Opinion: Donor 2015 Mustang IRS & rear brakes worth the savings?

[Brian76]

Pricing kit options and trying to establish a budget for the build. Wilwoods look sweet but are kind of expensive. Reading older posts, I'm not convinced they are necessary for street driving and occasional autox. I can find an entire rear end with 10-20k miles from a donor stang for $700-800 (shipping extra)

example:https://www.ebay.com/itm/2015-2016-2017-FORD-MUSTANG-3-55-IRS-REAR-AXLE-3-55-DIFFERENTIAL-5-0-COYOTE/263535298545?fits=Model%3AMustang&hash=item3d5bed87f1:g:1B0AAOSwj2Vanq93&vxp=mtr

Would like some feedback from anyone that's used the 13" rear brakes and anyone that's purchased an entire IRS/Brake system used. Would you do it again? are there any surprises that I maybe I haven't considered? Was is cost effective using donor parts rather than buy new from FFR?

[CraigS]

Yes the OE 2015 Mustang brakes are fine. I did a retrofit of the IRS into my MkII last winter and used the OE brakes. They are similar in overall design to the old rear discs in that they have the parking brake built in to the caliper. But they seem to work much better than my old ones. I run them w/ SN95 front calipers/rotors and they are a good match. My salvage yard rear even included the parking brake cables. The cable end at the caliper is different from the old style. So I used the outer sleeves I already had and the front foot or so of the old inner cable. I cut the inner cable from the 2015 parts so it overlapped the old cable and used three cable clamps per cable to splice it all together. This is the first time in 9 years that I have had park brakes that work correctly. I am using a CNC dual master cylinder setup so manual brakes. I did upgrade to Hawk pads for the 2015 calipers. I run 18-20 autocrosses per summer and these brakes have been the best I have had.

[Fixit]

I went the route of buying a "take-out" IRS assembly for my future build IRS (https://thefactoryfiveforum.com/showthread.php?27544-The-40-Watt-Garage-new-project-Mk4-arriving-in-mid-May&p=315673&viewfull=1#post315673)
I couldn't justify the cost of the Wilwood setup, and the parts/assy I bought were nearly as new as... well... new.

[q4stix]

I got mine for $1000 shipped but it was also with the 3.73 Torsen differential from the performance pack. Check out midwaymustangparts as a seller too. That's where I bought mine and they have at least one GT rear dropout with 3.55 and the aluminum diff just like your link for $550 plus shipping.

[Chris @ Forma]

We have used 3 used IRS units thus far. On our current build we are using low mileage '15 brakes. On the first two, our clients requested WilWood so we didn't re-purpose the brakes that we purchased. BTW, we also use 15 Coyotes as well as the Mustang trannies.

chris

[Brian76]

Thanks. I've read that the 2015 or newer brakes may need some tuning on lighter cobras. At the moment I'm assuming I would use the willwood master cylinder and 2015 front and rear brakes. what are your thoughts? Does using SN95 front brakes balance the brake system?

I don't know what tuning the brake system requires.

[cv2065]

I confirmed with FFR that a used IRS from a 2015 Mustang is exactly what you would get in the kit, just need to be looking for the correct gear set of 3.55, which is what they offer. Might seem common sense but wanted to verify that there were no modifications.

[edwardb]

I confirmed with FFR that a used IRS from a 2015 Mustang is exactly what you would get in the kit, just need to be looking for the correct gear set of 3.55, which is what they offer. Might seem common sense but wanted to verify that there were no modifications.

I've also heard from several sources that FFR supplies the 3.55 diff. Pretty sure the iron case version. They supply a new unit. That specific unit is available from Ford Performance, NIB street price around $800. The only modification required is to drill out the front mounting ears. Pretty simple. I assume you also know you need the knuckles and hubs from a 2015+ Mustang IRS, which FFR also provides as new parts. Street price for those is in the $450 range. The knuckles require a little modification, but it's pretty easy and thoroughly explained in the instructions.

Salvage (used) parts will fit exactly and will require the same mods. If you do get the iron case used it will be pretty rusty and most (including me) spend a little time to clean them up and paint. The aluminum case version, if you can find one, is 20-25 lbs lighter and won't be rusty, if that matters.

Looks like the used Mustang IRS complete assemblies have come down in price some. Truck LTL shipping does add some cost so make sure to consider the total cost. New parts are typically available with free shipping. When you buy the whole pallet, obviously there are a lot of unused parts. I thought I could recover some of the cost by selling the parts I didn't need. I sold a few things, but didn't make much. I've heard the same things from others. I did the used IRS complete assembly for one build and it worked out OK. Saved a little bit. But I ponied up for new parts on the Coupe build. Partly to avoid the hassle of used parts shipping, delivery, clean-up, parts disposal, etc. But also because I couldn't find exactly what I was looking for.

[CraigS]

Thanks. I've read that the 2015 or newer brakes may need some tuning on lighter cobras. At the moment I'm assuming I would use the willwood master cylinder and 2015 front and rear brakes. what are your thoughts? Does using SN95 front brakes balance the brake system?
I don't know what tuning the brake system requires.
Traditionally FFR have always lacked rear brake capacity especially when all Mustang OEM parts are used. Mustangs are front heavy so the brakes are designed for that. FFRs are slightly rear heavy. So using sn95 fronts and the 2015 rears is close to balanced because the rears are considerably larger than the old rear brakes were. I got the wrong sn95 calipers and had to get a different front MC size to get the system to work. I 'think' that the proper sn95 2 piston PBR caliper would be just fine. 'tuning' would be getting the correct caliper and/or adjusting the bias bar in the Wilwood MC setup.

[Clover]

Traditionally FFR have always lacked rear brake capacity especially when all Mustang OEM parts are used. Mustangs are front heavy so the brakes are designed for that. FFRs are slightly rear heavy. So using sn95 fronts and the 2015 rears is close to balanced because the rears are considerably larger than the old rear brakes were. I got the wrong sn95 calipers and had to get a different front MC size to get the system to work. I 'think' that the proper sn95 2 piston PBR caliper would be just fine. 'tuning' would be getting the correct caliper and/or adjusting the bias bar in the Wilwood MC setup.

This sounds more like a brake bias issue which can be adjusted with the Wildwood peddle boxes to apply more pressure to the rear brakes. The standard rear brakes are more then adequate for the roadster. Pretty much all cars are designed to have more braking capability in the front as your front tires do the majority of the braking. While the roadster has more weight towards the rear of the car when sitting stationary, that is not at all the case when you are braking. When you decelerate, the weight in the car goes forward and the front of the car squats. When the weight transfers, you get more traction with the front tires and are able to brake more with the front tires. You also use this for turn in while trail braking as you have the weight on the front tires and added traction. Even Porsche 911s, which are rear engine do the majority of the braking up front. Yes a 911 is more neutral, however, when you decelerate, the weight goes forward.

[CraigS]

This sounds more like a brake bias issue which can be adjusted with the Wildwood peddle boxes to apply more pressure to the rear brakes. The standard rear brakes are more then adequate for the roadster. Pretty much all cars are designed to have more braking capability in the front as your front tires do the majority of the braking. While the roadster has more weight towards the rear of the car when sitting stationary, that is not at all the case when you are braking. When you decelerate, the weight in the car goes forward and the front of the car squats. When the weight transfers, you get more traction with the front tires and are able to brake more with the front tires. You also use this for turn in while trail braking as you have the weight on the front tires and added traction. Even Porsche 911s, which are rear engine do the majority of the braking up front. Yes a 911 is more neutral, however, when you decelerate, the weight goes forward.
Yes you can adjust w/ the bias bar. But it can only do so much. If a Mustang is 60% front weight and it transfers 15% to the front it is then 75% front under braking. If an FFR is 45% front and it transfers the same 15% under braking it is then 60% front. 75% is a big difference from 60%. I base my comments on my experiences w/ my FFR. Take them any way you want or not.

[Brian76]

Here some price comparisons from a little searching online:

FFR:
New 13" ford rear brake kit=$700
New Ford center section = $1400
TOTAL=$2100

Take out:
Used complete rear end w/ brakes=$900 (w/ shipping)
Add New Rotors =$180
Add New Pads = $70
TOTAL = $1150

Buy Individually:
New Ford Center section (M-4001-88355) =$950 (w/ shipping)
New spindle & hubs (M-5970-M)= $400 (w/ shipping)
New GT rear brake kit (M-2300-MR) = $540 (w/ shipping)
TOTAL = $1890

Hopefully I'm not missing anything major. I think if i could find a low mileage take out and put a little elbow grease in the calipers it would be worth it. Gotta save $$ for quiet pipes :-)

[edwardb]

Here some price comparisons from a little searching online:

FFR:
New 13" ford rear brake kit=$700
New Ford center section = $1400
TOTAL=$2100

Take out:
Used complete rear end w/ brakes=$900 (w/ shipping)
Add New Rotors =$180
Add New Pads = $70
TOTAL = $1150

Buy Individually:
New Ford Center section =$950 (w/ shipping)
New spindle & hubs = $400
Used calipers =$100 (haven't found new OEM)
New Rotors = $180
New Pads = $70
TOTAL = $1700

Hopefully I'm not missing anything major. I think if i could find a low mileage take out and put a little elbow grease in the calipers it would be worth it. Gotta save $$ for quiet pipes :-)

Just to confirm, the $1400 option from Factory Five is the center section plus knuckles and hubs. Not just the center section.

$900 shipped for a comparable 3.55 iron case diff complete used assembly is a good deal. Better than what was available when I was shopping for them a couple years ago. Obviously the supply is better now.

There are better new prices than what you list. Google these part numbers: M-2300-MR Rear brake kit, M-4001-88355 Center section, M-5970-M Knuckles and hubs. Similar total as what you had but all NIB parts.

[Brian76]

Thanks Edwardb. I updated the list with the part #'s provided. Price went up a bit based on what I found with free shipping. I also found your IRS build from Cobra #3 which looks very informative.