Editing Jaguar XK engine (section)

==Description==

===Valve train===
The most recognisable feature of the XK engine is the [[DOHC|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 chamber]]s. 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 [[tappet]]s, 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.<ref>Simister, John. ''Legendary Car Engines'', Motorbooks, {{ISBN|0-7603-1941-3}}</ref>

===Cylinder head===
The cylinder heads were made of [[Hiduminium|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 {{convert|70|lb|abbr=on}} compared with a similar head made of cast iron. Special attention was paid to the gas flow, with [[Weslake|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.<ref>Thorley, Nigel. ''Jaguar - All the Cars'', Haynes, {{ISBN|978-1-84425-693-8}}</ref>  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.<ref>Porter, Philip and Isaac, Rowan. ''Original Jaguar XK, The Restorer's Guide'', Motorbooks International, {{ISBN|0-7603-1737-2}}</ref>  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{{cn|date=October 2019}} 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 <!-- "AND earliest 3.8-litre with straight-port head and 3 carburettors" The prior passage (set off by quotemarks) was moved here as redundant.  What is the difference between that left as is and that moved here with respect to 3.8-litre straight port heads with 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===
The block was made of cast iron (with the exception of some made of aluminium alloy for racing engines), with the crankcase split on the centreline of the seven-bearing crankshaft. The crankshaft was made of EN16 steel, heat treated prior to machining. An innovation was the fitting of a proprietary [[Metalastik]] vibration damper on the nose of the crankshaft to eliminate damaging crankshaft resonances. The design and materials of the bottom end of the engine hardly changed throughout its life, apart from the changes dictated by the respaced cylinders of the 4.2-litre version (also used by the 'new 3.4' litre version). The oil pump was changed after a couple of years from a relatively inefficient gear-type pump to a Hobourn-Eaton eccentric-lobe unit.

===Fuel system===
The pre-[[fuel injection|injection]] cars had either two or three [[SU carburettor|SU]], [[Weber carburetor|Weber]], [[Zenith Carburettor Company (British)|Zenith or Zenith-Stromberg carburettors]] of various sizes depending upon the model and market, although Weber carburettors were only available from the dealer or on the aftermarket, and not supplied by the factory. The exception to this was 2.4 litre cars up to the introduction of the 240, these earlier cars having twin [[Solex Carburetor|Solex]] downdraught carburettors. Some D-Type sports racing cars received fuel injection, but it did not become available on the road cars until 1978, beginning on certain US market models in order to comply with government fuel-efficiency standards. It was eventually extended across the range and to all markets.

===Power rating===
During the 1950s and 1960s Jaguar released [[Horsepower#SAE gross power|SAE gross horsepower]] figures, using the measurement system preferred by US manufacturers, as otherwise its cars would have appeared under-powered in comparison with US cars. This practice was superseded by reliance on the more realistic [[Horsepower#SAE net power|SAE net horsepower]] system around 1972, although Jaguar power ratings of that era may also be expressed using the very similar [[Horsepower#Deutsches Institut für Normung 70020 (DIN 70020)|DIN net]] rating system.  Since the conditions upon which the SAE gross figures were determined were not recorded, an objective comparison of the power outputs of the various models of XK powered Jaguar over time is impossible.

The 1965 4.2 XK fitted to the carburetted [[Jaguar E-Type|E-Type]] had a listed output of {{convert|265|hp|kW|0|abbr=on}} SAE gross; the 1986 fuel injected 4.2 XK fitted to the series 3 [[Jaguar XJ6|XJ6]] was listed at {{convert|202|hp|kW|0|abbr=on}} DIN net.{{citation needed|date=June 2018}}  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.