The Jaguar 420 (pronounced "four-twenty") and its Daimler Sovereign equivalent were introduced at the October 1966 London Motor Show and produced for two years as the ultimate expression of a series of "compact sporting saloons" offered by Jaguar throughout that decade, all of which shared the same wheelbase. Developed from the Jaguar S-Type, the 420 cost around £200 more than that model and effectively ended buyer interest in it, although the S-Type continued to be sold alongside the 420/Sovereign until both were supplanted by the Jaguar XJ6 late in 1968.
The 420/Sovereign traces its origins back to the Jaguar Mark 2, which was introduced in 1959 and sold through most of the 1960s. The Mark 2 had a live rear axle and was powered by the XK six-cylinder engine first used in the Jaguar XK120 of 1948. The Mark 2 was available in 2.4, 3.4 and 3.8-litre engine capacities.
In 1961 Jaguar launched two new models with the triple SU carburettor version of the 3.8-litre XK engineer and independent rear suspension: the Mark X (pronounced "mark ten") saloon and the E-Type sports car. Both cars used versions of the Jaguar independent rear suspension system, the Mark X having a 58-inch (1,500 mm) track and the E-Type a 50-inch (1,300 mm) track. In 1965 the Mark X and E-Type were updated with a new 4.2-litre version of the XK engine, still using triple carburettors.
In 1963 Jaguar introduced the Jaguar S-Type as a development of the Mark 2. It used a new intermediate-width, 54-inch (1,400 mm) version of the independent rear suspension in place of the live rear axle of the Mark 2. Other differences from the Mark 2 were extended rear bodywork to provide for a larger boot, a changed roofline for more rear seat passenger headroom, a slightly plusher interior and detail differences around the nose. The S-Type was available with either 3.4 or 3.8-litre XK engines (only 3.8-litre in USA) but in twin-carburettor form because the triple-carburettor setup would not readily fit into what was essentially still the Mark 2 engine bay.
James Taylor suggests four reasons why Jaguar boss Sir William Lyons might have decided to add yet another model to an already extensive Jaguar range:
Consequently, Lyons initiated development of a new saloon based on the S-Type, retaining its 54-inch independent rear suspension but adding a twin-carburettor version of the 4.2-litre powerplant and frontal styling more akin to that of the Mark X. The new car was released in August 1966 in the form of two badge-engineered models, the Jaguar 420 and the Daimler Sovereign equivalent.
The starting point for design of the 420/Sovereign was the Jaguar S-Type, which had been in production since 1963 but whose styling had never met with universal acceptance.
In styling terms, the 420/Sovereign was essentially an S-Type with that car's curvaceous nose made much more linear, the better to match its rear styling (which was not altered). Contouring around its four lamps was relatively subtle, with small peaks over each, and its flat frontage sloped forward slightly. The square grille with central divider matched that of the 420G, (which was the new name given to the Mark X at the time of the 420/Sovereign's release). The low-set fog lamps of the Mark 2 and S-Type were replaced by a pair of inner headlamps at the same level as the main headlamps. The inner lamps were lit on main beam only. Dummy horn grilles were added below each inner headlamp to break up what would otherwise have been a large expanse of flat metal on either side of the radiator grille. The tops of the front wheel arches were flattened to match the squarer lines of the nose. The slimline bumpers dispensed with the centre dip which had characterised the bumpers of the Mark 2 and S-Type. All this was done to improve the car's aesthetic balance compared with the S-Type and to create a family resemblance to the Mark X/420G, changes which Sir William could not afford (in either time or money) when the S-Type was designed. No attempt was made to give the 420/Sovereign the same front-hinged bonnet as the Mark X/420G and it retained a rear-hinged bonnet of similar dimensions to those of the S-Type and Mark 2.
Changes to the S-Type's interior to create the 420/Sovereign were driven mainly by safety considerations, with the wood cappings on the doors and dashboard replaced with padded Rexine and a wooden garnish rail on the tops of the door linings. The clock was relocated from the tachometer to the centre of the dashboard top rail, where it was powered by its own battery. The S-Type's pull out map tray below the central instrument panel was not carried over although the 420 retained the same central console and under-dash parcel tray. The seats of the 420 were of slightly different proportions from the S-Type, although they appeared very similar.
The 4.2-litre XK engine of the 420/Sovereign was fitted with the straight port cylinder head and 3/8-inch lift cams. Compression ratios of 7:1, 8:1 and 9:1 could be specified according to local fuel quality, the difference being obtained by varying the crown design of the pistons. The engine was fed by just two carburettors and developed a claimed 245 bhp (183 kW; 248 PS) gross at 5,500 rpm, which was 20 bhp (15 kW; 20 PS) less than the triple-carburettor version in the 420G and E-Type. The maximum torque of the engine at 283 lb⋅ft (384 N⋅m) was virtually the same as that of the triple-carburettor version yet was achieved at 3,750 rpm rather than 4,000 rpm.
The factory-quoted horsepower rating of 245 bhp (183 kW; 248 PS) was measured using the SAE (gross) system current in the USA at the time the 420/Sovereign was sold there. The SAE (gross) system excluded many accessory drives and often used non-standard induction and exhaust systems and so was replaced by the more accurate SAE (net) system in 1972, long after the 420/Sovereign had gone out of production. Reference states that the DIN horsepower rating of the 1977 USA specification 4.2-litre Series II XJ6 was equivalent to 180 bhp. The DIN system yields horsepower ratings which, for most technical purposes, are the same as those that would be obtained using the SAE (net) system. However, the 1977 test would have included power-sapping emissions equipment not present on the 420/Sovereign. Therefore, the SAE (net) power rating of the 420/Sovereign must have lain somewhere between 180 bhp and 245 bhp.
A novel mechanical feature that the 420/Sovereign shared with the 420G was Marles Varamatic power steering, which was offered as an option on the 420 but was standard on the Sovereign. Built by Adwest Engineering Co Ltd of Reading, England, it was a "cam and roller" system in which the non-constant pitch of the cam resulted in a variable steering ratio, with the lowest gearing being at the straight ahead, rising rapidly to either lock. The rise in gearing (equivalent to a drop in ratio from 21.6:1 to 13:1) occurred almost entirely within the first half turn of the steering wheel from the straight-ahead position. The effect was to give very light and relaxed steering at the straight ahead, with quick reaction when cornering. There was no adjustment in the behaviour of the steering in reaction to road speed. A very few of the last S-Types were similarly equipped.
Other mechanical refinements the 420/Sovereign had over the S-Type included:
Contemporary road tests indicate that the performance of the 420 and Sovereign was very highly thought of.
A Motor (UK) road test in May 1967 reported:
A North American perspective was provided by Road & Track, whose December 1967 report concluded:
A road test by Wheels (Australia) in August 1967 enthused:
In terms of performance measured under test conditions, 0–60 mph in under 10 seconds and a top speed of more than 125 mph (201 km/h) were typical. Such performance figures were superior to those of many of the 4.2-litre XJ6 models that followed. Among the few exceptions the testers took was to its 15–16mpg average fuel consumption, which even for the late 1960s was rather high. Combined with the modest size of its two 7 Imperial gallon (31.75 litre) fuel tanks, such fuel consumption gave the model a touring range of only around 250 miles (403 km).
Whereas the Daimler 2½-litre V8 released in 1962 differed from the Jaguar Mark 2 in having a genuine Daimler engine, only the Sovereign's badging and aspects of interior trim differentiated it from the 420.
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