Dear All,

Some time ago, I suggested that I might swap the rear end of my '95 XJR with 3.27:1 gears for a rear end from a 3.2 sport with 4.01:1 gears. Anyway the inner wishbone bushes on my original setup have failed, causing a clunk and replacement requires removal and diss-assembly of the whole unit. So I decided that it was time to try the swap as a quicker option, with the bonus of increasing acceleration.
I transferred the springs and dampers but left the rest of the replacement unit as is. I did notice that the ABS sensors where different, but as the ABS /Traction unit did not change part numbers during the production run I guessed that they would be compatible.
Anyway, I finished the swap and went for a test drive last weekend. The ABS/TRAC light came on and then went off after a short distance. Driving slowly around the estate everything seemed fine, speedo working etc so I pulled onto the main road, keen to test the acceleration. However once I reached about 28 MPH the car felt like it was being held back. The engine was showing positive pressure on my boost gauge and sounded like it was running fine, but the car would not go any quicker and surged slightly. I assumed it must be to do with the traction control as I once had similar symptoms due to smaller diameter rear tyres, so I turned it off. That did not make a difference and I went home disappointed!
I plan to swap over the original hubs/sensors this weekend to see if that helps.
I know at least one member on this forum has swapped his 3.27 for a 3.54 without issue, so I had hoped that the ratio change wouldn't be interpreted as, or turned into, a gearbox fault.
Do any of you gents know the situation with our cars?
Worst case I will just have to rebuild my original rear end and swap it over again, I just hope I haven't damaged my gearbox or ABS / Traction unit.
Any comments welcome.

Regards,

John

 

Is it changing gear correctly? I am wondering whether your ecu can't cope with the engine speed / gear/ road speed data, as it will be falling outside its parameters from the factory. Just guessing really, but hope this helps.

 

Thanks, I am unsure if it was the gearbox, the engine was not revving very high, say 2.5 K RPM max, so not stuck in first, but it could have been doing something crazy like trying to drive two gears at once. I agree the 4.01 might have put the data to the ECUs outside of the parameters, where the swap to 3.54 by another member could have been just within tolerance.
I only drove the car very briefly due to the obvious problem, but the fact that the problem remained after turning off the traction control would support a gearbox control issue, although the transmission fault / limp mode light did not come on. If that is the case, I will probably have to rebuild my original rear end and swap back. Guess I will know ether way when I swap the hubs to reunite the ABS/ Traction unit with its original sensors so the only difference will be the final drive ratio.

 

Just to update, in case anyone is thinking of doing something similar. Swapping the original hubs and sensors onto the replacement axle assembly seems to have cured the problem. No warning lights and the gearbox shifts normally. The car always felt like it pulled well and I have not driven it for a while, partly due to the axle swap but mainly because someone drove into it whilst it was on a car park, but yesterday it felt really fast.
The lower ratio makes the car feel like it has got lighter, for want of a better description. I have been unable to do any timed standing starts due to the wet road conditions but it must have made a difference judging by the feel. I have not been on the motorway yet, but as it is the standard ratio for the 3.2, I hope it wont be too buzzy. I estimated rpm at 70 mph will be 2700 rather than 2250 with the original final drive.
The only issue to date is a slight clunk from the rear on the 1-2 shift under full throttle. I guess this could be the A -frame bushes. The XJR has solid rubber, the na versions have large voids and I didn't replace them. I had filled them with seam sealer in anticipation of the extra wheel torque due to the supercharged engine and lower ratio, but should probably have a look for signs of movement.
Likewise, I have yet to evaluate the loss of the LSD. Straight line traction from about 20mph good enough in the wet with only slight wheelspin on the 1-2 shift. I am more concerned how it will affect handling, but can't afford a track day at the moment!
I will update once I have done a few miles in it.
Cheers,
John

 

John,
great to hear you are making progress! Re the A frame bushes, the XJR bushes are different, but they are not solid. They have voids too, which need to be correctly aligned when fitting, so must play some meaningful part in proceedings.

 

Hi, mine looked solid when I removed the original assembly, maybe the smaller voids were filled with 20 year's worth of road dirt so I didn't notice!
I will see how it gets on, I think polyurethane bushes are available, one member on this forum was fitting them a while ago so might try that if it becomes an issue. I hope to test the car out this weekend if I have time after cleaning it following bodywork repairs, so might try to use my go-pro to film the movement.