XK120, XK140, and XK150

XK120, XK140, XK150: The Sports Car That Made Jaguar

When William Lyons unveiled the Jaguar XK120 at the Earls Court Motor Show on 27 October 1948, it was not supposed to be the main event. The car on Jaguar's stand that mattered to the accountants was the big, ponderous Mark V saloon, and the low, voluptuous two-seater sitting beside it, bodied in aluminium over an ash-and-steel frame because volume tooling for steel panels was still unfinished, had been built chiefly to show off a new engine and to draw a crowd to the saloon. The new engine was the twin-overhead-camshaft XK straight-six, a unit Jaguar's engineers had refined through the war years, sketching an engine for a country whose factories still had blackout curtains. The roadster's name announced its ambition: XK120, for a claimed 120 mph top speed, then the fastest of any standard production car. Crowds at Earls Court ordered it, in numbers Jaguar had not planned for, and a car conceived as a low-volume flagship became, almost by accident, the company's defining product for the next thirteen years.

Jaguar had to prove the number. In May 1949 the company took a specially prepared XK120 roadster to a closed stretch of dual-carriageway autoroute between Jabbeke and Aeltre in Belgium. With the windscreen, hood and side-screens removed, an aluminium undertray fitted, and a small deflector shielding the driver, test driver Ron Sutton drove the car through a timed flying mile, recorded by the Royal Automobile Club of Belgium at 132.596 mph. The run was not a production-specification result, but it did what it was designed to do: it converted a number on a badge into a number confirmed by an outside timekeeping body, within months of the car's debut.

Demand forced Jaguar's hand on construction. Alloy-bodied cars built through 1949 and into 1950 gave way to pressed-steel bodywork as Jaguar brought the XK120 into volume production at its Foleshill works in Coventry. A fixed-head coupe joined the open two-seater for 1951; a drophead coupe followed in 1953. Across its run to 1954, the roadster remained the dominant seller, particularly in left-hand-drive form for the American market that would come to define Jaguar's commercial fortunes through the 1950s.

The same XK engine powering the road car was doing more dangerous work at the same time. Jaguar's competition department built the C-type for 1951, a lightweight tubular-framed racer over the XK's running gear, and won Le Mans outright at its first attempt; a second win followed in 1953, aided by disc brakes, then a rarity on any racing car. The D-type that followed pushed the same basic six further and delivered three consecutive Le Mans victories in 1955, 1956 and 1957. None of this changed a customer's road-going XK120, XK140 or XK150 into a race car, but the connection was not incidental marketing: the same twin-cam six underwrote both the showroom car's reputation and the racing team's silverware.

The XK140, introduced in October 1954, was a car of careful improvement. Rack-and-pinion steering replaced the cam-and-lever setup; telescopic dampers replaced lever-arm type; a busier grille and larger bumpers answered American taste. The engine now came in higher-output 'Special Equipment' tune as the effective standard, with a 210bhp 'C-type head' option. All three body styles continued, with the fixed-head's cabin stretched slightly for occasional rear passengers and luggage.

The XK150, arriving in May 1957, changed the car's silhouette more than either predecessor: a higher, wider wing line, a single curved windscreen, and a broader dashboard. The consequential change was mechanical: Dunlop disc brakes were fitted at all four corners as standard for the first time in the range, technology proven at Le Mans now standard on a car a private buyer could order from a dealer. The open two-seater arrived later, in March 1958, and from 1959 a 3.8-litre engine option, including a high-output 'S' tune quoted around 265bhp, gave the range genuine 130-plus mph performance to match its stopping power. When production ended in 1961, the XK150 was directly replaced by the E-type, which used a developed version of the same XK six. Between 1948 and 1961, the XK120/140/150 family had taken Jaguar from a coachbuilder's aluminium show special to a company whose sports cars were bought, raced, and admired on both sides of the Atlantic.

A note on production figures: body-style and total production counts come from later marque-history and registry compilations, chiefly Wikipedia's XK120 and XK140 articles and Supercars.net's XK150 guide.

Wartime Engineering: The Firewatchers' Six

Behind the XK engine's reputation sits a specific engineering team, not an anonymous drafting office. William Heynes had joined Jaguar's forerunner SS Cars in 1934 and was named to the company's main board as Technical Director and Chief Engineer in 1946, a post he held for the next twenty-three years. That made him the person most directly responsible for turning William Lyons's ambitions into working machinery across the XK120, XK140 and XK150, and eventually the E-type. Heynes assembled a small team to design a new postwar engine: production engineer Walter Hassan and Claude Baily worked the detailed design under Heynes's direction, with combustion specialist Harry Weslake brought in as an outside consultant on cylinder-head and porting work.

Jaguar's own oral history holds that the team sketched the outline of the twin-overhead-camshaft head that became the XK engine while on fire-watching duty on the roof of the Swallow Road works in Coventry during German bombing raids. The detail is repeated often enough across Jaguar histories that it has become part of the company's own founding mythology, though, as with any decades-old recollection, it rests on the participants' own memory rather than a contemporaneous record. What is not in question is the result: an engine strong enough to carry a road car to a genuine 120 mph and, in racing tune, to win Le Mans outright five times across the 1950s under the same basic design.

Lyons's Eye, and a Second Record at Jabbeke

The Jabbeke run described earlier in this account was not the XK120's only visit to that stretch of Belgian autoroute. On 20 October 1953, Jaguar returned with a further-modified car and a new driver: works test and development engineer Norman Dewis, who had joined the company the previous year. Fitted with a distinctive, glider-derived, bubble-shaped canopy over the cockpit and other streamlining well beyond the 1949 car's simple undertray and deflector, the car reached 172.412 mph. That was a faster confirmed figure than Ron Sutton's 132.596 mph four years earlier, achieved with a body no showroom customer could ever have ordered, but it was proof that Jaguar kept returning to the same public demonstration whenever it wanted an outside body to confirm a number.

Dewis went on to become one of the most important, if least publicly visible, figures in Jaguar's history. Across a 33-year career with the company he was the principal development driver behind the Le Mans-winning C-type and D-type, the XK140 and XK150 that followed the XK120, and later the E-type and the mid-engined XJ13 prototype; he is estimated to have logged more than a million test miles at an average speed above 100mph. His job sat at the point where William Heynes's engineering decisions and William Lyons's styling had to survive contact with an actual road, at speed, under one pair of hands.

A Jaguar for the Screen Idols

William Lyons styled the XK120's body himself, turning his own sketches into the finished aluminium shape shown at Earls Court within a few weeks, working by eye and proportion rather than through any formal design training or, at this stage of Jaguar's history, aerodynamic calculation. The result found an audience well beyond Britain's motoring press. Clark Gable became the model's most famous private owner, eventually acquiring four XK120s; one was reportedly a gift from Indianapolis Motor Speedway owner Tony Hulman while Gable was filming in the United States, and another carried him across Europe on an extended road trip before being shipped back to California and registered there. Humphrey Bogart, Gary Cooper, Tyrone Power, Lauren Bacall, Robert Mitchum and Rex Harrison were also reported owners, giving the XK120 a foothold in Hollywood that no earlier British sports car had managed, and lending its 120mph claim a glamour the bare number alone could never have supplied.