Sir William Lyons and his engine designers; William Heynes (Chief Engineer), Walter Hassan, Claude Baily, and Harry Weslake[1] are widely reported to have discussed a new range of replacements whilst fire-watching on the roof of the SS factory in Swallow Road, Coventry,[2] during German World War II bombing raids.

[3] 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 brake mean effective pressure (BMEP), the new engines would need vee-opposed valves operating in hemispherical combustion chambers.

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.

4-cyl engine development progressed as follows: 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.

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.

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.

There are some misleading claims of an intervening "XJ" 4-cyl prototype but it seems the only person who referred to them as such was William Heynes in a paper presented to the IMechE in 1953.

In 1968, Jaguar laid off 2,500 workers at both Radford and Brown Lane due to strike by Leyland workers over pay parity after the merger of Leyland with British Motor Holdings, which Jaguar estimated had cost them a loss of 160 cars a day at a cost of £320,000.

The unusual depth of the engine's cylinder head was dictated by the desire to make room for two generously-sized valves, whilst not excessively restricting the flow of gases into and out of the hemispherical combustion chambers.

To efficiently operate valves whose tops were such a long way apart, the dual overhead cam arrangement was found to be the most effective.

[7] The cylinder heads were made of RR50 aluminium alloy due to that material's high rate of heat conduction and light weight, the latter estimated by William Heynes to give a weight saving of some 70 lb (32 kg) compared with a similar head made of cast iron.

Special attention was paid to the gas flow, with Harry Weslake and Heynes designing a curved inlet port to impart swirl to the air-fuel mixture for improved combustion.

A supposedly more efficient "straight port" cylinder head, again with equal 35 degree angles on the inlet and exhaust was introduced later in the 1950s and this was painted pumpkin orange on the XK-150S.

Jaguar's colour-coding of its cylinder heads of the late 1950s and early 1960s can be determined from Heiner Stertkamp's[citation needed] list below.

See the complete table of XK cylinder head types as they relate to Jaguar and Daimler models at the end of this article.

[citation needed] Variables such as compression ratio, cam lifts and durations, and fuel consumption rate for each engine during testing would, at a minimum, be additionally required to develop any estimated comparisons.

The original 3.4-litre XK was used in the following road cars: In the mid-1950s, Jaguar lacked a compact saloon of the type represented until 1949 by the Standard-engined 1½ litre.

The Jaguar 240 was fitted with an uprated version of the engine, incorporating the straight-port cylinder head and twin SU carburettors delivering 133 bhp SAE gross.

For example, in 1965 the 4.2-litre with triple SU HD8 carburetors fitted to the US-specification E-Type was rated at 265 hp (269 PS; 198 kW) @ 5400 rpm SAE gross and a maximum torque of 384 N⋅m (283 lb⋅ft) @ 4000 rpm [13][14][15][16] whereas by 1969, now with twin Zenith-Stromberg carburetors, this had fallen to 246 hp (183 kW) SAE gross.

[17] The difference indicates the effect of the revised carburetion and other changes made to reduce exhaust emissions from US specification engines.

Jeff Daniels' reference states that the similar twin Zenith-Stromberg version fitted to a US specification XJ6 was rated by Jaguar at 170 PS (125 kW; 168 hp) using the DIN net horsepower system, the difference being largely due to the effect of the gross versus net horsepower rating systems.

After the arrival of the ribbed cam covers in 1967 the heads normally remained unpainted, which some sources describe as "silver".

The power output of the 2.8 was listed as 142 PS (104 kW; 140 hp) DIN net, which cannot be directly compared with the 133 bhp SAE gross of the last of the 2.4s in the 240 Mark 2.

This was later cured by a switch to stronger pistons but the engine was also a mediocre performer offering no real economy benefit and it was dropped in 1973, to be replaced by the ‘new’ 3.4.

All of the 2.8-litre XK engines used the straight port head unpainted, which some sources may describe as "silver", and had ribbed cam covers.

It used a straight port head and delivered 161 PS (118 kW; 159 hp) DIN net and far superior torque to the 2.8.