IRS complete kit or IRS after?

[natehall37]

Hey all, the title doesn't exactly ask the right question but, I didn't want to make it too long. What I am asking is...If I've got the desire and ability should I source a used complete IRS and fit it to the frame or just go the factory five route and get it pre-mounted?

From what I've been able to see on sites like E-Bay there are some pretty affordable "complete" IRS's out there, that look like they come with brakes and a third member. If I was able to secure one of these, how are they attached to the frame? Has anyone done a write up? I've got access to a pretty talented welder/fabricator thats done some pretty nice work on his own projects, I just don't know how the IRS's fit to the frames of these cars. I know that this has been a retrofit for a while so hoping to hear from some of you guys that might have tackled this in the past. My biggest question is, would this be "worth" the time and money or would it end up costing me way more in the end and should have just got it done a that the factory? Lets say for the sake of this conversation that I could get the whole (Including brakes and third member) IRS for $1000 shipped.

An additional question would be, what gear ratio would I look for? I'll most likely be going with a mild 351W/TKO600 combo.

Cheers!

[edwardb]

I personally haven't, but I've seen several retrofits of IRS into a solid axle car. It's highly custom and not supported by Factory Five. Maybe makes sense for an already built car where the change was desired and the will exists. But for a new build, makes no sense at all. The Roadster frame is highly specialized for the IRS setup. Highly recommend to start that way from the beginning on a new build. A complete Mustang IRS assembly (e.g. pallet) has only a handful of parts that will work. I know. My #8674 started with a 2015 Mustang GT complete rear suspension. All I used was the center section (differential) and the knuckles/hubs. It is possible to also use the brakes from the right model. But my build was with Wilwood brakes, so can't offer much about that. But likely usable from a complete Mustang IRS assembly. The rest, e.g. upper and lower a-frames, axles, the IRS frame, shocks, etc. are not compatible. When you select the IRS option, you get the proper chassis setup and all the rest of the parts except the already mentioned center section, knuckles/hubs, and brakes. Short answer to your last question -- a complete and good pallet for $1000? Yes, that's a good deal, but you'd still only use the parts mentioned. You'd probably be looking at a 3.55 or 3.31 ratio. The 3.15 is another option, but be careful. That diff can't be changed to any other ratio. But you need to look at the proper ratio based on a number of factors.

Just to give you an idea, this is the bare Roadster frame for IRS. And then the completed setup. This has the optional sway bar and double adjustable shocks, which you may or may not want or need. These instructions from Factory Five also give a lot of detail. https://www.factoryfive.com/wp-content/uploads/2019/03/IRS-2015-REV-D.pdf

https://hosting.photobucket.com/albums/ab234/edwardb123/Factory%20Five%2020th%20Anniversary%20Mark%204%20R oadster/Delivery/.highres/IMG_3276_zpsv2svcovn.jpg (https://beta.photobucket.com/u/edwardb123/p/0cd07561-96d6-4ba7-a2ee-7e2f67102e6d)

https://hosting.photobucket.com/albums/ab234/edwardb123/Factory%20Five%2020th%20Anniversary%20Mark%204%20R oadster/Independent%20Rear%20Suspension/.highres/IMG_3467_zpsjqh4024j.jpg (https://beta.photobucket.com/u/edwardb123/p/9de78b82-4e8c-40cc-8605-e8806282ffe8)

[CraigS]

Get it built for IRS. I did a retrofit of the FFR 2015 IRS into my old MkII so it is absolutely possible, but it is a ton of work. Also you need good welding skills especially in vertical and overhead welding. I am using the OEM rear brakes from my salvage rear suspension. I have the 315 gear and it works out great w/ my mild torguey 408 engine.

[mike223]

Get it built for IRS.

I have the 315 gear and it works out great w/ my mild torguey 408 engine.


+1

To be clear, you really want the IRS framework welded in while the frame is in FFR's frame jig - you'll have enough problems going cross eyed deciding what is "straight + perfect" back there without having to worry about the frame welds.

So your choices are IRS complete kit or IRS base kit.


I too am running a 3.15 gear with a torquey not too mild 393w.

I'd figure 3.27 to 3.55 max on a mild 351 / TKO.

I aim to gear fast cars for bumping the rev limit in 3rd in the 1/8 mile + 4th in the 1/4 mile.

Works for me.

[Aircontroller]

Thank you for posting this question natehall37....was wondering the same.

[Richard Oben]

Get it done at the factory. NO IRS will 'bolt in'.

[GTBradley]

When I bought mine last year the fall sale included $2500 worth of free upgrades, last summer it was up to $3000. That $2500 went directly towards my IRS, which made it a no-cost upgrade. You still need the center section, spindles and brakes, but this makes for a very easy time of installation. I am more than pleased with the ride of car with this upgrade. I really didn’t expect the car to ride this well.

[natehall37]

Thanks for the well informed replies guys, It's all kinda along the lines of what I expected. What I was hoping for was, "Heck yes! I don't know why more people don't do that." :D I know it wouldn't be an easy task, but I thought more of the structure and components might be compatible.

[BradCraig]

Definitely order with IRS. I went that way and bought an IRS pallet from Parts Farm in GA, should be here Monday. $800 shipped with Center section, hubs, knuckles, brakes....everything I need. With what it was going to cost for a solid axle with brakes it was a no brainer.

[chmhasy]

In order to build a roadster with IRS, you need to order the kit with the IRS option, you also need to source a complete 2015+ mustang IRS unit like the ones you see on ebay.
FFR IRS option welds all the correct mounts to the frame and provides the KONI coil-overs, tubular control arms, and CV axles. the parts you need to source are the center section, hubs, and brakes from the sourced Mustang.

so the cost would be
$2899 for the FFR IRS option and
what ever cost you would buy the mustang IRS unit.

[chuckster]

+1 on IRS I went with the Factory Five IRS option. I bought a salvage 2016 mustang IRS Assy. $800 delivered. I am very very happy with the ride everyone who rides in the car, and how compliant the ride is.

[Aircontroller]

Please disregard my question below. On the manual that Edwardb posted, it states Parts needed 2015 or newer!.....Thank you!



Gents, I wanted to get clarification.

Is the donor IRS strictly 2015 or is it 2015 and present?

Thanks!

[CraigS]

2015 until now, or to put it another way, 2015 to 2019. I just don't know enough about 2020 models to include them. I suspect they would work but don't know for sure.

[madkow70]

I went with FFR IRS option for the chassis. then sourced a used IRS rear-end from ebay. I found a 2018 complete rear-end with 11k miles for $900.

[Aircontroller]

Cool! I might consider going that route as well!


I went with FFR IRS option for the chassis. then sourced a used IRS rear-end from ebay. I found a 2018 complete rear-end with 11k miles for $900.

[ChasNMe]

does this include all that would be needed? does it matter if it is from an automatic? do all of the 2015 and newer come with torsen gears? thanks for the info. Jake
https://www.ebay.com/itm/2018-2019-Ford-Mustang-GT-Track-Pack-Auto-3-55-Torsion-Rear-End-Assembly-IRS-OEM/254433510649?hash=item3b3d6b44f9:g:keMAAOSw2Vdd3Aq 7

[edwardb]

does this include all that would be needed? does it matter if it is from an automatic? do all of the 2015 and newer come with torsen gears? thanks for the info. Jake
https://www.ebay.com/itm/2018-2019-Ford-Mustang-GT-Track-Pack-Auto-3-55-Torsion-Rear-End-Assembly-IRS-OEM/254433510649?hash=item3b3d6b44f9:g:keMAAOSw2Vdd3Aq 7

All you really need is the center section and the knuckles/hubs. You may be able to use the brakes, but I don't know enough about the 2015+ OE brakes to say one way or the other. Those looked pretty roasted anyway. The rest (upper/lower control arms, CV axles, IRS cradle, etc.) is all scrap metal. Nothing is compatible with a Factory Five build. I've tried selling these parts on Craigslist and eBay with little to no luck. Just not a market for that stuff. They used to only have the Torsen (misspelled in the ad BTW) with the 3.73 ratio. Looks like Ford is now selling the Mustang Performance Pack with Torsen in the 3.73 for manual and 3.55 for automatic. Since that Mustang is an automatic, looks like the 3.55 version. The Torsen is supposed to be an upgrade. You'll get different opinions, but should work very well for you. The automatic may take a different driveshaft adapter, but even that's changed over the model years. One way or the other, it will work.

[Aircontroller]

It seems like if we were to buy this ebay one just for the ctr section, spindles and hubs it’s not very cost effective.

FFR’s is close in price:

16668 - 2018 Mustang IRS 3.55 Center Section, Spindles, and Hubs (Watch Video) +$1,499.00



does this include all that would be needed? does it matter if it is from an automatic? do all of the 2015 and newer come with torsen gears? thanks for the info. Jake
https://www.ebay.com/itm/2018-2019-Ford-Mustang-GT-Track-Pack-Auto-3-55-Torsion-Rear-End-Assembly-IRS-OEM/254433510649?hash=item3b3d6b44f9:g:keMAAOSw2Vdd3Aq 7

[edwardb]

It seems like if we were to buy this ebay one just for the ctr section, spindles and hubs it’s not very cost effective.

FFR’s is close in price:

16668 - 2018 Mustang IRS 3.55 Center Section, Spindles, and Hubs (Watch Video) +$1,499.00

You can find IRS parts in salvage cheaper than that one. The Torsen (vs. standard clutch packs) adds value, which may or may not be of any value to you. When new, the Torsen diff is twice the price of the standard version like Factory Five sells. Which is fine BTW. Bigger question is whether you want to rehab used parts or start new, and not have any surprises.