I have a 71 XJ6 with a Supra 5 speed gear box in it. It was an automatic originally so has the auto diff 3.31:1 (?) which is not ideal for highway cruising. Good for grunt off the line but revs are a bit high when cruising. I have the opportunity to purchase a 2.88:1 Powerlock diff out of a V12 XJS. Couple of questions. Is it a straight swap; old one out new one in? Can it be done without dropping the whole rear end out? Any thoughts would be appreciated. Thanks

 

First, the entire rear end must come out, yes.

I *think it'll be a direct bolt-in swap but there are a couple of things......


Some of the mid-80s XJSs used a Dana differental....easily identified by lack of a drain plug at the bottom. If this is what you have you'll need matching brake rotors to go with it.

If you had a Ser III XJ6 I *know* it would be a direct swap but I'm not familiar enough with Ser I cars to know if there are any detail differences that might be a problem. I doubt it, but I'd wait for other others to chime in

Cheers
DD

 

I would say you will be OK, but what year was the XJS the replacement diff came off ?

 

Thanks Fraser

85 or 86 I think

 

AFAIK, nothing changed much on the rear suspension until the brakes were moved outboard in the 90s. The XJS was a very low production levels car, so essentially used the XJ saloon suspension with different spring rates and diff ratios, although the front subframe was different, I think.

However, as Doug says, there may be more to it, depending on what manufacturer supplied the diff. Anyway, a pair of new discs won't break the bank if you need to fit them.