Jaguar XK6 Engine - Early Development

Early Development

Prior to World War II, SS Cars (as Jaguar was then known) used engines produced by the Standard Motor Company; a 1.5-litre 4-cylinder and two 6-cylinder engines of 2.5 and 3.5 litres. Sir William Lyons and his engine designers; William Heynes (Chief Engineer), Walter Hassan and Claude Baily, are widely reported to have discussed a new range of engines to replace the pre-war Standard based units whilst fire-watching on the roof of the SS factory in Swallow Road, Coventry, during World War II and to have developed prototype engines immediately after the war. In fact, Jeff Daniels has demonstrated that Jaguar’s wartime engine developments went far beyond mere discussion and design, extending to the construction and testing of several prototype engines from as early as 1943.

The initial aim was to produce a series of engines of higher than normal output that would be able to stay ahead of the competition without revision for many years and which Sir William insisted also had to "look good". In 1942-43, a range of configurations was considered and it was concluded that, for good breathing and high bmep, the new engines would need vee-opposed valves operating in hemispherical combustion chambers. Two configurations of this type were selected for comparison in 1943 and the prototypes named "XG" and "XF". The XG 4-cylinder of 1,776 cc, first tested in October 1943, was based on the 1.5-litre Standard block and used its single cam-in-block to operate the opposed valves via a complicated crossover pushrod arrangement, similar to that of the pre-war BMW 328. The XF 4-cylinder of 1,360 cc used the now familiar dual overhead cam (DOHC) configuration and was first tested in November 1944. The XG was found to suffer from excessive pushrod and rocker noise and gas flow figures through its vertical valve ports did not equal those of the horizontal ports on the XF. Therefore, from these two options, the DOHC XF layout was selected and by October 1945 a larger DOHC 4-cylinder of 1,996 cc called the "XJ" began prototype testing and underwent many changes as its head, ports, valve gear and camshaft drives were perfected.

By September 1947 a 3.2-litre 6-cylinder version had been produced, called the "XJ 6-cylinder", which was intended to replace both Standard-based 6-cylinder units. Testing showed the need for higher torque at low speeds than this engine could produce and hence it was 'stroked' to form the "XK 6-cylinder" which, with its initial capacity of 3,442 cc, was settled upon for production in 1947-48. This engine first powered the Jaguar XK120, followed by the Jaguar Mark VII and a number of other Jaguar models in subsequent years.

The XG prototype soldiered on as a component testbed until 1948. There also existed an "XK 4-cylinder" of 1,790 cc, also first tested in October 1945 and remaining under development alongside the XJ and XK 6-cylinder units. At the time of William Heynes' paper presented to the IMechE in February 1953, the XK 4-cylinder was still referred to as being under development. It was only finally dropped as a possible production engine later in 1953, by which time it had been realised that Jaguar's image in the market had moved beyond the need for a replacement for the old 1.5-litre Standard 4-cylinder unit.

Because the 6-cylinder XK prototypes were found to be so much more refined than the 4-cylinder versions, in 1951 a 1,986 cc 6-cylinder version of the XK 6-cylinder was built to see if it would suffice as a smaller scale engine. By 1954 this had grown to 2,483 cc and it was this short-block version of the XK 6-cylinder that was fitted to the new compact Jaguar 2.4-litre (retrospectively known as the Mark 1) released in that year. None of the 4-cylinder prototypes ever advanced to production.

In his book, Jeff Daniels claims he could not find confirmation among the Jaguar archives that the 1,996 cc version of the 4-cylinder was called "XJ" but William Haynes' paper presented to the IMechE in 1953 makes it clear that this was indeed the name given to that prototype. The same paper does not mention the short-block XK 6-cylinder which by then was well into its development for the 2.4-litre of 1954. Instead, the paper refers somewhat misleadingly to both the 1,996 cc XJ 4-cylinder (clearly as an experimental engine) and the XK 4-cylinder, the latter being referred to in guarded terms, not declaring the engine's capacity and stating ″at the present full details cannot be given as the engine has not yet been released to the market″.

Thus were developed the two block sizes that formed the basis of all subsequent XK 6-cylinder engines; the shorter block being used for the 2.4 and 2.8-litre engines and the full sized block for both versions of the 3.4, the 3.8 and the 4.2-litre engines.