Fox T-Bird/Cougar Forums

I'm just putting this here for the time being as it's kind of just thinking out loud, but if mods feel it needs to be relocated so be it.

First I will preface by saying this is just something that has come to me over the weekend and I'm still mulling it over, but I may persue it.

Follow along now. 

Some of you know my main daily driver is an '88 Merkur Scorpio.  Some of you also know it's not "normal" in that it rides on 17x8" tribars, has 12" Vette front brakes and a performance suspension.  It doesn't ride like a "luxury sedan".  Anyway, on the suspension.  This past year I have coverted the front suspension to coil over and redid the rear suspension which included upping springs rates.  Stock on a Scorpio is 100#F/250#R.  Pretty soft.  After my work, the rates are 275#F/900#R.  No kidding this thing is stiff.  However......Even though the rates are MUCH higher than stock, the ride is not all that bad.  Especially in the rear.  There is a bump on my way to work that will completely unsettle my Fox cars at anything over 40mph (it will launch the back of the Z-7 into the air)....but in the Scorpio...it's just a "THUMP" at 50mph.

So with this in mind the gears started turning and when I got home I pulled out the tape measure just to compare with one of my Foxes.  Comparing it to my 80 XR-7 that is sitting behind it here is what I found.  The track of the Scorpio with the 245's on the Tribars is with-in 1/2" of the 80 XR-7 that has the 275's on the Cobra wheels on the wider SVO/Lincoln axle.  The spring locations are close, the front mounting points are close and the only thing that is really off is the width of the shock attachments.  They are narrower on the Fox.

Can you hear the gears turning?

Now before you go off on the "Cobra, this" or "Cobra, that".  Let me put this out here.  I am replacing my current Scorpio with another less rusty twin.  I will be completely stripping my existing car for spare parts and all of my mods.  I will be ditching a shell.  I will therefore have a complete spare Scorpio rear suspension.  So no cost there.

Anyway, about the Scorpio/XR4Ti rear suspension.  It is an IRS, but it is not like the Cobra or the Tbird stuff.  It is a semi-trailing arm set-up that has the uprights rigidly attached to the arms.  There are no upper arms.  It has it's pros and cons.

Pros:

Simple design
Cheap

Cons:

The diff
Different bolt pattern
Fab work required to install

Addressing the Cons.
The diff is an open and of odd pedigree.  There are stronger LSD Euro options available, but getting them can be difficult.  Guys have done 8.8 swaps, but the issue there is that route requires not only mods just to mount it, the axles/stubs have to be modded pretty heavily...and you're looking at bigger bucks.  Still another option that I'm looking into is a Ford 7.5 swap.  Granted it's no 8.8...but I'm not dumping the clutch at 5K on slicks.... Talking to one of the guys that have done the swap, there is still a little work, but the axle mod is pretty simple.
Different bolt pattern.  The Scorpio pattern is a 5x112.4 vs the 5x 114 of the "typical" Ford pattern we are used to.  On my Scorpio I just redrilled the wheels, but on this I wouldn't want to.  However, I do know it is possible to machine the Scorpio hubs and then press in the Cobra/Mustang studs.  The bolt circle is close enough that if you bore the hole out for the Cobra studs offset (ie not concentric) with the originals and the bolt circle and pressfit work out fine.
Fab work to install.  Now I think I can figure out how to make bracketry to mount the beam and the differential into the car....  Factory the Scorpio stuff is rubber mounted, but I would solid mount it just like I recently did in the Scorpio.

The only other thing I will need to do is tweak the trailing arm mounts to correct the rear alignment at ride height.  I need to do that anyway for the Scorpio (sits lower than stock), so I'm going to plug numbers in my Mitchell Geometry program and move things around to correct that.

OK, now "why" you say?  Well the obvious would be because I can.  This would not be slated for the silver car, but for one of the other two.  It might wind up under the 80 XR-7 just to try things out, who knows.  I'm curious about how the ride would improve as well as handling.  While it's not an SLA setup, it still feels leaps and bounds over the live axle in the car. 

Anyway...just more ramblings of a crazy man...:wtf: :screwy:

sounds kick ass!

lemme know if you need an extra hand for test fittin stuff

I dug up a pick of a Merkur rear suspension for those who are unfamiliar.  the Scorpio is the same except it has rear disks and is 5-lug

(http://www.turbochuck.com/images/Scorpio/Rear%20Susp/rear%20susp_1.jpg)

Try it:grinno:

So Chuck... this means camber will change with suspension compression.  Your ok with this?

I'm aware what the STA suspension does.  With the camber corrected at ride height, the change during compression won't be all that bad really.
I've been driving the Scorpio around with uncorrected geometry for a few weeks now, and it's really not a problem.

We're also not talking about HUGE changes as the travel is not going to be massive.

Even the Cobra stuff changes....

It will be liveable.  I'll know how much it's changing when I plug the numbers.

As an interesting side note.... with the current rear suspension, I picked up the new shortblock for the '83.  It was on a skid, so probably 200# of weight.  The back of the car only dropped 1/4" when I put it in the trunk :p

Finish the SHO transplant...then focus on the Merkur-madness. :D

Well the first order of business is getting the '83 back on the road... ;)
Then we'll see what takes up the next spot...

Sounds interesting but how much power can one of those rears handle?

It sounds like a great idea, but I wonder how much fab work will really be needed.  Keep us posted. :)

Well I would not be keeping the stock Merkur diff.  As far as the strength of the rest of it...it depends.  It shouldn't have an issue with 300hp. The halfshafts would be the only other concern.

Would that even bolt to the lower controll arm mounting points? If so you might have something there.......

You are seriously going to have the most badass lot of oddball cars when you're all through with these projects.  I say go for it.

Bolts?  Bolts?!  Chuck don't need no stinking bolts!

Well it's hard to say just yet.  The location looks to be close.  It may require cutting the mount of the beam and fabbing/welding a new mount to attach into the factory RLCA mount points.

so,, leme get this right,, you are or are not going to keep the axle setup on the rear?

the axles appear to be  to  type on the scorpio.  Are you thinking of keeping it cv axle? 

or
are you just planning on placing the piece parts excluding the rear diff and axles?

I'm not sure I'm following you.

The proposed plan is to remove the 4-link live axle set-up and swap in the semi-trailing arm IRS set-up(with required mods of course)

Sounds fun. If I did it I'd screw up the suspension geometry really bad. You know your way around this stuff so it shouldn't be so bad.

sounds bad ass and how far along is that sho project?

Waiting to find time to testfit the engine/trans in a car to build mounts.

Ok.  I did some more digging and the 7.5 diff route looks VERY doable.  Once I get the parts out of the donor car I'm going to look around for an MN-12 Tbird/Cougar 7.5 diff and axles to mess with.  Worse comes to worse, I can always swap it into my Scorpio.

The geometry issues will be solved again once I get the new car, check ride heights, etc and see what needs to be tweaked to get wheels pointing the right way when lowered. 

The first iteration would just be to use the set-up as it stands with only minor mods to facilitate installation and geometry correction and see what I think.

If I like it....jigs will be built and an 8.8 option persued as well as other things.....trust me, if this works I have plans.

Cobra this, Cobra that. :D

Hey you knew somebody would say it. :p

Seems like a lot of labor, time, and cash to accomplish something that could be done with a bolt-in swap.  Mounting points may be close, but being off by 1/4" is the same as being off by a foot - it has to be re-engineered either way.

Assuming a Thunderbird is 500lbs heavier than a Merkur (a WAG by the way) that rear end is going to ride much differently, and ride height will probably be significantly different as well.  Just increasing spring rate/height to get the desired ride height may throw off all the advantages of IRS.

Plus I always thought Merkurs were notorious for having fragile rear ends.  Seems like a big swap was to re-engineer them for live axles, which was a ton of work.  But the flimsy rear end was a problem well short of 300hp.  Again, this is all by my shakey turbo-4 message board memory, and that was close to ten years ago.

Cobra axles are fairly easy to come by, if a little pricey, but it bolts right in.  But hey, if a Scorpio rear tickles yer pickle, go for it. :D  I'd like to see it... and if anyone can make it work, it would be you.

my .20

You are correct in your memory being shakey ;)  I've owned Merks...

While I haven't verified all the dimensions yet...the only real gotcha would be the spring perches on the frame.  Everything else would be moot.

The curb weight of a stock Scorpio is 3000# (53%/47%) and they are VERY softly sprung.  Like I mentioned, I have tripled the spring rates (most likely quadrupled in the rear) on my Scorpio and the ride is BETTER than my live axle cars.  Ride height again is a non-issue.  Tweaking the geometry a touch to correct the camber/toe is minor.  Regardless, the Scorpio is lighter, but the minimal extra weight of the heavier Fox cars (my 83 is only 3250#) is not an issue. 
I have spent the past couple months with my Scorpio setup the way it is, and geometry tweaking aside, I like it.  I didn't up the spring rate to get the desired ride height, I upped the rate to up the rate as stock they are way too soft.  The rear springs on the Scorp are probably in the range of 900#....stock is 250#..... It's still nicer than any of my live axle cars.

As far as the live axle swap in a Merk goes...if your primary concern is drag racing....then yeah an IRS car is most likely not for you.  Thus all the Cobra guys g and moaning and swapping in live axles.  Folks weren't swapping live axle into Merks just because.  Most Merk owners chose it over the Mustang because it had the IRS.  The shame was that Ford US went with the py diff.

I already mentioned not wanting to use the Merk/Euro diff.  This is why my first change will be to the 7.5.  This can be done pretty cheaply.  No it's not an 8.8, but that can be done later

There is not going to be alot of cash outlay mainly because, well the rear susp isn't costing me anything.  I can get raw materials easy.  The diff swap should run no more than $300....I might have another $100 into springs.

The Cobra swap is a "bolt-in" once you correct all the issues with it.  I dealt with them, had my hands on them, designed parts for them....I wasn't impressed.  It took ALOT of work and tweaking to get them to actually perform.  If they could have put more rubber  into them they probably would have.  Where's the cost savings there by the time you buy good bushings and solid mount the carrier?  Also take into consideration it probably weighs 50% more than the Merk IRS.....at least.

Yeah but it sure is easy. :D

Be sure to give us a report back!

Also, is this more for performance, ride, or just to say you did it?

Yes :hick:

I think set up correctly it will offer all of the above.

The first stage is just to see how it works.  The more intense stuff will come later.

Well I must have grabbed the wrong info.  The curb weight of the Scorpio is 3250# (52%/48%).  So it's pretty much the same as the 83-86 cars.

Also, I have my other car here now, so once the weather starts to behave (ie...stop raining) I'll be able to get some better measurements and go from there.

Chuck, what kind of stuff have designed for the Cobra IRS?? I might be interested. Please post some pics of your work with this project. Also, where (how) do you manage to stretch your days??? So many projects......So little time

John

P.S. Let me know when you get those mounts I sent :)

All my work on the Cobra stuff was when I was back at KB.  I don't have any drawings with me.  I was working on another IRS project for a while (SLA), but that has since been axed due to other issues.

I haven't seen the mounts yet, but should any day.