The Jaguar XK is an inline 6-cylinder dual overhead camshaft (DOHC) engine produced by Jaguar Cars between 1949 and 1992. Introduced as a 3.4-litre, it earned fame on both the road and track, being produced in five hemispherical head displacements between 2.4 and 4.2-litres for Jaguar passenger cars, with other sizes being made by Jaguar and privateers for racing. A de-rated version was also used in certain military vehicles built by Alvis and Daimler.
Early development Prior to World War II, SS Cars (which changed its name to Jaguar in 1945) used three engines produced by the Standard Motor Company: a 1.5-litre 4-cylinder and 6-cylinder engines of 2.5- and 3.5-litres. Sir William Lyons and his engine designers; William Heynes (Chief Engineer), Walter Hassan, Claude Baily, and Harry Weslake 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,"Sir William Lyons - The Official Biography" by Philip Porter & Paul Skilleter, Haynes Publishing, during German World War II bombing raids. Rather than developing prototype engines immediately after the war, it is claimed that Jaguar's wartime engine developments went far beyond mere discussion and design, extending to the construction and testing of several prototype engines as early as 1943.Daniels, Jeff. Jaguar - The Engineering Story, Haynes,
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. 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 1,732 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.
4-cylinder engine development progressed as follows: XG Pushrod engine 73 x 106 x 4 1776 cc May to Nov 1944 XF 75 x 98 x 4 1732 cc Nov 1944 to Jun 1945 XK1 (first of Heynes’ 4 x XJs) 76.25 x 98 x 4 1790 cc Oct 1945 to Nov 1946 XK2 76.25 x 98 x 4 1790 cc Feb to Sep 1946 XK3 76.25 x 98 x 4 1790 cc Dec 1946 to Feb 1947 XK4 76.25 x 98 x 4 1790 cc Nov 1946 to Dec 1947 Gardner Engine (used in record-breaking MG car) 1970 cc 1948 XK Number 1 3-bearing crank 1970 cc 1949–1952 XK Number 2 3-bearing crank 1970 cc 1950–1952 XK 5-bearing crank 1970 cc 1953
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 3,442 cc to form the "XK 6-cylinder", which saw its debut in an open two-seat XK120 sports car at the 1948 London Motor Show. Following this the XK powered the Jaguar Mark VII and a number of other 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 XK 6-cylinder units. At the time of William Heynes' paper presented to the IMechE in February 1953,Heynes, W M. The Jaguar Engine, a paper presented to the Institution of Mechanical Engineers on 27 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 but Lt. Col. Goldie Gardner's speed record team did fit a 1970 cc version to the MG streamliner EX-135 in 1948 to take the 2,000 cc class record at ,"British Racing Green - Drivers, Cars and Triumphs of British Motor Racing" by David Venables, Ian Allan Publishing, 2008, on the Jabbeke motorway in Belgium.
There are some misleading claims of an intervening "XJ" 4-cylinder 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. Heynes stated there were many 4-cylinder variants following the XF but it was he alone who loosely grouped them as XJ. The last mention of XF was in July 1945 and the first mention of XK (XK1) was in October of the same year. This doesn't give much room for a series of XJ engines.There are no mentions of XJ in the archive (other than in Heynes’ paper). If there is a XJ, the first one is likely to have been referred to as XK1 internally. There were three others of nominally 1790 cc capacity called XK2, XK3 & XK4. It is likely these are what Heynes referred to as "XJ". The first "true" XK was called "XK Number 1" (distinct from "XK1") and was of 1970 cc nominal capacity with an 83mm/91mm bore/stroke.
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.
Production The castings for the XK block were manufactured for Jaguar by Leyland Motors. 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.
Description
Valve train The most recognisable feature of the XK engine is the dual cam covers atop the engine, which were a polished alloy until a change to ribbed black and alloy finish in early 1968, four years or so after the introduction of the 4.2-litre versions in 1964. 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 satisfy these two conditions, a relatively wide angle between the valves was initially chosen, with quite long valve stems. To efficiently operate valves whose tops were such a long way apart, the dual overhead cam arrangement was found to be the most effective.
The cam lobes act directly on bucket type tappets, which are adjusted by means of shims between the underside of the bucket and the top of the valves. Two duplex chains drive the camshafts, one from the crankshaft to a sprocket at the level of the combustion chambers, and one from the sprocket to each camshaft. The original sprung slipper design of the lower cam chain tensioner proved to be too weak and after a couple of years was changed to an improved hydraulic design, using engine oil pressure.Simister, John. Legendary Car Engines, Motorbooks,
Cylinder head 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 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. The same basic cylinder head layout was preserved throughout the production life of the engine but with many detail changes. Valve and port sizes and angles, camshaft lift, compression ratio and carburetion were frequently amended from model to model, depending upon whether power or torque was being emphasised.Thorley, Nigel. Jaguar - All the Cars, Haynes, Very early XK engines fitted to the XK 120 sports car and Mark VII saloon lacked fixing studs at the front part of the cam covers, where they covered the timing chains, which leaked oil as a result. From 1951 onwards, all XK engines had studs around the full perimeter of the cam covers.
The earliest cylinder heads are known as "Standard" or "A type" heads and are identifiable by their differing valve angles of 30 degrees inlet and 45 degrees exhaust, as well as by their unpainted aluminium finish.Porter, Philip and Isaac, Rowan. Original Jaguar XK, The Restorer's Guide, Motorbooks International, Around 1954, the "B type" head appeared, with equal valve angles of 35 degrees on inlet and exhaust. The "B type" heads are painted light "duck egg" blue (early cars) to light green (later cars) in the 2.4 and 3.4-litre models and metallic dark blue in 3.8-litre models. A very few XK 120s and XK 140s were supplied to customers with the "C type" cylinder head, which was painted red and carried a plaque on each cam cover stating "Jaguar Type C".
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. The cylinder heads in the Series 1 E-type and Mark X were painted gold whether the engine was a 3.8-litre or 4.2-litre. Around the time of the later series 1 E-Types and 420G in 1968, about the same time as the arrival of the ribbed cam covers, the practice of painting the cylinder heads ceased. Many sources still describe the subsequent unpainted aluminium finish as "silver".
Jaguar's colour-coding of its cylinder heads of the late 1950s and early 1960s can be determined from Heiner Stertkamp's list below. The only real anomaly is that the earliest 3.8-litre E-Types had an orange painted head (because they had 3 carburettors) but later changed to gold (because they had straight-port heads). Gold paint signified a straight port triple-carburettor setup from then until the demise of the Mark X. Silver/bare aluminium => 3.4 A-type head (Standard XK120, XK140 and MkVII with two H6 carburettors) Red => 3.4-litre with C-type head and 2 carburettors (XK 120 C and XK 140SE/M only) Light duck egg blue and later light green => 3.4-litre with B-type head and 2 carburettors (may appear as light blue, light green or light blue-green) Dark blue metallic => 3.8-litre with B-type head and 2 carburettors Orange => 3.4 and 3.8-litre with straight-port head and 3 carburettors Gold => later 3.8-litre and all 4.2-litre (until 420G and Series 2 E-Type) with straight-port head and 3 carburettors
Cars with the straight-port head and only two carburettors did not have painted heads. See the complete table of XK cylinder head types as they relate to Jaguar and Daimler models at the end of this article.
Engine block