I have a 1985 XJ6 with a stock LS1 engine. When I get on the car really hard in 1st gear, or when I do a full throttle 1-2 shift (my trans shifts VERY
firm) I get a thunking sound which I think is coming from underneath the rear of the car. It almost sounds like a drum roll. My best guess is that the IRS is rolling forward and the driveshaft is getting pushed up into the tunnel. There are marks there, but I'm not sure how fresh they are. I've been experiencing the problem since I got the car on the road a little over a year ago. It's not really RPM or speed dependent, but it occurs with heavy throttle and I suppose torque.

I recently rebuilt the whole rear end, including all new rubber. I used the extra-stiff rubber IRS cage mounts from SNG Barratt, and URO Parts radius arm bushings. I recently replaced both small-end radius arm bushings because they failed. I did drive the car a couple times with one radius arm removed. I took it nice and easy, though the noise did happen one time when I had to use a little more gas than I wanted.

Any thoughts? I think I have all good rear bushings on my car, and I'm not running a crazy amount of torque. I think there are other guys that run as much or more torque than I do without any custom parts that stabilize the IRS.

The large radius arm bushes are installed with the slit at the front and the rear, just like in the manual.

Think I damaged the IRS cage mounts when I drove it without a radius arm? I inspected them the best I could, and I did not see any signs of failure. I do not know how to otherwise test them...

Any info would be a big help. I am ready to beef up the engine, but I really need to resolve this first.

 

I had the same problem with my SBC 400. I had the same heavy drum beat on hard acceleration. It's known in performance circles that the Jag IRS rolls when under high torque loads and while surfing around the interweb I saw many ways others devised to control it. Most were solid or fixed and would have transmitted NVH to the body of the car. I chose a strut mounted to the cage bottom for a long moment arm that was bushed at the end for isolation. I fabricated it as a bolt on because I didn't know if it would work. One end bolts to the bottom of the IRS cage using the plate bolts and the other end bolts to the chassis using the bolt holes that were used for the center bearing support plate. The bushing I used is the small radius arm bushing pressed into some thick wall tubing that happened to be the right size. After I installed this I had no more drum beat, no more wheel hop and no additional harshness.

 

Thanks for the info. So this is "normal"?

Is that sound the tunnel getting beat up by the driveshaft, or is that the U-joints banging from being so far out of alignment under load?

I had my eye on that driveshaft plate, too. I guess those threaded holes are sufficiently strong to hold the IRS back?

I was looking at these - I think it might be a little easier for me with my current exhaust routing.
Lightweight Biscuit Bar - Speedway Motors, America's Oldest Speed Shop

 

As normal as having a Chevy engine in a Jag. It happens, other have the issue and many hotroders including the Cobra Replicas have this issue. The Cobra guys are the ones that have done the most development...but since it a cobra harshness isn't an issue for them.

Not ideal and probably wont accomplish anything unless you use 4 of them and set it up like a 4 bar live axel rear. You need stop the IRS from rotating around the center line of the wheel so a fixing bar needs to be solid on one end and long for leverage. The one you specified from speedway seems to be able to twist at both ends so as the irs moves it would simple move with it. Look at how jag solve the problem in the XJ40.

here is another good application...using the XJ40 support. I wanted to do this but exhaust routing was an issue and that piece was hard to find alone.
http://jaguar.fiboy.com/images/jagua...suspension.jpg

Have a look at these pictures. There is another tie plate idea in the photos. This guy does great work.
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