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The XK engine – 75 years of smooth power!

29 August 2023

Jaguar’s Legendary XK engine is 75 years old and is the bedrock on which the company was built. It continues to bewitch car enthusiasts the world over with its glorious blend of power, sound and beauty.  

History will record the 20th century as the era of the great internal combustion engine. The Chevy small-block V8 or Honda’s diminutive single-cylinder Super Cub bike engine may be the most numerous, with well over 100 million examples of each being produced, but most enthusiasts can name any number of classic engines, often just by colloquial names such as Alfa’s Busso V6, The Mezger engine in various Porsche 911s (which for some reason always has a definite article…?), Chrysler Hemi, Ford Crossflow, BMC A-Series, Colombo V12 or Lotus Twin Cam.

Jaguar’s XK engine ranks among them but may well be the most versatile; can you think of another power unit that has won Le Mans outright 5 times, the Alpine Rally Coupe d’Or (for three wins) and the BriSCA F1 stock car Gold Roof, but also powered hearses, limousines, sports saloons, the commonly accepted most beautiful sports car of all time (the E-type), a plethora of successful sports racing cars by companies such as Lister, HWM and Cooper, a whole host of kit cars, various record-breaking power boats, fire engines and a range of tracked armoured vehicles which included a light tank? We can’t, but we’d be interested to know… only the Buick-derived, Rover V8 comes close to that sort of versatility.

William Lyons’ Swallow Sidecar Company, known as S.S., had grown quickly from its formation in Blackpool in 1922 and by the mid-1930s had successfully relocated to Coventry and became a car manufacturer. By the late 1930s S.S. were producing a range of rakishly handsome cars using suitably modified chassis and running gear purchased from their neighbours, the Standard Motor Company. The 1936 SS100 3.5-litre sportscar offered spectacular performance for its price of £450, mixed with the sublime styling Lyons’ cars became known for. It also proved to be a very successful rally car which raised the company’s image and demonstrated the value of motorsport as a marketing tool.

War interrupted this success but proved key to the company’s long-term growth as, while engaged in war work building aircraft components, military trailers and, of course, sidecars for military use, Lyons and his team were able to plan a range of cars and engines for peace, should it ever come. The company also moved the engine-manufacturing machinery for the 6-cylinder unit from Standard’s factory to S.S. putting in place another building block to the autonomous future Lyons envisaged. During the conflict, the company was obliged to arrange small teams of employees to carry out fire-watching in case of an aerial attack. Lyons made sure that from 6 pm on a Sunday night was covered by himself, the company’s Chief Engineer William ‘Bill’ Heynes, Walter ‘Wally’ Hassan and Claude Bailey, who’d joined SS from Morris Engines in 1940. Their discussions ranged across the whole car at times but the central theme was to work on ideas for an engine whose design criteria Lyons had laid down, which were that it should have:

–  A good smooth spread of torque across a wide rev range.
– At least 160 bhp (which was quite a lot at the time).
–  A high level of refinement, so it could be used in the luxury saloons Lyons envisaged producing.
–  A stylish external appearance so customers could open their bonnet and feel they had invested in some high-quality engineering.
– Shared dimensions with a smaller engine so both versions could be made on the same machine tools.

The company wisely changed its name from S.S. to Jaguar in 1945 to avoid any reminder of Hitler’s Waffen-SS and reinstated production of some pre-war models to play their part in the UK government’s post-war plea to ‘export or die’. While doing this, the X for experiment engine programme, planned during the war, was taking physical form. Designs XA to XE went no further than the drawing board but design number 6, the XF, was actually produced as a 4-cylinder unit of 1360cc. It had hemispherical combustion chambers and twin overhead camshafts, a specification associated with exotic racing machines produced by Alfa Romeo, which were much admired by engineers the world over. Lyons liked this, it fitted with the high-tech image he wanted Jaguar to portray and encouraged his engineers to work on this concept. The XF suffered from a lack of block strength and the XG was really a blind alley idea using a Standard block and an in-block camshaft along pre-war BMW lines, which proved unrefined. A 2-litre 4-cylinder, the XJ, using many of the XF’s principles, was then constructed and a further 6-cylinder XJ of 3182cc was promising but lacked low-range torque. A longer stroke 3442cc version of this unit, with some other refinements, was built as the first XK engine and the design was finalised; a DOHC straight-6 with an iron block, an aluminium-alloy cylinder head with valves set at 70° and the plugs situated along the centre line in the path of the incoming gases. It was an impossibly high level of specification for a mass-production engine, but Lyons was ambitious for Jaguar and it proved to be one of his many masterstrokes.

Concurrently Jaguar had been developing what would become their new large saloon car, but the new engine was ready before the car that would be launched as the Mk7 in 1950. Lyons didn’t want to lessen the impact of the new engine in the stopgap Mk5 so decided to design a sports car based on a shortened version of the Mk5’s excellent new chassis, which featured independent front suspension. This, he reasoned, would be the ideal way to trial the engine and get some publicity as well as having the benefit that sports car customers were enthusiasts so would be tolerant of any teething troubles the new engine might have.

Lyons designed the new XK120’s body very quickly, working to the deadline of the 1948 motor show, October 27th, 1948. H.R.H The Duke of Gloucester opened the show and the car-starved public, who had been queuing for some time, poured into the first post-war UK motor show. It featured such notable new cars as the Morris Minor and Austin A90 Atlantic but there was only one star; Jaguar’s fabulous bronze XK120 was impossible to get near and they were flooded by orders. It was, quite simply, the greatest motoring media frenzy ever created by a car.

With hindsight, it seems unbelievable that Jaguar really expected to make only 200 XK120s as a testbed for the new engine. Who, after all, would want the fastest sportscar money could buy, wrapped in a body so beautiful it made grown men weep with joy and which won the first races it entered? Lyons may have designed some fantastically elegant and almost flamboyant cars but he was a curious mixture of ambition, caution and parsimony; known for walking around the factory turning off lights he felt were not needed.

Jaguar’s future was set that day at Earls Court, however, the planned 4-cylinder XK100 was never produced and after constructing 242 ash frame/aluminium skinned coach-built XK120s (now much-prized collectors’ items), a steel body was tooled-up and the XK120 became an enormous hit, globally. It was the must-have car for Hollywood superstars and amateur racing drivers alike, just as the E-type would be 13 years later, using a 3.8-litre XK engine. Such was the impact of the XK120 that within days the entire world knew the company that made a car as sleek as the cat after which they were named. In a world starved of glamour by war, there were few rivals and none that could get near the Jaguar’s beauty or pace for anything like the price, which was a very reasonable £1263. A Bentley MkIV saloon cost £2997 at the time.

The engine grew to 3.8 then 4.2-litres and was also produced as a 2.8-litre but the XK powered every car that Jaguar made from 1950 until the 5.3-litre V12 E-type made its debut in 1971. Those Le Mans-winning C and D-types, which helped to create Jaguar’s image in the public’s imagination, owe their very existence to Jaguar having an engine that could be an effective race engine. It beat Ferrari and Maserati’s V12s on track, but even when a twenty-year-old design it was refined enough to be a near-silent partner in what was then the quietest car in the world, the XJ6 Se1.

As the original XJ6 gave way to the new XJ40 in 1986 the XK unit was retired from mass production, replaced by a similar but more up-to-date straight-6, but it remained available in the Daimler DS420 Limousine until 1992.

Lyons had tasked his engineers to build an engine that looked beautiful and was advanced enough to be future-proof; 43 years of production shows they more than achieved those aims.

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