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How To: IRS install into a SN95 Cobra

Discussion in 'How-To' started by mysteed, Apr 24, 2015.

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  1. mysteed

    mysteed Well-Known Member Established Member

    Mar 16, 2005
    Adairsville, Ga
    How To: SN95 IRS Swap

    It's been a few weeks since I've completed this swap and I have to say it's one of the best modifications that I have ever made to my Cobra. With the IRS in the rear the car feels so much more comfortable on the street and taking it through the twisties has never felt better. Despite the setbacks that I encountered I would do the swap again in a heartbeat, in fact I plan to do anther IRS swap on my wife's SN95 in the future.

    Before I delve into the How-To in depth I thought I would offer a few pros and cons to the swap.

    Much smoother ride on the street
    The car handles a lot better in the corners
    IRS exhaust looks WICKED from the rear
    Very unique swap for our SN95's

    Cost of parts involved
    Time involved to do the swap
    All parts needed are IRS specific (we'll cover this later in depth)
    Added weight to the rear of the vehicle
    IRS not originally designed to fit our SN95's

    Items needed:
    IRS from a 99/01 Cobra or a 03/04 Cobra
    IRS specific exhaust from a 99-04 Cobra
    IRS specific rear springs from a 99-04 Cobra
    IRS specific rear shocks from a 99-04 Cobra
    IRS specific rear soft brake lines from a 99-04 Cobra
    IRS specific rear ABS sensors
    Universal 3/16”x40” hard brake line
    Universal 3/16”x20” hard brake line
    Universal 3 way brake line tee (O'Reilly's part number: 13044)
    Universal 3/16” male brake line fitting
    Ford fuel filler neck gasket (Ford part number: F8ZZ-9072-AC)
    Motorcraft fuel filter (Motorcraft part number: FG-986B)
    2 Grade 8 9/16” front subframe bolts (Full Tilt Boogie Racing part number: FT1600)
    Low profile IRS rear subframe bolts if running wider than stock tires (Maximum Motorsports part number: MMF-1)
    IRS specific rear cover brace (not required but definitely recommended)
    2 quarts of 75w140 gear oil (no specific brand but be sure it includes friction modifier)
    1 small 4 ounce bottle of friction modifier (if not included in 75w140 gear oil)
    4 Grade 8 9/16”x4” bolts with washers & nuts
    Full metric socket set
    9/16” deepwell socket
    9/16” wrench
    Assorted drill bits (including a 9/16” drill bit) & drill
    Sawzall or cutting wheel
    1 bottle of Loctite
    1 hydraulic jack
    1 standard size level
    Brake line flaring tool kit (available for rental at most parts houses)
    4 brake line copper washers (available at any parts house)
    Small can of spray pint (color of your choice)
    Small file or rough sandpaper

    Prepping for installation:

    Remove old gear oil & add just shy of 2 quarts of 75w140 gear oil (plus 4 ounces of limited slip additive if not included in 75w140 gear oil).

    Add IRS differential brace (follow directions specified from manufacture).

    Check all bushings for cracks or fatigue. Replace them now before IRS installation.

    Clean both the exhaust and IRS of any road grime (this is much easier to do while the IRS is out of the vehicle).

    IRS installation:

    Step 1: Remove old SRA, remove rear shocks, remove rear pinion snubber, and disassemble brakes (plenty of How-To's can be found on the web).

    Step 2: Replace fuel filter if not recently done. Once the IRS is in it will be a PAIN to do it.

    Step 3: Remove gas tank (optional but makes it a lot easier to work around).

    Step 4: Replace fuel filler neck seal on gas tank if removed.

    Once you have everything out of the way it should look like this:

    Step 5: The hard brake lines on your SN95 will NOT work with the IRS setup. You will need to cut the hard line coming from the front of the vehicle at about where it comes close to the upper control arm bracket. Take the IRS specific rear brake soft lines and bolt them up the frame where you unbolted your original brake lines. Use the 40” universal brake line to run a line from the drivers side soft brake line to about where the right side upper control arm bracket is. Then use the 20” universal brake line and run it from the passenger side soft brake line to where the right upper control arm bracket is. Be careful to make sure that the fittings on the hard brake lines stay at the end of the lines when bending the line itself (a little tape will do wonders for this).

    You should end up with a line that looks something like this from the rear of the vehicle:
    (Drivers side is on the left)

    Step 6: Using the brake flaring tool add your purchased 3/16” brake line fitting to the stock line and make a flare on the line. Once you are sure the fitting will seal, it's time to mock up your newly fabricated line into the car. If everything fits the way that you want it, tighten all the fittings.

    Here is my line mocked up in the vehicle. Please note that I routed the line on the back side of the fuel lines so as to minimize the possibility of contact with the IRS.

    Step 7: Locate the tabs that are above where the quad shocks bolt to. You will need to remove or bend those tabs so that the IRS rear subframe bracket will sit flush with the subframe. Be sure to do this with both sides of the vehicle.

    Here are the tabs:

    Here are the tabs bent out of the way:

    Step 8: Remove the rear subframe IRS brackets from the IRS and bolt them into where the factory quad shocks were originally using the quad shock bolts.

    Step 9: Using a paint pen or marker, mark where the underside of the bracket where the IRS needs to bolt to. After you have made your mark you will need to remove the bracket for the next few steps.

    Drivers side:

    Drivers side with the bracket removed:

    Passengers side:

    Passenger side with the bracket removed:

    Step 10: Using a small 1/4” or so drill bit you will need to drill in the four spots that you have marked on your frame. Once you have drilled your pilot hole, you can work your way up to using the 9/16” drill bit.

    Drill bits that I used:

    Drivers side after drilling with 9/16” bit:

    Passenger side after drilling with 9/16” bit:

    Step 11: Next we will be making the slots that will allow us to mount studs where we just drilled our two holes on the underside of the frame. First make two pilot holes about 3 inches apart from one another with the center between the holes being directly in line with the holes drilled in Step 10. After drilling the pilot holes step up your drill bit till you use the 9/16” bit.

    It should look like this after stepping the pilot holes up to the 9/16” drill bit:

    Step 12: Now using your sawzall cut out the portion of the frame between the holes.

    Step 13: Once you have cut out both holes on both sides of the vehicle make sure to clean up any sharp edges with a file/sandpaper/etc and then give the newly cut sections and the holes on the underside of the frame a quick shot of paint (I used black).

    Step 14: After the paint has dried it's time to bolt the IRS bracket into place. Go ahead and insert the 9/16” x 4” bolts and washers you purchased into the holes you've cut in the frame and have the thread of the bolt coming down through the bottom of the frame. Once they are through you can bolt up the IRS bracket with the existing quad shock bolts, then add the nuts to the bolts on the bottom of the frame. After you have hand tightened all the bolts and nuts make sure to torque everything down very tight (I torqued the nuts to about 100ft lbs).


    Step 15: Now that we have the rear subframe brackets attached it's time to turn our attention to where the front subframe mounts. The factory holes where the SRA lower control arms (and soon to be front IRS subrame) bolt to is 12mm in size. The holes will need to be drilled out to 9/16” in order to fit the larger bolts that are needed for the IRS to fit comfortably.

    Upper hole in picture has been enlarged to 9/16”. Note: Test fit your bolts to be sure they fit correctly and be sure there is no play or looseness between the bolt and the hole otherwise the IRS will shift and cause noise.

    Step 16: Once the holes on both sides have been drilled out and checked it's a good idea to take a long pry bar (or similar item) and pry the front control arm brackets apart a little bit. This will help a lot with installing the IRS assembly. Don't worry if you think you have pried on it a bit too much, as you tighten the subframe bolts the box will close up a bit.

    Step 17: The last thing to do before IRS installation is we need to modify the rear IRS subframe bolt. The factory bolt is just a little bit too long for our application so it will need to be shaved down about 1/4” of an inch. If this is not done it will prevent the bolt from seating completely in the rear IRS subframe bracket.

    Modified bolt on the left and the stock bolt on the right.

    Now for the fun part! It's time to install the IRS!

    Step 18: I did this installation with the vehicle on a 4 post lift but it will be roughly the same if you are doing this on the ground in the garage. Begin by manuvering the front of the subframe into the SRA lower control arm boxes where we previously enlarged the mounting bolts (I used an engine hoist for this but you may use a jack if working on the ground). Be sure to try and get both subframe ends into the boxes at the same time, if you try and do one end at a time the IRS will try to fight you the whole way. Once the bolt holes are lined up you may install your 9/16” bolts and nuts with the bolt head on the inside of the box (remember to have washers on both ends of the bolt). At this time do NOT tighten the bolts, just leave them a bit loose.



    Step 19: Next go ahead and install your IRS rear springs into the IRS spring perches and install your IRS ABS sensors if they were removed before.

    Here is why you CANNOT reuse your old springs (factory springs with ¾ coil cut in this picture) besides the obvious fact that the SRA springs are of a different size.

    Yes the exhaust is sitting ON THE GROUND in this picture. Anyways, back to the install!

    Step 20: With the front IRS subframe bolts in place (but not tightened) you may jack up the rear of the IRS subframe (I recommend underneath the pumpkin) until the rear subframe bolt holes are lined up with the previously installed rear subframe brackets. Once both holes are lined up install the bolts that we modified in Step 17 and then tighten the front subframe bolts from Step 18 (at least 100ft lbs). Once all the bolts are tightened you may remove any jacks or stands you have underneath the vehicle.

    Step 21: Now that the IRS is installed (YAY!) it's time to reinstall the your brakes and the IRS soft brake lines. Be sure to bleed the brakes after everything is installed and check for leaks at all of the new connections in the rear.

    Step 22: After the brakes are bled be sure to hook up the emergency brake cables. They may be a bit tight of a fit, but they will stretch a little bit. Also if you did not retain the factory emergency brake cable clamps on the rear lower control arm you can use a zip tie as a temporary solution. If you leave them loose they will rub against the tire.


    Step 23: Install your new IRS shocks in the factory location up top and connect them down to the IRS lower control arm. I found that a jack placed under the control arm will help bring it up the right height.

    Step 24: Since everything that is IRS specific has been installed you can reinstall your gas tank and fuel filler neck. The gas tank may be a bit tricky with connecting the fuel lines but you can reach the lines from the drivers side of the tank.

    Step 25: Reinstall your factory driveshaft & be sure to give the bolts just a drop of Loctite each.
    Note: Some people using the 03/04 Cobra IRS have reportedly had issues with the driveshaft yolk not bolting up to the pinion flange. You will need a Precision 448 u-joint to make it work.

    Step 26: Install the new exhaust to your existing midpipe (mine went in easiest when I installed the tailpipe hangers first and worked my way forward). Be sure to not tighten the bolts at the midipipe until you get the pipes to sit level. You can check this using a level on the exhaust pipes underneath the differential and then tighten the midpipe bolts after you are happy with the level of the pipes.



    Step 27: Test drive the vehicle! Be listening for any strange noises or any rubbing. If you hear nothing abnormal go drive the vehicle over some rough roads & marvel at how smooth the ride is compared to the SRA. :coolman:
  2. brucesvt

    brucesvt Well-Known Member Established Member

    Aug 24, 2005
    great write up, this needs to be a sticky....
  3. 03cobra#694

    03cobra#694 I Have LL’s Hammer Staff Member Super Moderator

    Aug 12, 2008
    SW FL.
    Yep, very nice write up indeed.
  4. Sheriff41

    Sheriff41 Member Established Member

    Nov 23, 2009
    Oahu, HI
    Well done!
  5. ZeroDCX

    ZeroDCX Active Member Established Member

    Sep 3, 2009
    Excellent write up and nice ride!

    Mods: Can this be moved to the "How to" section?
  6. ashleyroachclip

    ashleyroachclip Well-Known Member Established Member

    Jan 25, 2007
    Good write up, but I have done this to a Fox,91, and I am not sure the metal is very strong @your step 11-12.
    The cobra goes through the bottom side of the rail, but it has a doubled plate and the nut plate inside.
    I cut those off the parts car, was going to cut and weld it to my car, decided to just weld the bracket.
    It will never be a sra again as long as I live.

    But since it is bolted where the quad shocks once reside as well , it may be no issue.
    I just welded that bracket to the frame as well as it is bolted @ the quad mount .

    The ride and handling is certainly much improved over the sra.
    Last edited: Apr 25, 2015
  7. mikesyellocobra

    mikesyellocobra Member Established Member

    Jun 30, 2013
    Emmaus, PA
    Do you have an estimate of how long the whole project took from start to finish?
    in4thewin24la likes this.
  8. mysteed

    mysteed Well-Known Member Established Member

    Mar 16, 2005
    Adairsville, Ga
    Thanks fellas! It was a lot of work but it was sooo worth it. If your Cobra isn't a dedicated track car, I recommend the IRS wholeheartedly.

    I was also concerned initially when we decided to mimic the 99-04 Cobra's bracket installation. However, as I had access to my dad's "new" 01 Cobra we checked the thickness on his car at the rail compared to mine and it was identical. I overbuild my cars to last the test of time and if I had any doubt that the way we mounted it would not have worked, I would have welded the bracket to the frame as well.

    If I had this guide to go by and all my parts laid out and cleaned, this could easily be done in 8-10hrs (6hrs if you have access to a lift).
  9. 98cobra#1222

    98cobra#1222 that's just my game Established Member

    Jul 12, 2011
    Greensburg, Indiana, United States
    Awesome write-up. This will save a lot of headaches when it comes time to do my IRS swap. Thanks!
  10. Mi-Cyn

    Mi-Cyn Member Established Member

    May 18, 2009
    Middleburg, FL
    Awesome write-up:rockon:
  11. sho amo

    sho amo Member Established Member

    Mar 21, 2007
    olney, md
    Great write up. Ill be tackling this in the next year. Needs to be stickied!
  12. mysteed

    mysteed Well-Known Member Established Member

    Mar 16, 2005
    Adairsville, Ga
    Thanks fella's! Hopefully this will save some people from the headaches that I encountered.

    Does anyone who has done the swap before have anything to add?
  13. DVJ38

    DVJ38 One mod at a time Established Member

    Jan 7, 2007
    Wow, fantastic write-up! Thank you very much! Can't wait to use it some day lol.
  14. me32

    me32 BEASTLY SHELBY GT500 TVS Moderator

    Sep 6, 2005
    Fantastic write up.
  15. crj814x4

    crj814x4 Driving GOD Established Member

    Jan 8, 2007
    South Gate, CA
    Great write up. Been thinking about doing the same swap for a few years now. This should definitely be a sticky.
  16. rochoa21

    rochoa21 Member Established Member

    May 14, 2013
    Alameda, CA
    Beautiful color
  17. Shin

    Shin Member Established Member

    Jan 20, 2008
    Grand Cayman
    awesome write up thanks for taking the time!
  18. Blown_By_You

    Blown_By_You Richard Head Established Member

    Sep 22, 2005
    Great write up! As soon as i can find an 03 IRS for my 97, i'll be using this!

    Barry, please Sticky!
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