This is my concern: Will the IRS rear end in my 75 XJ6 Coupe hold up to the new power plant & transmission ???

I'm installing a 388cu.in SB Chevy in my XJ6. The engine produced 434hp & 468 lbs of torque on the engine dyno. I'm using a pro-built GM 700R4 transmission with a 3.06 first gear ratio & a custom built 2,000 - 2,200 rpm stall converter.

I plan on doing a little drag racing to show these owners of new Camaros & Mustangs what my little Jaguar is made of. In fact I have a little side bet with a buddy of mine who owns a 2012 Camaro SS - who says he'll beat me by 5 cars lenghts or better.

I've been building & racing American muscle cars for over 40 years, but this is my first experience building a Jag - so I'm asking the wisdom of other XJ6 owners. I found I can run a 275-50 drag radial with a special made wheels - so traction shouldn't be an issue. It's my belief the original half-shafts & or rear may fail. What if anything can I do to prevent breakage ?
Advise & opinions welcome - thank you.

 

My '71 XJ has a 383 that dynoed at 410 HP and 470# torque with a 200R trans. it used to have 3:31's and now has 2:88's. The rear has given no trouble at all since the new engine in 1997.

It will burn both tires at will and tries to lift the front end with the stock sized
tires.

I can stay up with the older Camaros with 6 speeds as they usually burn too much when they discover the Jag has good low end torque and usually leaves them at the beginning.

You will not beat the new Camaro...at best you will have a high 13 second drag racer without any power additives.

 

The rear end can handle it. Your torque will be limited by tire traction in any case. I'd like to hear more about how you can run 275's. I've got a traction issue. There are two week points.

1. The rubber rear cage mounts. my car with a SBC 406 has torn apart two.

2. The body mount on the rear trailing arm is weak and a rust prone area and must be re-inforced.
Here's a vid of a support tube I installed on an xj12c and its tied into a reinforced trailing arm area with a plate.

 

My rear cage mounts were replaced early on, they were just almost 40 years old. Since my cars (Series 1's) are both CA cars, we have no rust to deal with.

The trailing arm bushings were replaced at the same time as the rear mounts and the brakes completely rebuilt.

 

I took one of my totally rust free Series III V-12 with a slightly tweaked engine and a modified rear drive assembly to a road course race track here in Ontario. After ten or twelve laps chasing two fully race prepared Ferraris, managed to tear both rear suspension trailing arms right out of the body. They do need to be reinforced for any sort of real punishment.

 

I would like to thank those who have replied thus far. Seem's by popular opinion thus far, that rear itself is quite strong & that I should be more concerned with the trailing arms & mounts. I've used a similar engine / transmission combo in my 69 Corvette with
an IRS & had nothing but problems. Broken cases & twisted half-shafts. One by one I would address the weak areas & finally after countless hours plus approximately 6k $$$ I had all the bugs worked out.

What I would like to do if possible, is put a steeper gear in the rear 4.10's or 4.30's - although I don't know if these ratio's are even available for my rear. Maybe someone could shed a little light on the subject.

As for the gentleman's question as to the 275-50 tires. I have a guy that's been making me custom aluminum wheels for the last 5 or 6 years - any bolt circle or offset. Nothing real fancy, just functional - but pricey. Since the brakes are inboard,
inside wheel diameter isn't an issue - wheel size & correct back spacing is.

I'll check back later & again thank you all for your opinions / advise.

 

It should be able to take it. There's a guy in New Zealand, Andrew Robertson (sp?), that has a big block twin turbo SI car that he races. I am pretty sure he is running stock rear end parts with the exception of the trailing arms - he replaced them with a different setup.

I would really like to see this because I want a wider rear tire on my SIII. Those rear fender wells are pretty narrow. For reference, I had a 245/50ZR17 tire on the rear of my SIII. The tread width was 8.2" and the section width 10.2". The offset had the outer sidewall really close to the fender lip - so close that the outer sidewall would scrub the quarter lip when I would rip a hard turn in a parking lot. There was less than 1" between the inside of the wheel/tire and the bumpstop. That gives you a really small window with a 275 tire. If you're just going in a straight line it might be a little more forgiving, but if you plan on pushing through corners, I don't know...

BTW, I've heard of XKE guys shaving the bump stops to get some extra room. That might be an option for you too.

Yea, that trailing arm setup is not ideal for high-torque applications. My car with an LS1 ate a brand new small bushing on the radius arm in about 500 [hard] miles.

I am pretty sure that up to 3.54 was available from Jaguar. Any higher than that, I think you're going to be mixing and matching parts with a regular Dana 44 rear end.

Let us know how the rear tire endeavor goes - and please post pictures when you have something together!!

 

the serious Jag racers mount the rear cage solid to the body(no rubber mounts).

and really reinforce the torque box area, where the trailing arms mount ,and a steel plate to keep the rubber in place.

course you could go with HD Hiem joints!

 

Independent Rear Suspension Forum / 1972 XJ6 IRS Brakes

I sent this link originally, they do all kinds of Jag IRS mods there and can
help reinforce for road racing.

My only experience has been straight line stuff...with no problems.

Throwing a heavy Jag sedan around corners would make stresses on the trailing arm rubber parts and any other part not new or properly secured.

Jag rear ends came in other desirable ratios and a shop I have used in CA
has all of them. Curt Hamilton in Van Nuys, CA, Hamilton Auto Industries.
They have everything and can do anything to a Jag IRS.

 

i put ring and pinion from a JEEP Dana44 ,3.73 ratio into my 78 XJS rear,added an extra plate in the posi(powerlok), been great with a 700R4 overdrive.

its a street car gives a 2.60 final, nice cruiser, serious racing of any type costs big money if you are gonna be competitive.

 

to original poster, if you are serious about DRAG racing , just mount a 9" Ford rear, been done a thousand times or more,solid live axle, cant get much better for D/R.

ford came out with the 9" in 1957, go figure!

IRS has never been up for wheel lifting launches! and going straight at the same time!