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Thread: Mustang S550 IRS

  1. #1
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    Mustang S550 IRS

    I have been hesitant to start this thread because I know the implications of what I'm doing, but here it is. This is a ridden hard 300A that spent a bunch of time daily driven in Massachusetts and is on it's third engine. The rear has a nasty growl when you put a couple heavy dudes in the back, so it was time to make a change.

    First I went to the local junkyard and grabbed a P71 rear frame section. IRS is a 2018 PP1 rear with 3.73 Torsen and the heavy sway bar. Goal is to maintain factory rear air ride but we'll get to that later.






    Okay, so I can't delete old attached pictures, I'm out of space, and it wont let me embed google photos so lets start with links. ETA Okay, they're in but **** they're big. I'll keep working on it.
    Last edited by boom boom; 05-19-2020 at 09:43 AM.

  2. #2
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    Everyone knows Panthers are full frame cars on rubber body mounts so I'm not worried about NVH from the cradle but I still want it to be serviceable as an assembly. I ordered an ebay knockoff set of Ford racing solid cradle bushings- part number M-5872-M. The Ford parts are long discontinued and I didnt want to pay $300 for a BMR lockout set so $60 for the knockoff was the winner. A buddy sent me a youtube video of a trick way to remove the rubber cradle bushings: cut the rubber out with a hole saw, then cut the bushing with a sawzall, and knock the bushing out with apunch. Took only a few minutes for each one.








    Rear of IRS






    Bushing removed







    Bushing Cut


  3. #3
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    And here it is mocked up before I set all the ride heights. I spent a lot of time going through the dimensions before I undertook this project. The two big bonuses are the IRS fits inside the Marauder frame and at intersecting elevations. I have a crazy raggedy spreadsheet that includes all of my research. I'll clean it up at the end and post it when I have validated everything. Until then, here are some pertinent dimensions. The Mustang RWMSW is almost 2" more narrow than the Marauder. Much better than being wider... This crazy thing might actually work.

    Curb weight Dist F Dist R Wheelbase Track F Track R W excl Mirror RWMSW
    2003 Marauder 4195 55 45 114.7 63.4 65.6 69.875
    2008 GT500 3920 57 43 107.1 61.9 62.5
    2013 Boss 302 LS 3650 55 45 107.1 61.9 62.5
    2013 GT500 3845 57 43 107.1 61.9 62.5
    2018 GTPP 3860 55 45 107.1 62.3 64.9 75.4 67.9375
    2015 GT350 3798 107.1 61.8 64.4






    Early Mockup Front







    Early Mockup Rear






    Rear mount down


  4. #4
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    And here it is jigged up. That's all for today, sure wish I had a frame table for all of this. I'll follow up with the elevations soon and see about getting the picture size reduced.







  5. #5
    Join Date
    Mar 2007
    Location
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    This'll be wild if you pull this off. As a 4 time panther owner and S550 owner I 100% support your science experiment


    2016 Ford Mustang GT PP - Lund nGauge, BMR vertical links, MGW Shifter, BMR CB005

    2004 Mercury Marauder - Eaton M112, Billetflow 3.4, Accufab SBTB + Plenum, T-56 Magnum, 4.10s, Stainless Works LTs, 2.5" Exhaust, Borla Stingers, Ridetech Coilovers + Adj rear shocks, FX-R projectors, Wilwood 13" BNSL6 + FNSL4 BBKs, S55 Gauge Cluster
    440rwhp & 406rwtq Tuned by Adam @ Revolution Automotive
    2063/3214

    (Sold) 2003 Mercury Marauder - Stainless Works LT's, Stainless Works 2.5" Catback, JLT Intake, Mo's Xcal4 Tune
    5377/7838

    2000 BMW M5 - Evolve Intake, Evolve Alpha-N tune, Stoptech ST-40 BBK, UUC SSK, Dinan front STB, Dinan RSB

    1996 Ford Bronco - 4" lift, 35" BFG A/T's, Punisher Valve Body, MH1 Projectors

  6. #6
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    Mustang S550 IRS

    I like your motivation! From a quick look it appears four simple frame perches and she mounts up easily. Simple as welding in two crossbars nice and straight with gussets and then cutting out the unneeded center and capping off what remains. Done right and it would be stout and look factory.


    I honestly think retaining the air ride and finding shocks that won’t interfere as well as figuring out the gas tank and tailpipe routing would take way more effort than mounting the cradle up.


    Sent from my iPhone using Tapatalk Pro
    SOLD 12/31/2021

    Cobra short block / ProCharger F1-A / Cog drive upgrade / Manley valves / Brian Tooley Racing valve springs / 80# inj. / -8 fuel line / -6 return / Aeromotive 340 pumps and regulator / 2012 GT500 TR6060 six speed / twin disc clutch / Driveshaft Shop GT500 upgraded CV joint shaft / BA5000 blow thru MAF/ 4.10's / Ford Racing 31 spline set up and Girdle / Metco D.S. safety loop / Stainless Works headers, cats, and everything else out back / ClassGlass hood / Bob's hood strut mod / Hurst line lock / Metco control arms / Addco front and rear sway bars / Monroe severe duty shocks all around / Race concepts rotors / Badgeless grille / 35% tint / FBM's interior LED kit and ambient lighting kit / Pioneer FB700 deck / kenwood speaker 4 Ch. / Rockford Fosgate 10" sub and amp / Kicker 6 X 8's all around / back up cam / bluetooth / Pioneer Xm / 704 RWHP 603 RWTQ tuned by Mo's Speedshop.

  7. #7
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    Thanks for the encouragement. Four brackets, two spring mounts, some crossmembers, and new driveshaft. I too, at first, hoped it would be a slam dunk when I lined the IRS up under the frame but there are two big issues. One, the shocks are dead nuts under the panther frame rails. That's cool, but on closer inspection, they need to mount at close to the same height as the Marauder shocks so the frame rail has to go or have a 3-4" tube welded through it. The second issue is that the sway bar brackets and mounts are into the trunk pan. So the fuel tank has to go, the shocks/frame rails have to be dealt with, and the sway bar needs to be relocated. That put me on a new path....backhalf, but I really don't want to get at far into the weeds especially trying to do this outside of the car and without a table. So I wound up with the bracing setup that you see. I measured almost a dozen different S550 for ride height and also my 2003 vic (which has sagged to the perfect ride height) and my 300A. That gives me the dimensions I need to mock it up.


    Should I backhalf?







  8. #8
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    Here is the mounting height for the IRS. It is based on a 27.5" tire and a level lower control arm. This is the same setup as the PP1 cars and a GT350 at the Autoshow.


    Front Mount elevation






    Rear Mount Elevation -- More on this later. The cradle hits the front watts link bracket and the locating tabs are interfering with the frame so it isn't in the front to back position yet.



  9. #9
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    Mustang S550 IRS

    Personally I’m not scared off doing a full frame build, a kit or my own, Z cutting and shortening, doing geometry and altering front and rear everything, or notching for clearance or lowering.

    But, when I hear back half or front for that matter I cringe unless it’s done by a very reputable shop. I’m a Plumber/welder/fabricator and an ex mechanic (until I needed to start making money LOL) and knowing the rigidity that is comprised when piecing halves together without the proper knowledge and tooling I would let guys smarter than me tackle it..

    Something I don’t often say and that logic doesn’t always yield perfect results. I’ve seen two back halved cars at local car shows, both done by shops, and both looked like they would snap in half if they were ever fitted slicks!

    Of course that’s two out of the hundreds I’ve checked out..

    If your goal was tubbing I’d say proceed with caution and go for it. Just an IRS swap, not worth the work IMO.

    Look up Toomanyfords and check out his stout back half! John’s car is beyond a crazy build! And I’m quite sure he’d agree on not slicing it up. But then again, I absolutely love being told I can’t do something. It gives me purpose to go on and prove wrong.

    Sent from my iPhone using Tapatalk Pro
    Last edited by justbob; 05-19-2020 at 06:42 PM.
    SOLD 12/31/2021

    Cobra short block / ProCharger F1-A / Cog drive upgrade / Manley valves / Brian Tooley Racing valve springs / 80# inj. / -8 fuel line / -6 return / Aeromotive 340 pumps and regulator / 2012 GT500 TR6060 six speed / twin disc clutch / Driveshaft Shop GT500 upgraded CV joint shaft / BA5000 blow thru MAF/ 4.10's / Ford Racing 31 spline set up and Girdle / Metco D.S. safety loop / Stainless Works headers, cats, and everything else out back / ClassGlass hood / Bob's hood strut mod / Hurst line lock / Metco control arms / Addco front and rear sway bars / Monroe severe duty shocks all around / Race concepts rotors / Badgeless grille / 35% tint / FBM's interior LED kit and ambient lighting kit / Pioneer FB700 deck / kenwood speaker 4 Ch. / Rockford Fosgate 10" sub and amp / Kicker 6 X 8's all around / back up cam / bluetooth / Pioneer Xm / 704 RWHP 603 RWTQ tuned by Mo's Speedshop.

  10. #10
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    Just out of curiosity, how much is the weight diffrence between the IRS and a SRA?

    These cars are heavy enough without adding extra weight. Especially in the rear of the car.

    Are you using that back half for mock up only? Or do you plan on keeping it and stitching two frames together?

  11. #11
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    This is my rough math for the swap

    Stock Curb weight 4195
    2018 GTPP Curb 3860
    Weight Distribution F/R 55/45
    Stock Rear Corner 944
    2018 GTPP Rear Corner 868.5
    Add for IRS 110
    IRS Swppped Rear Corner 1019
    Rear Unsprung per side 132
    Rear Spring Motion Ratio 0.48
    Rear Shock Motion Ratio 0.7
    Shock Droop 1"
    Shock Bump 2"
    2018 GT PP Static Load 1534
    Swap Static Load 1848
    2018GTPP Wheel Rate 176
    Swap Wheel Rate 213
    2018 GTPP Spring Rate 728
    2018 GTPP Calc Rate 767
    2018 GT350 Spring Rate 900
    Swap Calc Spring Rate 923
    2018 GTPP Free Length 13.25"
    Spring Length at Ride 10.25"

  12. #12
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    A lingering question is the fuel sending unit. The tank needs to be moved to a horizontal position over the cradle. It will be somewhere around 34x22x6, which is a good size tank but can't accommodate anything stock. I haven't been able to find a reasonably priced 18-20 gallon fuel cell that would fit and I want to keep the stock EVAP and fill so I think it'll be a custom tank. In the boat world custom tanks are made everyday so I already have the tank estimates in and they are reasonable, but I need to settle on a fuel system.


    Future plans are a mild M122 so I need something that has increased delivery or that I can upgrade later. I saw a recommendation that a single Coyote fuel pump is good for 600 rwhp but the hat is at a funky angle and they changed the level sender resistance so I will source that pump and use a different hat if I need to. First thought was an Aviator fuel pump. Its not terribly expensive, close to the right height and orientation, plus has the correct connectors and resistance. It would be "drop in". The problem is that the plastic body would limit me if I ever wanted to upgrade pumps.







    Second Option is a Terminator fuel hat. This would give me many more options for upgrades and would also be "drop in". They are almost double the aviator price for a good used stock one. That, and I don't know the exact height. It would be really nice to get one in hand so that I could measure it but I can't justify $500 for a used part that might not even fit.






    Yet another option is something like the Aeromotive Phantom 340 kit. It is universal, can be run return-less, and is in the same price range as the terminator aftermarket but it locks me in to a single manufacturer.







    The last option would be to chop up my stock Marauder hat and move it to a vertical configuration. It would certainly be the least expensive but may make things more difficult down the road for upgrades.


    Last edited by boom boom; 05-20-2020 at 07:46 AM.

  13. #13
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    External fuel pump? Might give you the freedom to locate it where you want.

  14. #14
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    Definitely open to suggestions. I don't have any experience with external fuel pumps other than being very noisy so I haven't spent much time on it. I'm pretty sure I can reuse the stock fuel line so putting a mustang or aviator hat in seems to make the most sense for plug and play. It's also pretty easy to connect an external pump to a sumped tank.

  15. #15
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    While I let the fuel stuff percolate, here is some more shots of everything mocked up.

























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