XJ40 Autobox conversion

DamianZ28

DamianZ28

Active Member
Hi all, as you know I've recently converted my 74 V8 P6 to LT77 SD1 5spd manual, it was a BW35 Auto, l did the same 9yrs ago to my red one, transforms the car being manual........but 9yrs on and a tad older at 52 I'm thinking about possibly putting it back to auto, why l hear you shout? Well l just want the simplicity of driving it and auto keeps popping into my mind, dont get me wrong its great fun being manual with the newly rebuilt engine. So has anyone converted their V8 auto be it BW35/65 to the Jag XJ40 auto 4spd box? This a potential route im thinking of, my brother had his BW35 auto stag converted and its supurb to drive, so smooth and the 4th gear makes a big difference, think its an early box being non electronic if that makes sense.
So just a thought at the mo and too ask if anyone has done this conversion. My other thought was possibly putting it out there and asking if anyone would want to do a straight swap for my complete set up LT77 version so basically all the components l used to convert to manual from auto, so the actual box, bellhousing, flywheel, master & main clutch cylinders, braided clutch fluid pipe, pedal box, modified rear box mounting, gear stick, new clutch kit, new release arm (strengthend) and bearing, all new remote mounts ect ect. Even the modified mounting plate for the clutch master cylinder. In return id require everything single component from the auto car, so I'd would want an excellent condition BW65, Torque converter, flexplate, complete shifter, rods, rear mountings, trans fluid pipes, kickdown assessmbasically, all the nuts n bolts associated with removal of all the auto bit's id need as you'd get all the nessacary manual conversion bits. So food for thought if l were to change back to auto and if l were to use an XJ40 box.
 
That's the ZF 4HP in the Jag and many others. There's a sticky for it the forum. So the answer is probably a few 10s of people have done it, including myself. And the conversion is covered in great detail. #1 get your box complete from Andy Morgan ready to fit - save yourself a lot of messing around this way. Housing to tailshaftt, it's plug and play. You then basically need only a mount, a decent cooler and a speedometer drive.

The 4sp conversion in the Stag is in fact better than the P6 because the final drive gives you near perfect ratios. With the P6 with no overdrive option on the horizon for the V8, it got a tall final drive ratio to stretch 1:1 top for the motorway era - the Stag had an overdrive option, so "direct drive" is near where it should be and hence with the ZF, so is lock-up 4th. When you fit the ZF in the P6, the ratio is in fact even taller than the LT77 and taller than any RV8 application at 33.8mph/1000 rpm. It's not that it doesn't "work" (indeed it is transformative) it's that it's not optimal. And that's not just for the 4th gear obviously. In fact one of the reasons the V8 P6 is slower than you'd expect for a car of its weight is the other gears are not ideal also. I fitted 185/70s for a 6% reduction and the improvement is noticeable and 4th is still well over 30mph/1000rpm. The acid test for me was in lockup 4th it couldn't quite hold 80kph (50mph) which is the limit here as well as I'd like, it'd unlock on a slight incline. The gearing drop is enough that it cruises comfortably. The ZF also shifts a lot nicer than the BW as you'd expect with 30 years of mechanical auto transmission technology.
 
I lowered my rear diff ratio using a NOS 2200 gear set. Transformed the car even with the old BW35. next step is ZF4hp22/24.
There is one type of Jag box that has the rear mount on the left rear side. You don't want that one! AND make sure you get the switch gear with the box as they are very expensive. There is also a particular plug pin pattern that HGM use so either check their website or get the matching plug off the harness...
 
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mikecoombs said:
I lowered my rear diff ratio using a NOS 2200 gear set. Transformed the car even with the old BW35. next step is ZF4hp22/24.
There is one type of Jag box that has the rear mount on the left rear side. You don't want that one! AND make sure you get the switch gear with the box as they are very expensive. There is also a particular plug pin pattern that HGM use so either check their website or get the matching plug off the harness...
Click to expand...

I thought I was going mad being the only one talking about gearing, absolutely no one seemed to have been thinking about this. What Rover had at the time were two transmissions with a traditional 1:1 direct top. What they did with the V8 is boost the overall gearing with the final drive and put the that somewhere between where 4th and 5th should be to give it some "legs" on the motorway, the result is all the gears are all totally wrong - or at least highly compromided - the car must be a second or more slower to 60 than it should be. What I notice most even with a small drop is the "getaway" from stopped is much sharper, say 0-15mph.

With an LT77 or a ZF4HP you no longer need to do this as you have your overdrive gear. If you add overdrive gears to a stretched rear axle ratio, the result is like a 6th or 7th gear which is of less use and too big spaces between ratios.

We can talk about tuning the V8 which is fairly easy to add maybe 30% without effort and expense but the truth is it is far more important for a good experience on the road to have these gears where you actually need them at the speeds you actually drive on the road. With the ZF cruising through France at 130kph or 82mph, it about hit the peak of the torque and strangely felt like it wanted to speed up. We need this closer to Uk motorway speeds. If you look at the gearing of other V8 Rovers these are nearly 31mph/1000 instead of nearly 34.... So we need a 10% drop. 186/65s will nearly give this but look a bit silly....

I'd rather go with the 4 pot 3.54 and slightly bigger tyres to give the right combo.
 
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Here's a gearing chart I made some time ago - all the non-Rovers use the ZF here. The SD1 tops out just under 31 mph/1000rpm, so this is where we aim to push the overdrive - 33.85 is too high - 1.6 more than the LT77! But also, look how much taller the standard 1st gear is (10.31) with the BW35 than any other application - this may be to protect a weak transmission but this is some 20% higher than more modern cars with the ZF - 30% in the case of the Jag (7.1!) - so that's why initial get away is so sluggish...

Looking at SD1 gearing the fuel economy difference between the manual and auto V8 at motorway speeds must have been fairly spectacular!

@mikecoombs - I also considered the 3.54 final drive and this is in the chart above - this gives 29.45 which would be a nice result - that 3rd gear (1:1) is going to be a really nice ratio for b-road driving too. The stag is 27.1 in 4th which is perfect for a more sporty car with an engine that revs more easily.
 
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the difference in the actual gears is only one tooth. I think Rover were trying to get more speed to match the American V8s which were pretty much all bigger engines. They also relied on the 2-1 torque converter ratio to get off the line which slows everything down as the converter needs to build up speed under load... I think originally when the engine was first put into a test 2000 shell they left the original diff in place so performance would have been much better than the commercial version. Plus it had the original manifold and 4 barrel carb which would be better than SUs off the line anyway.

The diff swap was easy though you do need to be careful which ring gear you get as there is a slight difference in the mounting bolts. It's disscussed at length in the manual though and the last diffs were identical between V8 and 4pot bar the two gears

I've done major work on my engine to get better pickup but nothing has made as big a difference as the diff change. Its made the car feel significantly lighter to drive, especially in traffic and i'm using far less throttle.
 
I have a 3.54 lsd Jag going into mine, but still have an spare open Jag 3.54 on the shelf if you’re interested Damian?
Jim
 
I would be interested to hear from anyone who has opened a Rover 3.54 diff with the later multi ribbed case as to whether it contained 2 or 4 differential gears.
Thank you
 
mikecoombs said:
the difference in the actual gears is only one tooth. I think Rover were trying to get more speed to match the American V8s which were pretty much all bigger engines. They also relied on the 2-1 torque converter ratio to get off the line which slows everything down as the converter needs to build up speed under load... I think originally when the engine was first put into a test 2000 shell they left the original diff in place so performance would have been much better than the commercial version. Plus it had the original manifold and 4 barrel carb which would be better than SUs off the line anyway.

The diff swap was easy though you do need to be careful which ring gear you get as there is a slight difference in the mounting bolts. It's disscussed at length in the manual though and the last diffs were identical between V8 and 4pot bar the two gears

I've done major work on my engine to get better pickup but nothing has made as big a difference as the diff change. Its made the car feel significantly lighter to drive, especially in traffic and i'm using far less throttle.
Click to expand...

You really need to get that ZF in....
 
The v8s all had four pinion diffs. They use a split carrier whereas the 2000 units are two pinion in a unitary carrier (I think) . The 2200 used the V8's carrier with four pinion.
 
Thank you Mike
I always wanted to fit the blue car with the 3.54 diff, I reckon it would have been a real road rocket.
 
I'd like to think that somewhere in the future of my Estoura is a 4.6 running efi, with a ZF box and a 2200 diff.

I understand that to fit the 4.6 means changing the front of the engine to an earlier style, I'm less clear if this causes a problem with maybe crank reference sensor placement.

And, if I install a ZF how do I operate the S1 strip speedo?
 
Burgundyben said:
I'd like to think that somewhere in the future of my Estoura is a 4.6 running efi, with a ZF box and a 2200 diff.

I understand that to fit the 4.6 means changing the front of the engine to an earlier style, I'm less clear if this causes a problem with maybe crank reference sensor placement.

And, if I install a ZF how do I operate the S1 strip speedo?
Click to expand...

You'd likely use an electronic driver like the Dakota Digital.
 
Burgundyben said:
I understand that to fit the 4.6 means changing the front of the engine to an earlier style, I'm less clear if this causes a problem with maybe crank reference sensor placement.
Click to expand...

You can use the 4.6 front cover if you use a remote oil filter. When I was mocking mine up with standard engine mounting rubbers I added a 3mm spacer each side which gave more than enough clearance (the sump clears the crossmember without these, it just gives an extra bit for practical access to the remote filter sandwich plate)

The standalone coilpack ignition system I’ll be using is triggered from the factory bellhousing sensor which keeps things nice and neat too

Jim
 
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