Published by Cartechnical.co.uk 2011
William Lyons' SS car company made its name in the 1930s building stylish sports and saloon cars based on components supplied by the Standard company. But Lyons was keen to move on after the war with purposed-built chassis and engines. The first cars based on these new components were ready for the first post-war motor show, in London in 1948.
In-line and vee layouts of four, six, eight and 12 cylinders were considered for the new engine, each one given a two-letter code beginning with 'X' – for 'eXperimental'. Two in-line four prototypes were built, both using hemispherical combustion chambers and opposed valves to promote efficient combustion and good volumetric efficiency. The XF was an all-new 1360cc unit, with twin chain-driven overhead camshafts, while the XG was based on the old Standard engine but had valvegear similar to that used on the pre-war BMW 328. Conventional pushrods and rockers operated one bank of valves, while 'cross pushrods' over the top of the engine worked the valves on the other side of the engine. The XG worked well but the valve gear was noisy, so the twin overhead cam layout of the XF was chosen for the next development engine, the XJ, which was built in in-line four and six-cylinder versions. After testing, the four-cylinder engine was dropped and the six was enlarged by extending the stroke to 106mm, producing the definitive 3442cc XK six-cylinder engine.
Before the war engineering chief Williams Heynes had been working on independent front suspension to replace the beam axle and leaf springs that had been standard practice – and Standard practice – in the 1930s. Various layouts were tried before Heynes settled on double wishbones, the bottom wishbone being splined to a torsion bar spring that ran back to a mounting point at the bottom of the front bulkhead, the strongest part of the chassis. Deep box-section side members and a cross-shaped brace ensured that the new chassis had excellent strength and stiffness.
A saloon car based on the new chassis was scheduled for launch at the 1948 motor show – under the name Jaguar, as SS now had unfortunate Nazi connotations – but the XK engine was not yet available in sufficient numbers for a mainstream product. Instead, Lyons made a late decision to launch his new engine in a low-volume sports car, the XK120. Time was short, and the project progressed at a rapid pace: Lyons styled the car himself, taking just two weeks to turn initial sketches into a finished aluminium body.
The new saloon chassis was shortened by 18 inches to reduce the wheelbase and the cruciform bracing was swapped for a lighter box-section cross-member. The leaf-spring rear suspension and independent front suspension were carried over from the saloon, along with Burman recirculating ball steering and drum brakes all round. The first car had an XJ prototype engine but production cars were fitted with a 150bhp version of the XK six, which used a 7:1 compression ratio so that it would run reliably on the feeble post-war 'pool' petrol.
The XK120 was the star of the Earls Court motor show in 1948 thanks to its stylish and modern shape, its amazing 120mph top speed and a price of less than £1000 before tax. Jaguar had planned to hand-build all the bodies in aluminium, but received so many orders at the show that a deal was§ done with Pressed Steel to supply steel bodywork – which is cheaper as long as volumes are relatively high. While Pressed Steel tooled up Jaguar made 240 aluminium-bodied XKs: today they are rare and soughtafter cars.
Early road tests praised the high performance and supple ride but panned the dim headlights and cramped cabin. Another early problem was brake fade, exacerbated by the all-enveloping bodywork, steel disc wheels and small-diameter brake drums. The solution was to adopt disc brakes, which Jaguar used to good effect on its C-type competition car, but they would not be available on road Jaguars until the late 1950s.
In the meantime a two-seat fixed-head coupé was introduced, followed by a Special Equipment model with stiffer front springs, a thicker anti-roll bar and a claimed 181bhp thanks to high lift camshafts and a straight-through exhaust. A drop-head coupé with a fully-lined folding roof – much easier to erect than the roadster's kit-form hood – was added to the range in 1953.
The XK120 continued until 1954. Its replacement, the XK140, adopted heavy, MkVII-style bumpers with substantial over-riders, for better protection against audio parkers and the added showroom appeal of extra chrome. The headlights were upgraded and the nose mildly restyled with a one-piece grille, losing a little of its charm in the process. The engine was moved forwards in the car to liberate extra cabin space, rack-and-pinion steering was adopted and the dashboard was raised to make entry and exit easier. Though the revised car had lost a little of the XK120's grace, it sold even faster: more than 9000 found buyers in a little over two years.
The XK150 arrived in 1957, bringing a wider grille and bonnet with the front wings merged into the car's waistline to widen the cabin. There was greater luxury inside, servo-assisted Dunlop disc brakes and 190bhp (or 210bhp in big-valve Special Equipment trim). Later there was a triple-carb XK150S and the option of a 265bhp 3.8-litre engine.
Drive any of these cars today and inevitably they feel old-fashioned – they are all over 50 years old, remember – but they have an old-world charm to the way they look, an honesty about their handling and a musicality to their engine note that car makers today can still learn from. While the XK120 is the prettiest and the XK150 the fastest, many prefer the XK140's combination of sharper steering and classic looks, and they tend to be the most expensive of the three models.
The XK line put Jaguar firmly on the map as serious manufacturer of high-performance cars and continued until 1961, when they were replaced by another Lyons masterstroke – the Jaguar E-type.