I am after a facelift rear sub frame to change the inboard discs to outboard
my ratio, according to Haynes manual is 3.07 (I haven't looked yet) if I get a 4lt rear end it may well have a 3.54 ratio. will this be an advantage as far as fuel consumption is concerned or what would you more knowledgeable
folk go for, also will my 84 differential unit fit the facelift set up, thank guys

 

RGP
You have an HE version of the XJS and the diff is 2.88:1 ratio. The V12 facelift and 6 cylinder rear end diffs have lower (numerically higher) ratios as they also have compensatingly different ratios in their 4 speed boxes. I do not know if the V12 and 6 cylinder facelift diffs are the same ratio or not. The outboard diff if used with your three speed gearbox will substantially alter the speedo accuracy and also adversely affect fuel consumption, as the car will be running a considerably lower overall gearing.

I am sorry to say I do not know if your 2.88 diff will be a straight bolt-in to the outboard braked rear, but I think there is a good chnace it will, someone will know !

 

As Greg has stated, your fuel economy will decrease quite a bit. On the bright side, acceleration will be improved significantly, especially from a standing start

Also the car will be a bit noisier at motorway speeds as you will be at a higher rpm for any given speed. And not that you'd care necessarily but your top speed would be reduced a bit.

Also not sure if you could use your 2.88 diff as is with an outboard rear end.

Cheers,

Allan

 

cheers guys, just a matter of finding out the interchange ability of the dif now

 

All XJS and S1,2,3 XJ diffs are interchangeable. XJ40 and X300 diff internals are interchangeable with the XJS (pinion snout will need to be cut off and the nut machined, other wise a direct bolt in) My XJS has a 3.58 X300 hemisphere and gears.

Outboard brakes - you can use XJ40 X300 rear hubs and driveshafts. They have a spacer between the diff and shaft where the IB disk would be. You keep the XJS lower control arm. The lower outer fulcrum pin is smaller in diameter so you would need to modify the XJS lower control arm. The hardest part is modifying the handbrake to work with the OB drum.

 

so I can buy this f/l crossmember and fit my prefacelift differential straight into it?

 

Yes. But why would you want to do that unless you are rebuilding the diff. If you get a rear subframe from a FL 6.0L the rear will be 3.54 if its a 5.3 it will have 2.88. I think the early FL still used IB brakes. But everything is interchangeable. If you can source a rear form an OB car the hanbrake cable will fit.

I have a full X300/XJ40 rear and am planning on converting my IB to OB brakes in the future. While I had the rear out and in bits I test fit the X300 hubs and shafts. The drive shafts and unis are different in the X300 but bolt straight up to the diff with the spacer from the X300.

 

I have had a word with Graham Hallet from grublogger parts. and he advised against it, as the work involved wouldn't warrant the improvements gained, there is no problem with the one I have, so I have put this project is on the back burner for the time being, definitely worth a thought in the future though.

 

RGP
If I have this right, to do the swap you (a) unbolt your old diff, remove the FL diff, and bolt your old diff into the facelift outboard braked setup, and then (b) the so-made up axle unit will then go straight into the car with no further machining or modification?

The main point of doing this would be to get a much better and more accessible handbrake, I take it? Plus disc and pad maintenance a good deal easier too?

Am I correct about all these points? Thanks

Greg

 

hi Gregg don't start me off again.lol
email coms rec from Graham

- are you sure? Personally I wouldn't bother, because you would have to change the diff as the 3.54 is too low for the 3-speed auto box. So it means a complete strip-down and rebuild. Bearing in mind the going price (£650) for an outboard rear it seems a big outlay just to get easier rear brake maintenance
Happy though I would be to sell you an outboard...

 

I see this as a "six of one, half dozen of another" situation.

If you have inboard brakes a system overhaul (calipers, rotors, pads, parking brake) is a good bit of effort....a full weekend project for most of us. But, when done, you'll be "good to go" for many years, especially since, in most cases, XJSs are no longer subject to the wear of daily use.

The OB brake conversion certainly would make future repairs much easier...but it's not as though the project is a freebie. There's labor and expense involved in the conversion itself.

I guess a person would have to weigh the options and decide for himself.

Cheers
DD

 

that's what I mean Doug all that was done about 3000 miles ago, (3 years) and is fine, I was just really looking for something to do I suppose, so I will start next year with adjusting the valve clearances because when I fitted new heads to it I set the clearances to 15 thou after reading xjs help advice for improvement in consumption?? but I cant stand the tapperty tap from an otherwise super smooth engine, I just ordered 2 half moon alloy inserts from Ron, so I am now committed, cant wait now till spring,

 

i would be keeping the 3.54 and installing a 4 speed auto. Give you much better punch off the line and lower cruise RPM as well.

I currently have 3.58 and the 3 speed. Until my 4L60e is ready.

 

Warrjon,
How is the four speed to be controlled, as the pre facelifts do not have the electronic controller? Can you build a four speed with a vac/mechanical system like the 3 speed's?

greg

 

Yes - there are basically 2 different GM 700 4 speeds the 4L60 (aka 700R4) is a non electronic unit and the later 4L60e requires a TCU. I decided on the 4L60e and will use a Mega Shift controller to run it.