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Hey all, let me preface by saying I have a fox body I would like to install an 03-04 Cobra IRS into for the purposes of improving the car's compliance over broken pavement. The car is a 100% street car, so I'm not concerned with lap times, or 1/4 mile. The car makes 600whp and is backed by an auto trans and I roll around on 275/40/17 ET Street R's. Right now I run stock fox upper control arms, MM lowers with poly bushings on the chassis side and the articulating bushing on the rear end side, Bilstein HD's, and H&R sport springs. When I do the IRS I will go fully poly (to cure wheel hop) and I'll run stock 03-04 Cobra spring rates on Bilstein HD coilovers.

Does anyone have any before/ after feedback regarding the IRS? Obviously a bone stock 99-04 Cobra will ride waaaay better than a lowered SRA fox. But how does a stock spring/ poly equipped IRS car ride? Even if the poly harshens things up a bit, my thinking is there is NO way it could be harsher than a solid axle car.
 

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Rides great. I've got the entire Full Tilt Boogie delrin setup in mine minus the subframes which are poly since those are non articulating. Tokico D specs, Steeda springs, no isolators with the aluminum diff bushings and the Full Tilt tie rods. Eibach swaybar with solid links.

NVH is really not increased a ton... I was kinda shocked. Ride is great if you pick the right springs. Car flies around corners and 0 wheel hop.
 

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I Swapped my 86 GT with a Cobra IRS years ago, it's all stock cobra components Shocks, springs (uncut) and bushings. I love the ride in the rear. I barely have wheel hop with a stock 5.0 and T5.
 

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Well with a stock 5.0 with the E7 heads you won't make a ton of power to hop the wheels too much. If you plan on adding power you NEED to do the complete bushing kit for it or you'll snap halfshafts. Poly for the subframe bushings, delrin everywhere else, aluminum front diff bushings and use the shims to get the front of the pumpkin raised as high as it will go without touching anything and also replace the stock tie rods with aftermarket rod ended ones.
 

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I have 18x9s with 275/35/18 and I have plenty of room even without spacers. With spacers I have a ****ton of room. If you are drag racing with fat tires out back you usually stick with the SRA anyway. I'm setting the car up for tracking so 275 all around is good. I might go 265/40/18 next go around like they did with the 00R.
 

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So, while I was modding the 86 Fox my DD was an 01 Cobra. The Cobra had stock bushings but H&R Super Sports with 00 Cobra Bilsteins. Ride was better than stock but too low. Still had bad axle tramp and the rear tended to squat on acceleration causing massive negative camber which reduced traction. But compared to the stock Fox, even with mild front C/Os, the Cobra was sublime. Until I went to a PM3L on the Fox.

I already had an MM PHB and their LCAs on the car with Bils all around. The PM3L replaced the PS upper and the DS was removed. Rear springs were replaced with H&R T/As at 370-410# progressive. Ride was improved by at least a magnitude. This because all the bind inherent to the converging 4-link was removed. Because the IRS doesn't have bind, this is also why it rides better.

So over several years driving both the Fox (later with an MM T/A swap) and the 01 Cobra, I can say the ride was only marginally better for the Cobra. With a stick axle converted to a T/A or one of the other options, the ride improves so dramatically that an IRS is not needed if ride quality is the only goal.
 

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Did you replace the IRS tie rods with Full Tilt or MM units? Because yeah with the stock tie rods you will get that squat when you punch it hard. The aftermarket ones reduce or eliminate it completely depending on your power. Also rubber bushings in the control arms will contribute to **** moving around that shouldn't along with the 3 point mounted pumpkin on rubber will move around a lot without solidifying it with the solid front mounts and the FTBR delrin unit out back. Did you replace the cross axis joints with new ones? I've driven a good deal of these cars with a stick axle and nothing has ever approached the fully redone IRS I have now.
 

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I was going to do the FTB pieces on the Cobra because I was going to track and A/X the car. But after I went to the PM3L on the Fox, the Fox handled so much better than the Cobra and was lighter and easier to throw around. So scrapped the FTB plan and focused on the Fox, ultimately doing the full MM catalog.

FWIW, I now DD an S550 GT with PP. While its IRS is light years better than the old Cobra (well the whole car is), the Fox is still more nimble and with the T/A rides nearly as well. The Fox is old tech so the new S550 platform with IRS has more potential and can still retain ride quality civility. I can't say that about an IRS equipped Fox or sn95/New Edge.
 

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There are two things here that are being confused with each other.

Squat on acceleration. This is a function of the antisquat percentage in the rear suspension design. I'm not aware of any IRS products that change this from stock.

Toe change. This can be from a couple of different sources. The most common is bumpsteer. This is from the rear view geometry of the suspension. The less common cause is from a lack of link stiffness and subframe in the IRS causing deflection under loads. These loads can be from applied engine torque, cornering, brake torque, etc. Using stiffer tie rods will reduce this toe change under acceleration and cornering.
 
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