Jaguar XK120

Jaguar XK | Legendary engines

Jaguar XK | Legendary engines

Powering everything from Le Mans racers, to tanks, serving for some 43 years from its debut in 1948, is one of the most significant internal combustion engine families not only of Jaguar’s history, but perhaps of all time. We of course refer to the Jaguar XK straight-six engine, the 75th anniversary of which will be celebrated at the 2024 Goodwood Revival.

The engine’s tenure reflects perfectly the brief set by Sir William Lyons back when development of a new engine began, as early as 1942: higher than normal output, that could stay ahead of the competition without revision for many years.

And he had the perfect team for the job, in William Heynes, Walter Hassan and Claude Bailey. As Jaguar (formerly SS) had used four- and six-cylinder Standard engines in the pre-war years, these configurations were drawn up, prototyped and tested, with both single and twin-cams investigated.

Eventually, a six with two overhead cams was found to be the best solution, thanks to the increased versatility and refinement of the configuration, as well as the added benefit of more cylinders being associated with a more premium image. For production sportscars, a longer block good for a bit of extra capacity was added to afford it more torque. And so the XK straight-six was born, debuting in 3.44-litre, 162PS (119kW) form in the 1948 XK120 and would later be seen in numerous (confusingly-named) Jaguar saloons, including the Mark VII, VIII, Mark 1 and Mark 2.

Sir Lyons might have specified ‘without revision’ but the XK’s incremental evolution was arguably underway before the 120 had made its debut at the 1948 London Motor Show.

The hunt for power was on, with power outputs leaping past 180PS. Then, using the C-type (not that C-Type) ‘red head’ for the XK120C adding higher-flow carburettors, improved porting and larger exhaust valves, for up to 213PS (157kW). As compression rose and porting improved, that rose to more than 250PS in the XK150 SE of just a few years later.

The so-called ‘short-block’ 2.4 (more like 2.5-litre thanks to being 2,483cc) powered the lower-powered Jaguar Mark 1 and Mark 2 saloons of the mid-‘50s to late-‘60s.

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It was the power and performance of the XK engine and the slippery bodies of the XK-badged sportscars that yielded the succession of top speed records, with which Sir Lyons was obsessed. That obsession culminated in Sir Norman Dewis piloting a highly modified XK120 to a 173mph speed record on 20th October 1953.

The XK engine was of course the heart of a number of legendary racing cars and powered Jaguar to some truly historic results – victories among them. Beginning with the Le Mans-bound C-Type, the XK got fruitier cams and triple carburettors for over 200PS. The C-Type took victory at Le Mans on its first go in 1951 and again in 1953, though the latter can in addition to the reliability and performance of the XK, be put down to the revolutionary disc brakes.

For 1954, Jaguar introduced the D-Type, perhaps its most famous XK-engined racing car and certainly its most successful. Using a revolutionary monocoque construction and more advanced aerodynamics, the XK was the carryover element, albeit getting improved oiling, valving, breathing and fueling, in addition to both an increased displacement to 3.8 litres for 1957 and reduced displacement (per FIA mandate) to 3.0-litres in 1958.

While the D-Type scored Jaguar its Le Mans hattrick in 1955, 1956 and 1957, it’s the 1957 result with D-Types locking out the top four, in addition to coming sixth, that’s the greatest testament to the XK engine’s sturdiness. Its wings were clipped for 1958 though, with the FIA’s 3,000cc mandate. Though racing versions of the E-Type road car of the 1960s found some success, the top-flight career of the XK ended with the D-Type. In that glorious decade of racing, the XK racked up five Le Mans wins among numerous other sportscar victories the world over.

Nevertheless, powering what Enzo Ferrari described as “the most beautiful car ever made” isn’t a bad post-racing gig, right? Yes, the XK lived on into the 1960s as the heart of Jaguar’s newer-generation sportscar, the E-Type, with 3.8-litre (as seen in the latter years of the XK150) and 4.2-litre versions in the Series 1 and Series 2. Though the V12 took over as the flagship the E-Type in the 1970s, the XK lived on in Jaguar’s saloons.

A 2.8-litre version of the short block served throughout in the late 1960s and early 1970s in the XJ6, before being replaced by the ‘new’ 3.4-litre engine, with a straight-port head and stiffer bottom end to the original.

Having already powered the Mark 2, S-Type, Mark IX and Mark X in 3.8-litre form, the 4.2 carried on in Daimlers and Jaguar XJs for the next two-and-a-bit decades, gaining in some cases modernities like Bosch fuel injection, until being replaced in the early 1990s. The last Jaguar to use the XK6 was the 1987 Jaguar XJ6, surviving in the Daimler DS420 until 1992.

Speaking of non-Jaguar applications, the XK was popular in everything from the Lister and Tojeiro racers of the late 1950s, to the FV101 Scorpion Tank. It was also used in Panther’s cars, alongside the V12.

Even the XJ6 that largely replaced the XJ began life as an XK, though revisions to the block and internals technically informed its new designation. The bottom end of the XK engine went largely unrevised, all the way from its 1948 introduction, to its final outing in 1992.

Due to demand Jaguar itself even began remanufacturing the cast-iron 3.8-litre block – as seen in the original E-Type – in 2020, with the engine supplied with no less than a 12-month warranty. Mission accomplished eh, Sir William?

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