred in my previous post scenario?
    You want me to disagree with Rich? Not today.

    I don't have anything to add to what I already wrote in post #3, plus what others have written.
    Build 1: Mk3 Roadster #5125. Sold 11/08/2014. Build 2: Mk4 Roadster #7750. Sold 04/10/2017. Build Thread
    Build 3: Mk4 Roadster 20th Anniversary #8674. Sold 09/07/2020. Build Thread and Video. Build 4: Gen 3 Type 65 Coupe #59. Gen 3 Coyote. Legal 03/04/2020. Build Thread and Video
    Build 5: 35 Hot Rod Truck #138. LS3 and 4L65E auto. Rcvd 01/05/2021. Legal 04/20/2023. Build Thread. Sold 11/9/2023.

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    Senior Member cv2065's Avatar
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    Quote Originally Posted by edwardb View Post
    You want me to disagree with Rich? Not today.

    I don't have anything to add to what I already wrote in post #3, plus what others have written.
    LOL...Thanks again for the input...

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    While I have not started my build yet, I went with the iron center section for 2 reasons. 1.) I am okay sacrificing the minimal additional weight for the strength. Initially I will only be throwing about 400 rwhp at it, but like to keep my options open. 2.) I really wanted 3.73 gears and I found a unit with 14K miles from a performance pack 2016 Mustang GT. This was much cheaper than building my own with the gears set I wanted and Torsen. Of my 3 current vehicles, one has a built 9" with traction lock, one has a built 8" with a detroit locker and one has IRS with a Torsen. After driving all of them, I really wanted the IRS & Torsen differential in my MK4. I find of all 3, the Torsen is much more streetable than the locker (for obvious reasons) and much more predictable than the Trac-Loc.

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    Senior Member CraigS's Avatar
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    i would go w/ the gear ratio you want and not worry if it's in aluminum or iron.
    FFR MkII, 408W, Tremec TKO 500, 2015 IRS, DA QA1s, Forte front bar, APE hardtop.

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    I am going with the 3.55 and aluminum case. RWHP should be 525 range and do not feel there will be any issues.

  16. #16
    Senior Member cv2065's Avatar
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    Ended up finding an iron 3.55 center section for $400 shipped, that was a takeoff from a new 2015 Mustang and only had 50 miles on it. I'll get the M-2300-MR rear brake kit and M-5970-M knuckle/hubs...all for about $1300 shipped. Not bad considering they are basically all new parts. Thanks again for the input.

  17. #17
    Forte's Parts Connection
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    Hi,
    I sell a new 3.55 iron cased IRS with knuckles & hubs for $1260.00. New aluminum case assembled add $600.00.
    All parts in stock and ready install.
    Mike Forte: Forte's Parts Connection / Framingham, Mass. 01702 / 508 875 0016 / [email protected] / fortesparts.com / Facebook Instagram: @fortespartconnection Est: 1981 dealing performance parts to build your dream car. A REAL SPEED SHOP with parts, price & knowledge. Developer of the early Ford & FE Tremec and Mustang firewall adjustable quadrant kits, external slave clutch release conversions & cable release conversions. Tremec Elite, FORD RACING & QUICKTIME DISTRIBUTOR!

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