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Kirkham Motorsports

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  • 1 Post By PaulProe
  • 1 Post By Tom Wells

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Old 05-05-2021, 03:56 PM
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Default IRS from S550 Mustangs for a Cobra ?

Ive been studying these S550 rear IRS set ups and they are becoming cheap and plentiful since they are fitted to 15’-21’ Mustangs. Has anyone attempted to pitch their solid rear axle and try to graft one of these to their Cobra’s? Ive watched a few YouTube videos of guys doing this to other cars and thought this might be a neat path to take on a Cobra. The added bonus is there are loads of aftermarket parts to make these subframes and diffs live with big power (800+ hp). Heck the factory GT350 setup would be strong enough for my big block for daily street duty. It seems a reasonable approach. Granted its a little heavier and some chassis welding is extensive but the ride and handling would be better. The Jag IRS and BMW IRS on some Cobras have their own pros and cons too, this S550 solution just seems strong, practical and cheaper. And FFR offers a version on their kits too.

Thoughts? Experience?
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Old 05-05-2021, 06:45 PM
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Track width is considerably larger than the Cobra's I'm using one in a Hurricane Motorsports build. Uses the pumpkin and uprights. All other parts are scrapped and replaced with new pieces

Paul
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Old 05-06-2021, 04:38 AM
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Like Paul said, the width of the IRS for those years is a bit over 66" wide. You are going to have to narrow it by close to 10" to get into Cobra territory. I think the earlier Mustang Cobras from 98-04 were around 61" wide. Still pretty wide as I looked at one of those years ago.
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Old 05-06-2021, 06:17 AM
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Our new Backdraft is coming with an S550 rear end
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Old 05-06-2021, 09:56 AM
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Have you looked at the latest Factory 5? They use the latestMustang IRS.
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Old 05-06-2021, 12:49 PM
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Quote:
Originally Posted by Tom Wells View Post
Have you looked at the latest Factory 5? They use the latestMustang IRS.
Don't they modify them pretty extensively? I know they used to do that with the old T-bird IRS units they used. They had their own control arms, spindles, and axles.
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Old 05-06-2021, 06:04 PM
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When you say they have a S550 rear-end, that is typically referring to the differential and the uprights. Ford Performance sells both into the replica market for that specific reason.

I know that Hurricane and Factory Five both use those components. Was unaware Backdraft has gone that direction also but doesn't surprise me. They used to use a BMW system but Ford is readily available.

With that said, all take a different set of control arms and half shafts. No way to use the Ford stuff, it's too wide.

Same applied to the older T-Bird units. Differential and uprights were standard Ford components. Control arms and half-shafts were special.
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Old 05-08-2021, 05:12 AM
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Thanks for this discussion. Yes Ive seen some cars retrofitted with custom control arms to make it fit their desired track width while using the S550 center section. What makes a retrofit of a Ford IRS to my West Coast Cobra so curious is that my axle width from axle flange to flange is about 63”. Yep 63. Thats because the West Coast replicas are wider and longer than originals and the rear hips on these cars are huge. It allows 12 inch wide wheels and 13 inches of tire tread on the asphalt. A whole lotta “Rosie”.
But I digress. The WCC uses a rear axle assembly from a full size ford 73-79 car.
One post I found on line ( not confirmed) for modern Ford IRS systems for flange to flange dimensions
Were:
S550 Mustang = 65 inches
Crown Vic = 65.6 inches
Ford Elplorer = 58.6 inches

My research continues.
Thx
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Old 05-08-2021, 10:07 AM
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No matter which factory IRS you use, you will need to narrow it to fit the Cobra. You may or may not want to use the OEM uprights. If you do use them, make sure the upright you use provides enough room for a large supporting bearing ID. Some of the uprights (Ford calls them knuckles) leave the drive hub bearing register with a thin supporting wall that can crack in performance applications.

This is a pic of the current offering from Ford;

Not shown clearly in the picture is the backside of the wheel hub. The bearing surface that rides in the bearing anchored in the knuckle is quite thin. For daily drive usage not a big problem. For performance applications a high probable point of failure especially on the T-Bird generation of IRS designs,

The other thing you want to be sensitive to is the distance from the axle centerline to the front of the pinion. The newer Ford IRS centers have an extended pinion. Normally this would not be a problem. On our cars it aggravates another drivetrain issue — driveshaft angle.

With drive shafts that are typically 12 inches long and sometimes shorter, the car builders predisposition to use equal length half shafts produces a significant angular offset from the transmission tail shaft in the horizontal plane.

Depending on chassis design and space available you want to build your iRS with a centered pinion if at all possible. U-joints will last longer and the "V" component of an NVH measurement will drop dramatically. Translation: the car will drive with much less vibration and U-joints will last much longer.

A nice touch on the new generation IRS's from Ford is the use of CV joints instead of U-joints. Try to adapt as much of this as you can — again reduced NVH and improved driving experience.

A centered pinion means unequal length half shafts but this is not a problem, although it complicates the half shaft ordering process. That said most shops will work with you to get proper length half shafts made.

If your chassis style has the IRS center section tightly enclosed in a chassis construct that centers the IRS housing your pinion is automatically going to be offset towards the passenger side of the car. If you want to center the pinion in one of these chassis you will end up doing some re-design and re-engineering. Not impossible but more work.

The earlier T-Bird IRS center might be easier to hang in the car and they definitely will have a shorter pinion snout so driveshaft misalignment is further reduced. If you elect to use the earlier generation IRS center get the one out of an 2010 or so Elxplorer, it is a little more robust but has much larger mounts at the nose of the pinion, see below, The Explorer center is the one on the left.



The center section on the right is from an 03/04 SVT Cobra which, absent a few cosmetic differences, turns out to be indistinguishable from other aluminum 8.8 IRS housings Ford offers with the exception of the Explorer housing.

There is a very robust rear cover available from FRPP that will fit the housing to address the paper thin OEM cover breakage problem. This is what the FRPP rear cover looks like;




In the end you might find the T-Bird generation of IRS could a better fit for your build.

No matter which path you pursue you are going to find yourself in the custom half shaft swimming pool — there is no escape.



Ed
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Old 05-08-2021, 04:33 PM
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kitcar,

Here's the link to the Factory Five instructions page: Instructions

Scroll down to the #15942 download for the IRS install PDF. It's pretty explicit and detailed about the install and lists the parts they supply. If you'd like to do it their way, possibly they'd sell you the parts they use.

Just another option,

Tom
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Old 05-08-2021, 07:11 PM
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The old Tbird irs are basically junk for what you wan. The 2006 and up Explorer are way stronger and way easier to get. No matter what you will end up making your own adjustable control arms and shortening the axles to the desired track width. i looked very long and hard for quite awhile before making the decision not to.
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Old 05-09-2021, 12:56 AM
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Quote:
Originally Posted by 74proii View Post
The old Tbird irs are basically junk for what you wan. The 2006 and up Explorer are way stronger and way easier to get. No matter what you will end up making your own adjustable control arms and shortening the axles to the desired track width. i looked very long and hard for quite awhile before making the decision not to.

With the exception of pinion length and chassis attachment points the strength of the two different generations of Ford 8.8" rear ends are essentially the same.

Ring and pinion torque transmission capacity is a function of ring gear diameter and pinion shaft diameter. The pinion shaft needs to be of a large enough diameter to transmit the torque delivered without shearing off. The reason for the old Pro Series 9" Ford gear sets was the larger diameter pinion shaft, improved 9310 metallurgy not withstanding.

Once the pinion shaft was upgraded the subsequent changes were to the ring gear diameter, gradually increasing it from 9 to 9.5 and then a 10 inch ring gear. The larger diameter ring gears both had a higher ultimate torque handling capacity while also allowing for the use of larger hypoid teeth on both the ring and the pinion. The last incarnatioin for that design after the 10 inch ring gear was the final 10.5 inch ring and pinion. To accommodate the rapidly increasing torque in the fuel motors, Strange took a big step upwards in terms of strength and torque capacity by going to a 12⅓ ring and pinion that is the current strong man.

Adequacy is a beautiful concept. A properly set up 8.8" ring and pinion is way stronger than you will ever require in a replica. These cars are so light and the ability to hook up any significant power without spinning the tires negates the need for larger gear sets.

The original Cobra's and even modern day Vipers very successfully use the 8.5" Dana ring and pinion. A road race car does not require the kind of brute torque handling capacity a drag race car needs. In the FWIW bucket even the drag race 8.8 rear ends properly set up go for many racing seasons in 3600 lb. low 8 second cars without any significant maintenance other than the periodic oil change.

The new 8.8 rear ends from Ford in current showroom vehicles are no stronger than the previous 8.8 rearends. They just use different packaging because they are adapted for a new chassis and new subframe along with using a CV joint instead of a traditional u-joint connection between the driveshaft and the pinion.

In terms of fitment, the earlier design is much easier to work with if you are fabricating. With the availability of the new style in aluminum there is essentially no longer a weight penalty for choosing the new rear end design however there is also no upside strength-wise. The internals are virtually identical strength-wise.

The seductive draw the new style has is the widespread availability in Mustang chassis for a half dozen or so years making salvage yard sourcing easy. After you get done sourcing the complete IRS cradle you still have to narrow it to match the Cobra rear track and then buy custom length axles.

If you have ever tried this you will quickly recognize it is easier to build a new IRS subframe than modify the OEM unit and of course you still have to deal with the extended pinion length that aggravates the already severe drive shaft angle offset in the horizontal plane.

In the end this is one of those beauty is in the eyes of the beholder deals however, strength is unchanged. What is not unchanged is the difficulty in correctly adapting it to the chassis of the target vehicle.


Ed
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Last edited by eschaider; 05-09-2021 at 08:05 AM.. Reason: Spelling & Grammar
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Old 05-09-2021, 08:04 PM
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Great info guys! Thanks Tom and Ed!
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