Custom Jag IRS strength? (Pics inside)

[750hp]

How strong is a Jag IRS when some of the weakness issues have been addressed? I'm putting my supercharged motor into my Arntz. It makes 870hp and 705lb/ft on an engine dyno.

I can buy this Jag rear, but want to know what success or failure you've had with modified engines and/or sticky tires with Jag rears.

The IRS for sale has custom chrome moly tubular lower control arms and thick wall tubular half shafts. Is it worth considering, or should I be looking at a different design entirely?





Thanks

[patrickt]

We've had a couple of members at the local club twist the stub axles off. If that rear has the plain old stock stubs, they won't hold up to your kind of horsepower with sticky tires.

[Cobra #3170]

Do you know if the stub shafts and gears are upgraded, that would make a difference.
I am not at your power level (850 and 640 NA) but have had no failures with Hoosier A6 335x30x18 because the car is so light it can't hurt itself.
I am quite sure drag radials would break something on a hard launch though.
I am running an aluminum Salisbury very similar to the Jag unit with 30 spline
stub shafts and 3:31 gears, stock half shafts with drag race U-joints.

[lippy]

I have the Jag rear end with the standard strengthening ERA does on it. I've been told by ERA it can handle 500-600HP as long as I don't drive like an a-hole and launch really hard with drag/sticky tires. The photo you have looks like it has more strengthening than mine, but you are talking about an awfully powerful car, so I'm sure you could break it if you drove really hard.

[mickmate]

That does look like a great set up with diagonally braced tubular arms and vented brakes and all the good stuff. There is the issue we've discussed about the top of the hub carrier having no for and aft support. The other consideration is that stock outer stub axle. It sounds like you have the equipment to test for the weakest link. You don't want it to be one that drops your tank or metal against the road.

[DougD]

Why even mess with an independent rear? If you want to drag race, put a 9-inch in there. Problem solved.
Otherwise there will be another thread here soon detailing what broke and asking for opinions on how to fix it.

[RICK LAKE]

750HP If you are looking at higher than these numbers, I would look at a quick change rearend. If this rearend has 30 spline stub shafts and you watch the abuse, it will hold. You didn't say how much booster you are looking for and tires you want to run. 1 thing missing is a carrier cap support cover. This has 2 clamps to support the carrier caps inside the center section. If you have a booster controller to not allow the banging problem to the drive train. I would go with this rearend. Still no slicks or 5,000 rpm hole shots. What clutch are you looking at? This is also a BIG thing. IF the clutch has no slip in it, It will short live the rearend. Is the Clutch disc have a spring hub? This helps control shock loading of the drive train. I have been running the same Jag rearend for 15 years in auto cross and track racing. Controlled driver will extend the life of any car or parts on the car. I did loss a stub shaft bearing at 4 miles when new. My 482 is in the 600/500 torque to hp range. I did goto a street twin setup. Its soft on the drivetrain but does lock it up good. Last notes, what the ratio in the housing? If it's a 2.88, It will not last. I like Cobra #3170 have been running a 3.31. IMO this is the best ratio for BBF cars. Is the stock posi in this or a locker. That stock posi will need to be blueprinted and clutch packs setup tight. There are 4 different carriers for a Jag rearend. What trans are we going with?? Some one put some good coin into this rearend, see if the cost is worth it? You are looking at about 3G's for a rebuilt jag rearend with a 30 spline setup and solid rotors and calipers. That setup in front of you is about $1,500.00 alone. You gave no price on this. Good luck with your choice.

[DanEC]

Originally Posted by 750hp

I'd like to see the couple of gorillas that wrestled hat 350 lbs or so unit into that tight space in that shelving unit.

[Morris]

Inboard brakes are not good.......temps get to high......

[Bernica]

Originally Posted by patrickt

We've had a couple of members at the local club twist the stub axles off. If that rear has the plain old stock stubs, they won't hold up to your kind of horsepower with sticky tires.

Like this? Not my car, but a good representation of what PatrickT is discussing. Sticky tires, big HP...something's gotta give!

[joyridin']

Personally, with that type of HP, I would be looking at something like this.

[strictlypersonl]

I suspect that Pat was discussing the inner stub axles, not the half-shafts, which, in spite of your picture, are pretty bulletproof. HD stub axles are available for about $1600/pair.

What worries me a little is that those lower control arms aren't very stiff in torsion, leading to some potential wheel hop.

[patrickt]

Originally Posted by strictlypersonl

I suspect that Pat was discussing the inner stub axles, not the half-shafts, which, in spite of your picture, are pretty bulletproof. HD stub axles are available for about $1600/pair.

Yes, the inner stub axles. It was right around this time last year that DCDoug twisted his off (a CSX car), on the drag strip, but the other was an ERA. The ERA car had the optional rear, as I recall, and was 1000 times easier to perform the repair on. I kibitzed on DCDoug's job and it was a borderline nightmare changing that stubby out on a CSX car. Here's that thread: Need help diagnosing a clutch/tranny problem

[Double Venom]

750-Hey! haven't talked to you in forever. Remember DV II? Not sure it is the quick change Rick's talking about but, the special V-10 Dyno'd well over 750 HP, and we ran a quick change 11". Nick had gears for 2.88 all the way to 373's. As far as I know he is still racing it and has never had a rear end problem. ?? He has ran 17" what M/T calls Street Slicks on it at times.
DV

[patrickt]

MdMan352's Contemporary


DCDoug's CSX


Fullchat289's ERA

[Bernica]

Ouch! All three look painful and a total PIA to fix.

[750hp]

Thanks a million for all responses - there's so much good information here that I'll try to summarise everything into one response.

The Arntz is my old workhorse and is the car I currently drive to work everyday. The current mild motor and 4 speed and 3.54LSD Jag IRS have been a cheap, reliable and fun combination for the time I've owned the car. I bought a more radical and all round "nicer" and more authentic looking 427FE Contemporary with the idea of selling the Arntz, but the attachment is too great and I couldn't bring myself to sell it.

The Jag rear in the Arntz needs replacement radius rods as the bushed rod end at the hub end developed a crack at the mounting point. With some general creaks and groans that come with the fact that the rear end has even in the car for over 30 years, I need to look forward towards to an impending engine change, and get the rear end ready. I want to continue to drive the car every day with the new mechanicals. Other than the usual car required with a solid roller cam, the rest of the engine is relatively low maintenance. I only say this so that I don't give the impression that I'm turning the car into a single purpose drag car. That won't be further from the reality.

The motor is a 398 cubic inch small block Ford with a YS Vortech (DV, this is the same motor that OZVENOM and I were installing into a previous Cobra way back when we met you for the Fling back in 2002). I'd like to use a tweaked TKO600 with a sprung hub clutch assembly, and hoped the custom Jag rear would solve the rear end requirement to upgrade or rebuild. The idea of having to replace both the inner and outer stubs, plus then install or of Mickmate's great Watts Linkage systems, then replace what I already believe to be an unsuitable center section, well that's all getting a bit expensive with the ongoing uncertainty about the half shafts, unis and who knows what else.

If I was to consider a quick change "live" rear I'd be inclined instead to use what has worked for me in the past and that I have a lot of spare parts for; a 35 spline, 4 link 9" with 1350 unis to never have to consider parts breakage again. Perhaps with a Mumford Bar locating arrangement. I hate the idea of potentially sacrificing ride quality to retrofit a live axle to an IRS car. Nobody but my Dad and I and handful of passionate Cobra enthusiasts even really know what an Arntz is over here, so resale value of any mods isn't a consideration. The main issue will be the drastic engine setback in the Arntz and a live rear end would possibly lead to the worlds shortest driveshaft and uni joints that bind at full suspension travel.

Patrick, thanks for DCDoug's repair thread. I think that single handedly turned me off the idea of trying to work with what I have!

[Mdman352]

If you launch like this off idle....



Using these....



The first to go will be these.....



Then these.....



then these.....






then these....



then the "STRENGHTENED" ones.....



So....unless you have LOTS of these




Really though, the Jag rear was designed for 230 HP and 700 series tires. The design leaves something to be desired (and lots of parts on the track) unless it is used as intended, which is cruising. I have a mild motor (500+/- hp and much more torque. You can spend all kinds of money HOPING it will hold together or you can accept reality and move on to something more suited for the intended purpose. After all, what good is 750+ hp if you can't hook it up and use it??? FYI...I also have a 750 HO Cobra in my garage and it is NOT fun to drive.

[1985 CCX]

Dave

Great rendition of what not to do......................

Cobras use IRS, best for track. Use solid axles for drag way.
Yes, Dragonsnakes were IRS still they were 289's and yes IRS was Beefier than Jag unit.

Debate continues.....

[mickmate]

Great summary from Davids school of hard knocks. I must point out his tubular shafts that twisted were driveshaft tubing which is too light on tubing wall. The diff is a big torque multiplier and you need to do some simple maths with your ratio when calculating a tube wall thickness. Then throw in a factor of safety (FOS) on your numbers. Use seamless DOM mechanical tubing.
Then there's 3159, do we know what diff or upgrades that was running or were even available?