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…
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.
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.
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.
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.
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.
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.
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 email@example.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 firstname.lastname@example.org. John will build a copy of Warren’s custom rear cross-member and ship to anywhere in the world.