IRS Question

[cv2065]

Hey all. As I'm planning my build out, I was thinking of buying a complete donor IRS from a 2015 mustang to save a little coin. I sent FFR a note asking if this would be straight bolt on or did I need anything further. Answer I got back was that I still had to buy the $2500 IRS kit in order to get the custom length axles, control arms, coilovers and framework for the chassis. Does this sound correct as I thought that it might be a direct bolt on with maybe some added things for customization, but sounds a little more involved from a money perspective. Thought someone who's done it could shed some light. Thanks.

[Clover]

There are two main components to the IRS. The $2500 kit from factory five is one component. You also need the center section and knuckles from the 2015+ mustangs. I do think you need to buy that $2500 kit to be able to use the IRS pieces from the mustang. From the mustang, you can use the center section, knuckles, brakes, and a few other bits which you can get through factory five for more or from a salvage yard. If you go the salvage route, there are multiple options for the gears and either an aluminum or steel case. The most expensive is the 3.73 gears with a torsion limited slip differentials from the mustangs with the track pack. I think the diff that FF sales is 3.55 with traction lock instead of torsion.

[edwardb]

The answers you received from Factory Five and Clover are exactly right. The #16598 option for the 2015+ Mustang based IRS provides the proper frame configuration (it's extensive), the upper and lower control arms, toe arms, Koni coilovers, CV axles, and all necessary hardware (also extensive). What's not included in that option, as described already, is the center section (differential), knuckles and hubs, and brakes. You can use the Mustang brakes from an IRS donor, buy new Mustang brakes which they also offer, or of course there's the Wilwood upgrade. Really nice, but not inexpensive. None of the other parts from a complete IRS donor pallet from a Mustang is compatible or remotely fits. The track is narrower than a Mustang so the CV axles don't fit. The upper and lower control arms and design in general of the Mustang IRS setup is very different than what Factory Five designed.

[cv2065]

Exactly what I needed to know. Looks like I may be going with a Moser setup, which I'm Ok with as well. Thanks again!

[CraigS]

Be sure you make an apples to apples comparison. I don't have all the answers but what does FFR charge for the 3 link kit? What will you pay for the Moser axle assembly? What about brakes on the Moser axle. I know they don't come w/ the axle itself. I know the $2400 plus the salvage yard parts seems to add up quickly, but it can include brakes and the OEM 2015 brakes are a LOT better than old style Mustang rear brakes. I don't know what engine you are planning but it seems the 315 ratio 2015 salvage yard parts are the least expensive-well under $1k. That ratio is fine w/ my 408. i am just trying to make sure you have ALL the info. It would be a shame to go solid axle and later realize you only saved yourself $600 or $700.

[edwardb]

Be sure you make an apples to apples comparison. I don't have all the answers but what does FFR charge for the 3 link kit? What will you pay for the Moser axle assembly? What about brakes on the Moser axle. I know they don't come w/ the axle itself. I know the $2400 plus the salvage yard parts seems to add up quickly, but it can include brakes and the OEM 2015 brakes are a LOT better than old style Mustang rear brakes. I don't know what engine you are planning but it seems the 315 ratio 2015 salvage yard parts are the least expensive-well under $1k. That ratio is fine w/ my 408. i am just trying to make sure you have ALL the info. It would be a shame to go solid axle and later realize you only saved yourself $600 or $700.

Agree 100%. Look at it carefully and consider all the options. Might be less than you think, and especially as a percentage of the overall cost. Having had both solid axle and IRS, there is a big difference final product. No complaints with the solid axle. But the IRS is a more pleasant car to ride in and drive. I'm just a street cruiser, so can't comment about performance differences. But the newer IRS is supposed to also be the highest performance option.

[cv2065]

Thakks again for the details. i’ll do a tally and see where I’m at cost wise. If it’s within $500 or so, i’ll run with the IRS

[cv2065]

So, did the tally, and if I'm getting donor parts for the differential, hubs, knuckles and brakes, and the rest from FFR, I'm looking at about a $700 savings going with the Moser setup. Doesn't seem like a lot, but I would be getting brand new parts for everything with Moser. In the long run, I would assume that the value of the car would exceed $700 with the IRS option. Still thinking about it, but most likely will go IRS.

By the way...Do I need the sway bars with the IRS system, or should I just wait and see how it handles once I get to that point?

[edwardb]

So, did the tally, and if I'm getting donor parts for the differential, hubs, knuckles and brakes, and the rest from FFR, I'm looking at about a $700 savings going with the Moser setup. Doesn't seem like a lot, but I would be getting brand new parts for everything with Moser. In the long run, I would assume that the value of the car would exceed $700 with the IRS option. Still thinking about it, but most likely will go IRS.

By the way...Do I need the sway bars with the IRS system, or should I just wait and see how it handles once I get to that point?

I think you're making a good decision figuring out how to go that way. Yes, an IRS car will have better resale than a solid axle. Many are built without sway bars. They're an easy add later. The front is pure bolt-on. The chassis mounts are standard on every chassis. The sway bar is attached to the LCAs with longer lower bolts on the coilovers. For the rear, the IRS LCA has the mounting points standard. The chassis mount though requires two added brackets held by two suspension bolts on each side. You might want to get those brackets added and build the car with those in place. Wouldn't be terrible to add them later. But kind of a pain taking those big bolts back out. The brackets look like this:

http://i867.photobucket.com/albums/ab234/edwardb123/Factory%20Five%2020th%20Anniversary%20Mark%204%20R oadster/Independent%20Rear%20Suspension/th_IMG_0150_zpslldjumio.jpg (http://s867.photobucket.com/user/edwardb123/media/Factory%20Five%2020th%20Anniversary%20Mark%204%20R oadster/Independent%20Rear%20Suspension/IMG_0150_zpslldjumio.jpg.html)

[cv2065]

You might want to get those brackets added and build the car with those in place. [/URL]

Awesome. Do those brackets have a part number that I can ask for?

[edwardb]

Awesome. Do those brackets have a part number that I can ask for?

Looks like 16000 and 16001. You can see them in these instructions. https://www.factoryfive.com/wp-content/uploads/2017/08/SWAYBAR-2015-IRS.pdf

[Jeff Kleiner]

... Looks like I may be going with a Moser setup, which I'm Ok with as well. Thanks again!

OR....rather than the big ticket Moser straight axle you can pick up a used Fox or pre-1999 rear end, rebuild or have it rebuilt with the proper length axles + brakes and be even farther ahead $ wise however this doesn't answer the question of whether 3 link or IRS is more appropriate for your intended purposes.

Jeff

[cv2065]

Got it. Great call on getting these installed ahead of time!!