Heavy metal on the racetrack
What an interesting piece by Martin Buckley (Backfire, February) on the 1970s XJC. I helped Ralph Broad run Broadspeed for the duration of the Jaguar ETCC contract, and the firm did have experience of cars ‘heavier than Minis or 800kg Escorts’ – for some years it successfully prepared Vince Woodman’s 1100kg 3-litre Capri, and the BSCC Dolomite Sprints. It had also previously done work on the BMW CSL, subsequently copied by its ETCC rivals.
Heavier weight was not a limiting factor. The suspension failures that caused the XJC’S demise were inevitable but also avoidable. The suspension was derived from the 1960s E-type: 1050kg cornering at 0.8g maximum – a bit different from 1700kg, plus fuel, at 1.3g. The front stub-axle deflected enough to stuff the pads back into the calipers, needing vigorous pedal pumping to restore braking. That problem had to be fixed, so Broadspeed designed new uprights with large-diameter stiff rotating stub-axles.
At the rear the problem was not immediately obvious, but gross deflection of the small-diameter driven hubshafts made fatigue failure inevitable. Yet BL was not prepared to fund a redesign. Bob Tullius’ XJ-S had no such problems, so why did we? Maybe because the American XJ-S had narrow rims to meet more restrictive regs, didn’t corner at 1.3g and was far lighter…
The 5.3-litre unit made 580bhp when fully bedded in; Jaguar was said to have had trouble getting the standard V12 to produce more than 450bhp, which is why Harry Mundy was sceptical. The reason was our redesigned cylinder heads, with a combustion chamber in the head rather than the standard flat head.
Getting 550-580bhp provided interesting challenges. Any block will distort when you torque down the head/mains, but when it’s aluminium – rather than nearly three times as stiff cast-iron – and a long, complex block with open deck for wet liners, the problem multiplies. It needed delicate preparation: torque the head and main-bearing caps, hone the bearings into line, then fixedhone the bores back to cylindrical. Problem solved, helped by Vandervell kindly supplying thicker steel shells around its VP2 metal. And the bigend bolts had to be torqued higher than standard to avoid their coming loose through the higher inertia loads at well over 8000rpm.
Lubrication was also a challenge. The stock V12 had a large-diameter gerotor oil pump around the front of the crank – a neat design. Sadly, rotor pumps absorb more power than gear pumps – in the V12’s case, about 20bhp at 8000rpm according to Bob Knight. And the huge-diameter mains needed high pressure to keep them afloat at any revs, as well as giving higher friction losses. The answer: replace the oil pump with an external belt-driven one (from Cosworth).
That was the era when synthetic oils became available from the major companies. Unfortunately, the BL PR department arranged a contract with a firm whose oils weren’t so good. After an expensive test-bed failure, the company agreed to let us put the oil of our choice into its cans to avoid red faces. The other problem was oil surge because of high-g braking and a long sump. A small cyclone air separator in the pressure line helped mitigate this.
I don’t remember stripped gears in the transmission, and the brakes worked fine. With open-top calipers and a water spray when the brakes were applied (using a brake-light switch), the temperature came down about 300ºc. A caravan water pump supplied a spray nozzle in the eye of the disc, and evaporation did the cooling. An elegant solution.
I also don’t recollect the need for ‘different compound tyres on each side.’ Or that tyre wear was a problem. In hindsight, had Jaguar consented to our rear end redesign the programme need not have been terminated. Easy to be wise after the event, but what interesting times they were!
Colin Mynott
Crick