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  1. #1
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    Default 86’ IRS swap or solid axle

    I have an all stock 86’ that I want to upgrade suspension on because my factory stuff is wore out. The car is not my daily but I want to continue to be able to take it anywhere and occasionally drag race it.

    A friend of mine has a Tubular K member/arm setup up with coil-overs, SN95 Cobra front brake setup, caster camber plates, and 03 Cobra IRS with 3.55 gears, cat-back for the IRS and some Bullet wheels and tires he will sell me for a good deal.

    I know typically IRS is not better for drag racing. My occasionally drag racing will be once or twice a year at most! More importantly to me is ride quality and drivability. Would the IRS swap give me this and would I still be able to drag race the car on occasion without terrible wheel hop?

    Any help you can give is appreciated!!!

    Here is my car!!!




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  2. #2
    Moderator wraithracing's Avatar
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    With the exception of the tubular K member that is the same essential suspension that is on both my 79 Pace Car and now my 86 GT. Ride quality is improved if the shocks and springs are matched properly. If not, honestly not much difference in my experience. The biggest issue with the IRS is the MUCH WIDER width of the rear suspension. Wheel fitment becomes an issue and you generally have to run much deeper offset wheels. I run a 36mm offset on 17x9 Cobra R's on my Pace Car and they work, but honestly for a bit more wheel lip clearance I would like at least a 40mm offset. My 86 GT is getting flared rear quarter panels to match my Maier Racing front flared fenders, so wheel offset and sizing doesn't apply here.

    To make the change you will have to do some brake line work in the rear, (I recommend the Maximum Motorsport kit), you will want the 94/95 Cobra brake M/C and (again the MM brake line kit helps), make sure the CC plates your buddy has are 79-89 style and not the 90-93 style as they are not interchangeable. You will need the rear exhaust hangers for the 99-04 cars to fit the IRS exhaust (Pypes mounts from LMR are the least expensive option). If the Bullitt wheels are the standard 8" wide with 30mm offset they will work in most cases and should be fine if you don't want wider wheels or tires.

    The IRS will hold up to drag racing if you don't get completely crazy. Honestly the full bushing kit from either Maximum Motorsports or Full Tilt Boogie Racing is the way to go. HUGE Improvement in the overall handling, ride, etc. of the IRS. Not a cheap option, but well worth it. Urethane or Delrin subframe bushings should be the bare minimum to start with. The rear subframe mounts are somewhat of a PITA to mount correctly, but can be done on a Fox, check out my 86 build thread for one option. http://vb.foureyedpride.com/showthre...r-Street-Build The Steeda or BilletFlow rear cover brace is mandatory for drag racing and really should be on all Cobra IRS. Truthfully the front and rear differential bushings should be swapped for urethane or delrin (MM or FTBR again). The pinion drive shaft flange is different on the IRS if your buddy hasn't swapped it already to the Fox sized, so keep that in mind. Front subframe (lower control arm) on the body bolts were 12mm on Fox and generally the 03/04 Cobra used 14mm, so keep that in mind when ordering bushings and which bolts you are using as the sleeves are different and if using the 14mm bolts you have to drill out the holes on the body.

    Depending on the deal from your buddy it sounds like a no brainer to make the swap. It's not exactly a bolt in, but reasonably close. So if you have the tools and skills to make the swap its just time consuming. Definitely changes the whole characteristics of a Fox and can be a lot of fun to drive. Hope that helps and good luck!
    ​Trey

    "I Don't build it hoping for your approval! I built it because it meets mine!"

    "I've spent most of my money on Mustangs, racing, and women... the rest I just wasted."

    Mustangs Past: Too many to remember!
    Current Mustangs:
    1969 Mach 1
    1979 Pace Car now 5.0/5 speed
    1982 GT Stalled RestoModification
    1984 SVO Still Waiting Restoration
    1986 GT Under going Wide Body Conversion Currently

    Current Capris:
    1981 Capri Roller
    1981 Capri Black Magic Roller Basket Case
    1982 Capri RS 5.0/4spd T-top Full Restoration Stalled in TX
    1984 Capri RS T-top Roller
    1983-84 Gloy Racing Trans Am/IMSA Body Parts

  3. #3
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    Default

    Quote Originally Posted by wraithracing View Post
    With the exception of the tubular K member that is the same essential suspension that is on both my 79 Pace Car and now my 86 GT. Ride quality is improved if the shocks and springs are matched properly. If not, honestly not much difference in my experience. The biggest issue with the IRS is the MUCH WIDER width of the rear suspension. Wheel fitment becomes an issue and you generally have to run much deeper offset wheels. I run a 36mm offset on 17x9 Cobra R's on my Pace Car and they work, but honestly for a bit more wheel lip clearance I would like at least a 40mm offset. My 86 GT is getting flared rear quarter panels to match my Maier Racing front flared fenders, so wheel offset and sizing doesn't apply here.

    To make the change you will have to do some brake line work in the rear, (I recommend the Maximum Motorsport kit), you will want the 94/95 Cobra brake M/C and (again the MM brake line kit helps), make sure the CC plates your buddy has are 79-89 style and not the 90-93 style as they are not interchangeable. You will need the rear exhaust hangers for the 99-04 cars to fit the IRS exhaust (Pypes mounts from LMR are the least expensive option). If the Bullitt wheels are the standard 8" wide with 30mm offset they will work in most cases and should be fine if you don't want wider wheels or tires.

    The IRS will hold up to drag racing if you don't get completely crazy. Honestly the full bushing kit from either Maximum Motorsports or Full Tilt Boogie Racing is the way to go. HUGE Improvement in the overall handling, ride, etc. of the IRS. Not a cheap option, but well worth it. Urethane or Delrin subframe bushings should be the bare minimum to start with. The rear subframe mounts are somewhat of a PITA to mount correctly, but can be done on a Fox, check out my 86 build thread for one option. http://vb.foureyedpride.com/showthre...r-Street-Build The Steeda or BilletFlow rear cover brace is mandatory for drag racing and really should be on all Cobra IRS. Truthfully the front and rear differential bushings should be swapped for urethane or delrin (MM or FTBR again). The pinion drive shaft flange is different on the IRS if your buddy hasn't swapped it already to the Fox sized, so keep that in mind. Front subframe (lower control arm) on the body bolts were 12mm on Fox and generally the 03/04 Cobra used 14mm, so keep that in mind when ordering bushings and which bolts you are using as the sleeves are different and if using the 14mm bolts you have to drill out the holes on the body.

    Depending on the deal from your buddy it sounds like a no brainer to make the swap. It's not exactly a bolt in, but reasonably close. So if you have the tools and skills to make the swap its just time consuming. Definitely changes the whole characteristics of a Fox and can be a lot of fun to drive. Hope that helps and good luck!
    Wraithracing thank you for the detailed response it is very helpful. All of this complete setup is still currently on his car. I forgot to add its Koni front struts , Bilstein rear shocks with Eibach springs, also a bump steer kit is included. I’ll have to ask about the caster/camber plates on what year they are for.

    He is offering it all for $1800.

    I definitely don’t want my wheels sticking out past my wheel wells and don’t want to add flares.

    He did tell me about the the master cylinder change, and brake line.


    Sent from my iPhone using Tapatalk

  4. #4
    Moderator wraithracing's Avatar
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    If the setup is still on his car and its a 79-93 Fox, then that should give you an idea of how it all fits and looks. Personally not a big fan of mismatched struts/shocks, but that doesn't mean they don't work. Again the CC Plates are the same from 79-89 then Ford changed the strut towers for the 90-93 models.
    ​Trey

    "I Don't build it hoping for your approval! I built it because it meets mine!"

    "I've spent most of my money on Mustangs, racing, and women... the rest I just wasted."

    Mustangs Past: Too many to remember!
    Current Mustangs:
    1969 Mach 1
    1979 Pace Car now 5.0/5 speed
    1982 GT Stalled RestoModification
    1984 SVO Still Waiting Restoration
    1986 GT Under going Wide Body Conversion Currently

    Current Capris:
    1981 Capri Roller
    1981 Capri Black Magic Roller Basket Case
    1982 Capri RS 5.0/4spd T-top Full Restoration Stalled in TX
    1984 Capri RS T-top Roller
    1983-84 Gloy Racing Trans Am/IMSA Body Parts

  5. #5

    Default

    When I swapped out my SA for a slightly modified IRS I could not believe how different the car went down the road, it was a night and day difference. I was able to still use the SN-95 wheels that I had at the time (99 GT wheels), but I will echo what was stated above, once you want to go with a wider rear tire "off the shelf" mustang wheels are not really an option any more. I had to do a lot of measuring to make sure my wheel and tire combo fit under my car without looking like a "roller skate".

    18x9 with a 275 tire

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    1986 SVO 1E (IRS'd and too much $$ spent making it turn and stop)

    1979 Pace Car #2179 (Awaiting GT500 swap)

  6. #6
    FEP Senior Member Patrick Olsen's Avatar
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    So what offset are those Traklites? On Motegi's website I see +35mm for the 18x9", but that doesn't seem like it would be a deep enough offset considering how much wider the IRS is.
    '89 GT convertible - not a four-eye
    '82 Zephyr Z7 - future track car

  7. #7

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    Quote Originally Posted by Patrick Olsen View Post
    So what offset are those Traklites? On Motegi's website I see +35mm for the 18x9", but that doesn't seem like it would be a deep enough offset considering how much wider the IRS is.
    18x9 35mm offset = 6.38 BS

    The IRS is not that much wider then a SVO 7.5 axle. I used the slim head bolts from MM and do not have any issues running a 275 (I did roll my fenders).
    1986 SVO 1E (IRS'd and too much $$ spent making it turn and stop)

    1979 Pace Car #2179 (Awaiting GT500 swap)

  8. #8
    Moderator wraithracing's Avatar
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    Cobra IRS is 60.3" according to most sources, although some list it as 60.6" wide.

    84-86 SVO rear track width is 58.3" according to many sources, but the 94-98 track width is often listed as 58.7" so . .

    79-93 Mustang/Capri rear track width is 57" (NON 1993 Cobra)

    The Cobra IRS and the 99-04 rear axles are wider than the SVO by at least 3/4" per side. So that can definitely affect wheel offset and clearance.

    I run a 17X9 36mm offset Cobra R replica on my PC with the IRS and even with 255/40/17 I am super tight to the wheel lips even being rolled. Personally I would like at least a 40mm offset for added wheel lip clearance. I have no inside clearance issues, but again that's a 255 not 275. Running the 275 might require the low profile rear subframe bolts for additional clearance, but won't know that for sure until the new engine for the PC is ready to be installed.

    Here's a 295/30/18 on 18X10.5 with 22mm offset and the Cobra IRS. Oh WAIT that doesn't fit under the stock rear quarters.

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    Last edited by wraithracing; 08-05-2021 at 08:49 PM.
    ​Trey

    "I Don't build it hoping for your approval! I built it because it meets mine!"

    "I've spent most of my money on Mustangs, racing, and women... the rest I just wasted."

    Mustangs Past: Too many to remember!
    Current Mustangs:
    1969 Mach 1
    1979 Pace Car now 5.0/5 speed
    1982 GT Stalled RestoModification
    1984 SVO Still Waiting Restoration
    1986 GT Under going Wide Body Conversion Currently

    Current Capris:
    1981 Capri Roller
    1981 Capri Black Magic Roller Basket Case
    1982 Capri RS 5.0/4spd T-top Full Restoration Stalled in TX
    1984 Capri RS T-top Roller
    1983-84 Gloy Racing Trans Am/IMSA Body Parts

  9. #9
    FEP Senior Member Patrick Olsen's Avatar
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    My understanding has always been:
    "normal" Fox length = baseline axle length
    '94-98 SN95 = +0.75" longer than baseline on each side
    SVO = 1.25" longer than baseline on each side
    '99+ SN95 / IRS = 1.5" longer than baseline on each side
    I don't know how all of that compares to the advertised track widths of the various models, I just know these are the axle lengths that I've seen talked about for many years on the intarwebs.

    I ran the M179 Cobra R wheels (17x9" +36mm) with 275/40s on SN95 axles and had to roll the fender lips flat to avoid contact when hitting a big bump on the highway. [The drive home from the shop before I rolled the fender lips resulted in big chunks being taken out of the sidewalls of my tires!] Had I tried that wheel/tire combo on axles that were another 0.75" longer no amount of fender rolling would've fixed the problem.

    But I suppose with the IRS there's some camber gain under compression, so the top of the tire tucks in, something that clearly doesn't happen with a solid axle. That means the wheel fitment with the IRS can be a bit more "aggressive" and not cause issues.

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