Dear All,


Some time ago I posted regarding the possibility of swapping a 4.01 diff from a 3.2 sport onto my '95 XJR automatic to replace the standard 3.27. This was driven by two factors, one the original had play in an output bearing and the inner wishbone bushes had failed, two I had a complete rear end from a 3.2 sport which I had managed to collect for free a few years ago.
Anyway, I actually got around to doing it over Christmas, swapping the complete assembly. The only technical issue was that I found that I needed to swap over the original hub carriers with ABS sensors to avoid the traction system playing up.
Anyway, fun part first, acceleration off the line is noticeably improved, I estimate sub six seconds to 60, although I have not had chance to time it with my G-tech yet. The car feels lighter and more eager. No straight line traction issues on a warm dry day with high fiction tarmac at the traffic lights, despite the loss of the LSD. This is with Falken FK452 tyres, which I like. I have not had the time or money for a track day, so can not comment on the overall affect yet. Wet / greasy / loose surface road obviously a different matter.
The downside, one of the best things about the XJR is its lazy performance and broad power band giving high average speeds without you trying / noticing. This is spoilt slightly by the lower ratio, and at first was more of an issue to me than I thought it would be, especially as 3 K rpm would be about 95 mph and now it gives about 75-80 mph making the natural cruising speed much lower with no benefit and probably increased fuel consumption although I haven't been on a long journey to properly evaluate, again will let you know. That said, I used to have a 3.2 sport with that ratio (the gearbox ratios where similar), and didn't think that was too low geared at the time, in fact the XJR felt too lazy when I bought it, hence my confidence in the swap. Now that I am getting used to it, I am starting to like the 4.01 a bit more, helped by the fact that I have been driving my wife's MX5 a lot.
So in summary so far, probably the cheapest way to knock down your 0-60 or 1/4 mile without adding extra stress to the engine or transmission and helping your old Jag beat newer machinery. Not without its downsides, if I drove my car everyday, I would stick with the standard ratio or maybe the 3.54 from a NA 4.0 would be a better compromise.


Regards,


John

 

In my XJR/6 I went from the 3.27 to the 3.58 and was pleased with the result. A bit more acceleration and response...enough to feel the difference...but only about 200 rpm increase at 70 mph and loss of 1 mpg at the same speed.

I never set out to make the change, actually. However, my original diff failed (at just 75k miles....grrrrrr) and by sheer luck I found a Coventry West rebuilt 3.58/LSD for just $400 (shipping included) on Ebay! Simply, it was the most affordable alternative at the time.

Cheers
DD

 

Hi Doug,


I remember your original post on the subject, and your words of warning re the 4:1 being a possible step too far, based on experience with old muscle cars. I will try to time the acceleration if it is dry over the weekend.

 

Very interesting. I have wondered about doing this on my 1997 XJ6 with 74,000 miles since 90% of my driving (5,000 /yr) is for short drives on uncongested city streets with highways jaunts limited to 5 -8 minutes as a result of my work.

My hope was that it would make acceleration snapier (prior car was XKR 2002 coupe), and perhaps the change from open diff to LSD would also be noticeable in tight turns under power (in rainy conditions I enjoy getting opposite lock in low speed turns, heh heh, )

 

I missed that bit on the first reading.

Did the 3.2 assembly not have ABS sensors, or were there a different
number of teeth?

You may actually find that the change increases your gas mileage.
I read tonight about a Chevy Astrovan that benefited by 2 MPG
when a similar jump in gear ratios was performed. 3.23 to 4.11
with the 4.1L V6 and possibly the same transmission as you
have.

The thought was that the engine was running in a better part
of the powerband when on the highway.

 

Weisberg, I guess you have the 3.58, so personally not sure given the amount of effort needed to change the diff and your low mileage. I had a worn rear end and a complete axle assemble on the floor to swap over. I have since started to dismantle the original for storage / rebuild. Some of the bolts are very hard to undo / withdraw and everything is very heavy. The 4.01 diff are found on the 3.2 cars which I don't think you got in the States, but they are most common in the UK so should be readily available but quite heavy to ship, just the diff must weigh over 50 Kg. Also the 3.2 cars are least likely to have the Powerlok option. This is something that I did not address.


Plums, the ABS sensors had the same plugs for the car but looked different in that the lead was moulded into the original '95 XJR ones, but could be unplugged from the 3.2 ones which I think may have been a '97. I did not count the teeth. The difference could have been model year or the traction control option. If you keep the original setup on your car it should be Ok.


The only other consideration is that my XJR has the 4L80e transmission, the n/a 4L has the ZF unit. I believe some transmission controllers take a wheel speed v gear / revs signal and can be upset by lower final drives because it thinks that the transmission is slipping. Has anyone swapped ratios on the N/A 4L?

 

I am wondering if having an extra gear will make a car with a 4.0 diff a lot of fun to drive. Of course, you'd have to pay for the fun by having to shift the gears yourself

 

Do you mean fitting a 4.01 in conjunction with a manual five speed? I guess with a manual there would be less to be gained as I expect first gear will be lower then that of the automatic and the extra gear would make the ratios closer anyway. If the engine had a narrow powerband at high revs, then it would be worthwhile if performance was your priority, but the six cylinder Jags are the opposite with a moderate peak power delivered over a broad rev range, which works well with taller gears and suits the car. A manual conversion would no doubt be a good way to improve both performance and economy, and make for a more involving drive, but I would probably keep the standard ratio in this case.

 

This above observation about "less gain with manual transmission" is entirely true if you limit your choices to Getrag 290 as installed by Jaguar with 3.553 first gear ratio. The automatic ZF 4HP24 has 2.48 first gear ratio. If you were to opt for a Tremec trans, say TKO or T56, they can be had wit 2.66 first gears. Plus, with 2 overdrive gears you should be able to cruise at reasonable RPMs.