The Jaguar XJ is a series of full-size luxury cars produced by British automobile manufacturer Jaguar Cars (becoming Jaguar Land Rover in 2013) since 1968 across four basic platform generations (debuting in 1968, 1986, 2003 and 2009) with various updated derivatives of each. Since 1970 they have been Jaguar's flagship four door model. The original model was the last Jaguar saloon to have had the input of Sir William Lyons, the company's founder, and the model has been featured in countless media and high-profile appearances. The current XJ (X351) was launched in 2009. It is one of the cars used by the British royal family and an armoured version is used for transporting the UK Prime Minister.
The original first-generation of the XJ was produced for a total period of 24 years, with two major facelifts in 1973 and 1979. Retrospectively, these are often known as "Series" XJs among the Jaguar enthusiast community.
The XJ6, using the 2.8-litre (2,792 cc (170.4 cu in)) and 4.2-litre (4,235 cc (258.4 cu in)) straight-six cylinder versions of Jaguar's renowned XK engine, replaced most of Jaguar's saloons – which, in the 1960s, had expanded to four separate ranges. Apart from the engines, other main assemblies carried over from previous models were the widest version of Jaguar's IRS unit from the Mark X and the subframe mounted independent front suspension first seen in the 1955 Mark 1 with new anti-dive geometry.
An upmarket version was marketed under the Daimler brand as the Daimler Sovereign, continuing the name from the Daimler version of the Jaguar 420.
The car was introduced in September 1968. Power-assisted steering and leather upholstery were standard on the 2.8 L De Luxe and 4.2 L models and air conditioning was offered as an optional extra on the 4.2 L. Daimler versions which were launched in October 1969, in a series of television advertisements featuring Sir William. In these advertisements, he referred to the car as "the finest Jaguar ever". An unusual feature, inherited from the Mark X and S-Type saloons, was the provision of twin fuel tanks, positioned on each side of the boot / trunk, and filled using two separately lockable filler caps: one on the top of each wing above the rear wheel arches. Preliminary reviews of the car were favourable, noting the effective brakes and good ride quality.
In March 1970, it was announced that the Borg-Warner Model 8 automatic transmission, which the XJ6 had featured since 1968, would be replaced on the 4.2-litre XJ6 with a Borg-Warner Model 12 unit. The new transmission now had three different forward positions accessed via the selector lever, which effectively enabled performance oriented drivers to hold lower ratios at higher revs to achieve better acceleration. "Greatly improved shift quality" was also claimed for the new system.
Around this time, minor changes were made as well, such as moving the rear reflectors from beside to below the rear lights; on the interior the chrome gauge bezels were changed for black ones, to cut down on distracting reflections.
In 1972, the option of a long-wheelbase version, providing a 4 inch increase in leg room for passengers on the rear seats, became available.
The XJ12 version was announced in July 1972, featuring simplified grille treatment, and powered by a 5.3 L V12 engine (coupled to a Borg Warner Model 12 transmission). The car as presented at that time was the world's only mass-produced 12-cylinder four-door car, and, with a top speed of "around 140 mph" (225 km/h) as the "fastest full four-seater available in the world today". Although it had been the manufacturer's intention from launch that the XJ would take the twelve-cylinder engine, its installation was nonetheless a tight fit, and providing adequate cooling had evidently been a challenge for the engineers designing the installation. Bonnet/hood louvres such as those fitted on the recently introduced twelve-cylinder E Type were rejected, but the XJ12 featured a complex "cross-flow" radiator divided into two separated horizontal sections and supported with coolant feeder tanks at each end: the engine fan was geared to rotate at 1¼ times the speed of the engine rpm, subject to a limiter which cut in at a (fan) speed of 1,700 rpm. The fuel system incorporated a relief valve that returned fuel to the tank when pressure in the leads to the carburetters exceeded 1.5 psi to reduce the risk of vapour locks occurring at the engine's high operating temperature, while the car's battery, unusually, benefited from its own thermostatically controlled cooling fan.
3,235 units of the first generation XJ12s were built. A badge-engineered version, the Daimler Double-Six, was introduced in 1972, reviving the Daimler model name of 1926–1938