1967 - 1969 Mustang IRS or Solid????

[venomous99]

I will be building a 1967 or 1969 Mustang from my 1999 Cobra. I have not yet decided which car, one is farther along and the other has sentimental value. Either way, they will be transferring my drive train, which includes a built DOHC twin turbo set up, a 6 speed auto and currently IRS. Since the Cobra is my daily driver - I build them to drive them - I need to determine a path forward. I will be upgrading the rear so I can have a driver while most of the project is built, as the current 28 spline IRS held as long as it could. So, since I have to upgrade, and I also intend to transfer this to the project car someday, I would like to get opinions from the pro's on a path forward...

I can upgrade to the 31 spline, delrin, coil overs, etc... Build a corner carving IRS that will hold 800 plus HP....OR I can go to a solid axle, and build it. Can anyone inform me which will perform better in these older body styles? I only want to do this once, i.e. I do not want to build an IRS onlyt to find it would have been optimal to build and solid. Any information would be greatly appreciated. Thanks.

Ken

[exwestracer]

My main concern would be a lot of cutting for minimal gain in performance. Putting this:

Under a 67 or 69 is going to mean removing a large amount of floor. So, is this a 10/10ths performance build? If not, a 9" and torque arm or 3 link can be made to handle pretty damn well....and you get to keep the whole floor pan.

[MrQuick]

if done properly you can keep the trunk floor....there's a silver one running around here with the cobra set up. well planned and executed.....why did i just think of "and, if properly used, it can remove the fingerprints"

[ace_xp2]

DVS gets away without cutting, they've even got a kit for it:
http://www.davestriblingrestorations...&id=7&Itemid=8
There was a lot of discussion on the IRS vs. Stick axle setups over on Corner Carvers. If I remember right, the mustang irs isn't the best design as it was built after the chassis was. So they had to make it fit in a chassis built for a stick axle.
That left it with I believe a less than ideal camber curve, and I think its got some bumpsteer. However, I recall there being a post/link about it being built up which lead to it being tit for tat with a 3 link, excelling in some areas but losing in others.

[exwestracer]

DVS gets away without cutting, they've even got a kit for it:
http://www.davestriblingrestorations...&id=7&Itemid=8
There was a lot of discussion on the IRS vs. Stick axle setups over on Corner Carvers. If I remember right, the mustang irs isn't the best design as it was built after the chassis was. So they had to make it fit in a chassis built for a stick axle.
That left it with I believe a less than ideal camber curve, and I think its got some bumpsteer. However, I recall there being a post/link about it being built up which lead to it being tit for tat with a 3 link, excelling in some areas but losing in others.

As soon as I saw your post, I immediately thought about ride height. I looked at DVS' site, and they do mention that the ride height for the IRS is "stock". They offer a modified cradle that allows the car to drop 2", but that's about it. If the OP wants the car "pro-touring" low, I still don't think it can happen with the stock floor (and correct link geometry)... I've never done one of these on a Mustang...only going off our experience with a second gen Camaro and 350Z IRS...

[venomous99]

Thanks for all the great information.

I am still in the planning phases, but this gives a lot of food for thought. Although, my original plan was to transfer all components from the Cobra, I am not sure the trade off is worth it, especially considering that I do want the car "pro-touring" low and would like to go to track events. I am not sure how much I would have to give up by installing a lowered IRS, i.e. ride height, geometry, ground clearance, etc...

I guess my next step will be to talk to DVS, and do some research on solid setups. Once again, thank you for the quick and insightful responses.

[SVTforme]

Although I have not done this on a Mustang.. I can tell you what it involves on a 1968 Torino. My goal was to run a certain ride height and ensure proper geometry. To do this - I had no problem cutting and/or moving anything that got in the way.

There were several things that were a pain when installing an IRS with wide rear tires (315's) and getting the car low.
- Upper control arm pivot points. Had to notch the trunk floor to clear.
- Upper control arms get close to the frame rails under full jounce. If you want lower - you need to notch frame rails. Thankfully for the ride height I wanted for my Torino, this was not necessary.
- If you want to go low, you need to modify mounts on the IRS cradle as they hit the stock frame rail and limit you from going any lower.
- Spring and shocks - in order to run wide tires and not add fender flares, tires needed to go inboard. Original plan was to run coil over shocks, but there was no room for these as the would hit the tires. Next problem was springs are in the way when using the stock spring puckets. Solution - run smaller springs (60mm ID).

In the beginning, I paid DVS a visit in Indiana. They have put together a nice package on this for someone that is not interested in fabricating anything for this mod. If you want to get the car low, you need to have them section your IRS cradle. I am not sure if they maintain the factory suspension position, and I do not know what spring/shock rates they are using. In the end I decided I wanted a more "factory" appearing install, so I made my own mounting brackets. I also wanted to be able to run certain spring and shock rates and to maintain the stock motion ratios for each of them.

As for how I installed things - I made a jig to hold the IRS in a stock ride height position of a 2004 Cobra. By stock I mean the optimal position the suspension was designed for. Then I lowered the car body to get the desired ride height which required cutting the stock mounts off and notching the trunk floor. After the relative locations were decided on, mounting brackets were made up to connect the dots. To do the equivalent to this on a new Mustang would require moving all of the suspension points on the car up the same amout you lower the car.

The only thing you really have to give up here is a bit of weight..as they are clearly a bit heavier than a solid axle. I don't have any track time on the setup yet as I just got the car up and running, but I will in the spring.

Here are some of the highlights.. sorry to hijack your thread with my car . Was this a lot of work - sure was!! You could easily cut lots of corners compared to what I did.

Check out my thread if you want to see more pics and info.
https://www.pro-touring.com/showthre...489-TwinTorino

[venomous99]

Wow, really nice build and very well documented. Thanks for sharing this, you are in no way hi-jacking. This gives me a lot to think about and research. I know there already exists a huge controversy over the whole IRS vs Solid axle set-ups, so i will stay away from that topic. I have, however, done the research on modifying my IRS for optimal performance, as my original intention was to track my Cobra. I guess I am a little selfish, since I did so much leg work on the IRS, I really wanted to keep it. So reading your thread has given me confidence this can be done, although it may require some cutting. If so, I will have to make that decision once I know how much and where exactly.

Once again, thank you.

[mizfitjon]

What we did in my 68 coupe was mounted the cradle. Notched the frame rails for the coilovers to clear. Made box for the upper shock mount in the trunk. At a ” pro touring” stance the control arms were angled up alot. Pretty much to mae travel as the lower arm was about to hit the bottom of subframe. To fix this, slide a jig table underneath with the car on a lift. Welded fixtures in to hold the irs and then unbolted it and raised the car up and off it. Then cut 2” out of rear uprights and just cut front arm. Put each mount into the body mounts lowered car back down onto irs. Had to heat and tweak cradle a bit to fit mounts. Once lined up welded it all together. This way kept pinion angle, centered, etc. no angles were changed just sucked it up and gained me the perfect amount of suspension travel at ride height.

[mizfitjon]

Next step was the ”big” fix! the cobras irs track width is way to wide for our classic mustangs. Esspecially with atleast a 305-315 tire. So to be able to suck the tire up in the wheel well without adding terrible big bulky flares you need to push out the quaters. We originally tried to bubble it out by seperating the quarter except for at the door jamb and rear and then pushing at center of wheel well to bow it out. Couldnt really get it to bow, so we widened the rear quarters about 3” on top and about 5” on the bottom. No body lines were altered also in this process. But with the rear wider you then have to widen the bumper etc. for tons of pics on how we did it check out my pics in cars for sale cool ”cool mustang project”