Carole Nash Classic Insurance Specialists

Your Cars : P6B ZF gearbox transplant – Part Two

AROnline reader, Warren Loveridge, from Christchurch, New Zealand owns a unique 1972 Rover 3500 named Brown Rover. However, when the original Borg Warner 35 transmission blew up, Warren opted to replace that with a later, modified ZF unit. Here, in the second part of a revised version of an article which was originally published in the October, 2014 issue of Driving Force, the Rover P6 Club’s magazine, Warren concludes the tale of how he undertook the conversion…

The rebuilt hybrid ZF awaiting its tail-housing, governor, valve body, sump and front pump

The rebuilt hybrid ZF awaiting its tail-housing, governor, valve body, sump and front pump

Brown Rover was now transported to John Weir’s workshop at Canary Automotive and perched on a hoist. John began constructing the custom cross-member while I scratched my head over a multitude of details, such as transmission cooling lines, the shift selector, the filler tube and the throttle valve/kick-down (control) cable.

John solved the first problem for me by getting custom lines made that plumbed the ZF into the existing P6 radiator cooler. Meanwhile, Andy had been working away in darkest Wales and established that a Jaguar XJ40 shift cable would allow the existing P6 selector to operate the ZF’s shift lever.

I had a special kickdown cable made that mated the throttle end of the existing P6 cable to the gearbox end of a Land Rover cable. John constructed a fill tube using the top and bottom of the Land Rover tube, joined by a flexible hose that negotiated the route up into the engine bay. The cross-member began to take shape, with my now empty (and thus lightweight) spare case coming in handy for a trial fitting.

Work begins on the custom cross-member

Work begins on the custom cross-member

I was also busy with Rover beautification. Forever battling obsessive tendencies, I have developed a rule: of the never-ending list of Brown Rover jobs, I undertake strictly one at a time, and I do it to the highest standard I can: a happy medium that maintains my sanity. Not everything gets done, but everything that gets done gets done properly. In this instance, while the car was gearbox-free, the tunnel was stripped and repainted and everything associated with it, including the propshaft, was cleaned, painted, repaired or replaced. Nobody will ever see it, but Brown Rover has a beautiful transmission tunnel, and I sleep well at night.

The custom cross-member before final tidying and powder-coating

The custom cross-member before final tidying and powder-coating

Finally, the ZF was bolted permanently into place. John fabricated a new exhaust mid-section to negotiate the ZF and put it aside for me to fit later, and then Brown Rover was rolled onto a trailer and transported home for the final effort.

The ZF in place, with temporary Land Rover sump for protection. The new cross-member is bolted to the existing tunnel brackets

The ZF in place, with temporary Land Rover sump for protection. The new cross-member is bolted to the existing tunnel brackets

Several subsequent Saturdays disappeared sorting out all the minor details. First, I had to fit the XJ40 shift cable. The mounting was easy; not so was the fiddly adjusting required to make the P6 selector operate the ZF lever throughout its range (PRND321). But finally it was done, the original selector was retained, and the car remained standard inside.

I fitted the new cooler lines, attached the prop shaft, mounted the exhaust, connected the kick-down cable and installed the filler tube. The valve body and freshly zinc-plated BMW sump were fitted, the transmission filled with the correct amount of fluid, and the level scratched on a modified Land Rover dipstick.

Dakota Digital speedo transducer ready to be installed

Dakota Digital speedo transducer ready to be installed

To install the Dakota Digital kit I had the supplied cable modified to fit the back of the P6 speedometer, but it was otherwise easy, the pickup wires passing through the original speedo cable grommet from the transmission tunnel into the interior.

At last, I was ready to turn the key…

On the hoist, the engine had been spun over to test the starter motor/flex plate alignment and make sure the torque converter and front pump turned freely. Now I had to actually start the car and see if it would move. I was very nervous, so I took a deep breath and turned the engine over with the ignition disconnected until I was satisfied that the gearbox wouldn’t immediately implode or fall out. With a second deep breath, I reconnected the coil and started the engine. The V8 burst into burbling life and no awful noises came from the gearbox, so leaving the car warming up, I walked away to make a cup of tea and wait for my hands to stop shaking.

Returning, I sat in the car and stared apprehensively at the selector. The car was parked nose out of the garage, so with a third deep breath I selected D. The engine note changed abruptly and the car strained forward against the brakes. Oh! I slowly released the brakes and… Brown Rover moved forward! I cannot overstate my excitement at this moment. I had completely stripped an automatic gearbox, modified it, reassembled it and… the car MOVED! A few metres out of the garage, I applied the brakes and selected R, and… Brown Rover moved backwards! Astonishing! Back in the garage, I stopped the engine and grovelled underneath, checking carefully for leaks and other signs of catastrophe, then sat down to finish my cup of tea and bask smugly in my own brilliance.

That afternoon Brown Rover and I made numerous short excursions around the block as we progressively tested the gearbox, my confidence growing with each circuit. The kickdown cable needed several adjustments before I was happy with the shift points, and the selector cable required more tinkering. The Dakota box had to be calibrated, which was easily accomplished using a GPS to confirm road speed. These problems solved, Brown Rover and I were ready for the big time! On a quiet semi-rural road not far from home, we built up some speed.

Dakota Digital transducer installed under dash

Dakota Digital transducer installed under dash

Now came my first disappointment… Fourth gear failed to appear! Yes, fourth gear, the entire point of this whole project, was MIA! My stomach churned. Solving this problem proved difficult: suffice it to say that the valve body was removed and replaced several times before it was resolved. Immediately afterwards I discovered that the torque converter would not lock up. In both instances my saviour was once again Andy, providing remote guidance and, eventually, the solution. I am unable to adequately express my enormous gratitude to this selfless member of the wonderful worldwide P6 community.

Brown Rover was also required to pass New Zealand’s strict compliance test for modified vehicles in order to become road legal once more. As far as the NZ authorities are concerned, you can install any gearbox at all: they don’t care. However, if you modify or replace the original cross-member, or reshape your car’s structure to accommodate your new gearbox, the whole installation is subject to inspection. As a builder and repairer of race cars, John knows his stuff, so his cross-member passed with flying colours. I wasn’t so pleased with the enormous fee, but such is bureaucracy, and Brown Rover soon sported an official low-volume certification plate.

There were further niggles to sort. For example, on the first decent outing, carrying passengers and a picnic, it became apparent that the P6’s standard piggy-tail radiator cooler wasn’t up to the task of cooling the ZF, so I fitted a good quality aftermarket cooler. Soon, however, these problems were all behind me and it remained only to drive and enjoy the car.

Low-volume certification plate showing approved new cross-member

Low-volume certification plate showing approved new cross-member

And what is it like? Well, in a word, transformed. Even before that wonderful upshift into fourth gear, it is clear that the ZF is much smoother and more responsive than the BW35. There is no slush. It shifts quickly and smoothly up and down as conditions dictate. With the kickdown cable adjusted to taste I can control upshifts and downshifts in virtually manual fashion by just feathering the throttle. The ZF is a delightful box, and I am thrilled with it.

However, the real improvement is on the open road. The V8 now spins at a lazy 2100 rpm at 100 kph. Overtaking, a gentle prod of the loud pedal unlocks the torque converter. A little more pressure prompts a downshift to third, and that delicious V8 growl fills the cabin as the car winds up in what, with a ratio of 1:1, used to be top gear. Soon we’re back in fourth, then the torque converter locks up, and the engine noise returns to a relaxed throb. When accelerating, I admit that I get a little a thrill every time the box shifts to fourth, and another as the torque converter locks up and the revs subside further. The novelty still hasn’t worn off!

Regarding fuel consumption, throughout my early road testing the car was suffering from carburettor issues which ruined my attempts to get good comparative mileage measurements. The full effect of the new box on consumption will be ascertained after an upcoming carburettor rebuild. However, with the BW, I’d generally achieve mileage in the low 20s on the open road, with a long flat run edging things up toward the mid 20s if I was careful. With the ZF, I’ve recorded 26-27 mpg in mixed driving. I expect that with properly sorted carburettors, the far side of 30 mpg might be achieved. It is a useful improvement, but the principal aim of the conversion was to transform the car’s refinement and driveability, and that aim has been completely realised.

Brown Rover has, without any loss of character or its essential P6B-ness and with no irreversible changes, taken a giant leap forward. The P6, Rover’s finest moment, is already a thoroughly useable classic car, with superb qualities belying the half-century since its introduction. Brown Rover in particular is now, at 42 years of age but with a modern gearbox, a truly effective open-road cruiser, capable of carrying me and my family long distances up and down the country in total comfort. I look forward to many more years and many more miles behind the wheel.

Finally, Brown Rover’s ZF conversion would not have been possible without the invaluable assistance of the following:

Andy Morgan (“Eightofthem” on the Classic Rover Forum) at 02920866645 / 07941361146 and awm3@sky.com can supply help, complete transmissions, parts and upgrades as can other ZF specialists.

Dakota Digital Inc., suppliers of the ECD-100 electronic speedometer cable drive and SEN-4165 magnetic pickup used in Warren’s project: 4510 W. 61st St. N.Sioux Falls, South Dakota 57107; 1-800-593-4160 and www.dakotadigital.com

John Weir, Canary Automotive Limited, 23 Heathcote Street, Woolston, Christchurch 8023, New Zealand. Phone +64 3 384 2860, or email canary.automotive@xtra.co.nz. John will build a copy of Warren’s custom rear cross-member and ship to anywhere in the world.

Clive Goldthorp

Clive claims that his interest in the BMC>MG story dates back to his childhood in the 1960s when the family’s garage premises were leased to a tenant with an Austin agency. However, back in the 1920s and 1930s, his grandmother was one of the country’s first female Garage Proprietors so cars probably run in his genes! Admits to affairs with Alfa Romeos, but has more recently owned an 06/06 MG TF 135 and then a 15/64 MG3 Style… Clive, who was AROnline’s News Editor for nearly four years, stood down from that role in order to devote more time to various Motor Racing projects but still contributes articles on as regular basis as his other commitments permit.

12 Comments on "Your Cars : P6B ZF gearbox transplant – Part Two"

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  1. Big John says:

    Stunning work. I’m sure more regularly used P6’s will benefit from such an elegant upgrade.

  2. Andy says:

    Good write up. Sounds like a huge amount of work for a single extra ratio, but high 20s mpg from a big 1970s carburetted V8 saloon is amazing.

    In ten years time you can have a go at fitting the new 8-speed ZF transmission.

    • WarrenL says:

      Ah, but that one extra ratio (and the torque converter lockup, which is almost like a fifth gear), has made such a big difference to the car. Definitely worth all the work. A year later and it still puts a big smile on my face every time I drive it.

  3. Richard Davies says:

    Good to hear you managed to get it all working without too many headaches, considering in needed a selection of different parts to be compatable.

  4. kennyg says:

    Just to put things in perspective, I owned a 56reg Fiat Croma with the 150bhp twin cam diesel and Aisin 6 speed transmission and torque converter lock up. After all that and driven gently it only managed 33mpg overall and possibly 36mpg in summer. Torque converters eh!

  5. Bilbo says:

    This is the kind of modification I respect- it doesn’t adversely affect the character of the car, but makes it much more useable in daily traffic. I wouldn’t be surprised if more P6s get treated to this new gearbox.

    • WarrenL says:

      That, Bilbo, was exactly what I was aiming for. I should also point out that it is entirely reversible. The only “modification” made to the car itself was a bit of meat trimmed off the transmission tunnel brackets to allow the ZF to swing into place, but not enough to affect the refitting of a BW35. Of course now that I’m used to the ZF, there’s no chance of that ever happening in my lifetime.

  6. Jemma says:

    Given its been so successful it might be an idea to get someone like Rimmers interested in doing a full kit to do a conversion. That does seem a pretty huge gearbox, but then I suppose it had more space for RWD, the 4 speed Renault AD8 was about half the length
    This touches on something I have thought for ages – even more so after getting the replacement for the Safrane.. auto gearboxes seem to have improved but manuals are going the other way – the ratios are abysmal on alot of cars and what I wouldnt give for a Sceptre manual/OD box in the Accent – poor little thing churns its guts out at motorway speeds with the 4 1/2 speed crunchy monstrosity.
    Ditto engines – even fitting an EFI kit would improve power and MPG and in the case of the P6 theres bits floating around that bolt straight in – what price environmentalism then?

    • WarrenL says:

      Photos might be deceptive, Jemma. It’s not that big. Interestingly, from what I’ve seen of attempts by others, the fitting of EFI Rover V8s doesn’t do much, if anything, for the economy and power of a P6. The most effective 4.6 conversion I know of uses the original SU carburettors, rejetted for the exra capacity.

      • Stewart says:

        The fitting of EFi should if done correctly produced a marked improvement, it did when they fitted it to the range rover! although you do need the EFI heads and pistons

  7. Mike C says:

    This article would benefit greatly from links to or transcripts of the forum technical discussions that helped complete this project.

  8. Owen says:

    I would be interested in approximate costs of fitting the ZF and approximate time involved. Many Thanks. Owen.

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