In this second instalment, we examine the XJ6’s technical package.
Sanctioned in 1964, XJ4 was intended to launch in 1967, which seems in hindsight to have been a rather optimistic timescale. The project team would be led by Bob Knight, Jaguar’s senior development engineer and one of the finest conceptual minds of his era. The Browns Lane engineering department at the time was something of a collection of minor fiefdoms, most of whom worked somewhat independently, as Jim Randle discovered to his amazement when he joined the project team from Rover in 1965. [As related to this author].
“When I came to Jaguar, I couldn’t see how they made a car, there was no bloody programme at all! It gradually dawned on me – you got people who were around the company – people like Tom Jones, Cyril Crouch and whatnot. They took their bit of the car; they didn’t talk to anybody, they’d just get on with it. And they often fell out with the guy who was doing the next bit! But you didn’t need anyone producing a programme.”
From his overseeing promontory, engineering director Bill Heynes did not envisage anything particularly radical, XJ4 being more about refining a successful formula. So while the double wishbone front suspension closely resembled that of existing models, it was rubber-mounted upon a box-section subframe and would embody an anti-dive geometry which not only reduced weight-transfer upon braking by 50%, but allowed softer road springs to be used. The Burman power-assisted steering was by rack and pinion – a first for a Jaguar saloon.
At the rear, Bob Knight (in conjunction with Tom Jones) oversaw the design of a new twin-link suspension design, mounted directly to the body and employing the driveshafts as the upper links as before. However, this new design employed a torque tube arrangement for the drive-line, with the aim of channelling stresses through the strongest section of the floorpan. The new system was lighter, and displayed a noted improvement in handling and secondary ride characteristics; a weakness of the older E-Type-derived unit, which had been rather hurriedly created in 1957.
Designed from the outset to utilise the all-alloy V12 engine, then in development, the only version of the in-line XK six intended for the car was a 3.0 litre version, developing a net 185 bhp and believed to have been particularly sweet-running. Additionally, a 60° 3.5 litre V8, to be derived from the twelve cylinder tooling was also being investigated.
With Knight tasked with ensuring the new car exceeded Jaguar’s already impressive standards, Jaguar’s resident chassis guru employed everything he had learned about NVH suppression, employing the car’s major masses – its suspensions, axles, engine and gearbox as attenuators, to cancel out unwanted noise and vibrations.
A double-skinned bulkhead further insulated the passenger compartment, while painstaking attention to suspension bushes and damper settings saw new standards in ride isolation and handling being set, despite the pioneering adoption of purpose-designed 70-series low profile Dunlop 205/15 radial tyres, which appeared colossal at the time. Also in the quest for quietness, large quantities of soundproofing material were fitted throughout the cabin.
However, problems soon beset XJ4 development. The most pressing being a resonance of the torque tube, which led to a good deal of head-scratching, as Jim Randle explained. “The original XJ6 system was totally dependent on the angle of the drive link. That system [the alternative Bob Knight design] had a forward-facing tube with a mounting into the drum. The firing order put this on to resonance, and we had a boom”.
Complicating matters further was not only the multiplicity of projects being handled simultaneously, but also the fact that Jaguar’s engineering resource, although of a very high order, was woefully understaffed for the tasks at hand. Owing to the fact that the XJ body was almost as low slung as that of an E-type, XJ4 proved something of a packaging miracle. But that didn’t mean the job of incorporating all that heavy and space-taking hardware was anything but a massive headache for Jaguar’s overstretched development team.
By 1967, with the programme now well behind schedule, the resonant boom being generated by the experimental torque tube suspension system became a potentially ruinous drag on engineering resources, defying all attempts at a cure. With vast amounts of development time lost, Knight abandoned further development.
This entailed, not only the late adoption of the existing rear suspension and final drive unit into the XJ4 bodyshell, but a hasty redesign of the underbody by Pressed Steel, who somewhat fortuitously were prepared to accommodate this last-minute alteration within a now very tight timescale. A further complication being that a good deal of proving and refinement work would need to be reprised.
Lyons was furious, but there was nothing for it but to defer introduction until the Autumn of 1968. A further setback came with repeated delays to the V12 engine. In early 1968, with launch material and sales brochures already in print, it became evident that the V12 would not be ready for the September launch. In fact, delays would continue to bedevil this engine programme, ultimately deferred until 1972.
An unintended knock-on was the lack of a larger engined variant. Already the 3.0 litre XK unit had been subsumed into a 2.8 litre version, intended to dip below European tax brackets, so the venerable, if rather compromised 4.2 litre XK unit from the 420 saloon was press ganged as a temporary stop-gap. But while the installation proved relatively trouble-free, the taller power-unit wouldn’t clear the XJ’s low bonnet.
To Lyons’ intense dismay, his exacting bonnet line was sacrificed and a central bulge added. Sir William reportedly detested the addition, expressing his surprise to a subordinate during the model’s launch that nobody had noticed. Ironically, this feature, not to mention the 4.2 litre unit itself, not only lasted the duration of the XJ6’s career, but would become over time an essential component of Jaguar stylistic iconography.
©Driven to write. All rights reserved.