Jaguar XK6 engine
From 1949 to 1968
The Jaguar XK6 dual overhead camshaft (DOHC) inline 6-cylinder engine was introduced by Jaguar Cars in 1949 and manufactured through 1992. It was produced in five displacements for Jaguar passenger cars, with other sizes being made by Jaguar and privateers for racing. In contrast with contemporary BMW and Mercedes engines, the original XK engine was relatively unstressed and produced high torque thanks to its more advanced valve and head configuration.
Prior to World War II, SS Cars (which changed its name to Jaguar in 1945) used 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 and Claude Baily, 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, during German World War II bombing raids 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 1,732 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-cyl engine development progressed as follows:
- XG Pushrod engine 73 x 106 x 4 1776cc May to Nov 1944
- XF 75 x 98 x 4 1732cc Nov 1944 to Jun 1945
- XK1 (first of Haynes’ 4 x XJs) 76.25 x 98 x 4 1790cc Oct 1945 to Nov 1946
- XK2 76.25 x 98 x 4 1790cc Feb to Sep 1946
- XK3 76.25 x 98 x 4 1790cc Dec 1946 to Feb 1947
- XK4 76.25 x 98 x 4 1790cc Nov 1946 to Dec 1947
- Gardner Engine (used in record-breaking MG car) 1970cc 1948
- XK Number 1 3-bearing crank 1970cc 1949-1952
- XK Number 2 3-bearing crank 1970cc 1950-1952
- XK 5-bearing crank 1970cc 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 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 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 but Lt. Col. Goldie Gardner's speed record team did fit a 1970cc version to the MG streamliner EX-135 in 1948 to take the 2,000 cc class record at 177 mph, on the Jabbeke motorway in Belgium.
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 Haynes in a paper presented to the IMechE in 1953. Haynes stated there were many 4-cyl 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 1790cc capacity called XK2, XK3 & XK4. It is likely these are what Haynes referred to as "XJ". The first "true" XK was called "XK Number 1" (distinct from "XK1") and was of 1970cc nominal capacity with a 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.
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.
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 Bill 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 Weslakedesigning 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. 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. 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".
The logic behind Jaguar's colour-coding of its cylinder heads of the late 1950s and early 1960s can be difficult to fathom and sources often get it wrong. It can be determined from Heiner Stertkamp’s reference that the colour-coding follows the pattern listed 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
- 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.
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 Metalastikvibration 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.
The pre-injection cars had either two or three SU, Weber, Zenith or Zenith-Stromberg carburettors of various sizes depending upon the model and market. The first engines to receive fuel injection were some D-Type sports racing cars but fuel injection did not become available on the road cars until 1978. The US market was the first to receive fuel injection on certain models, with the switch away from carburettors eventually extending across the range and to all markets.
During the 1950s and 1960s Jaguar used the SAE gross horsepower measurement system used by US manufacturers, as otherwise Jaguar cars would have appeared under-powered in comparison with US cars which always quoted their horsepower using the SAE gross system. Gross horsepower was flattering since non-standard exhaust systems were allowed and most of the usual belt-driven ancillaries were removed or discounted from the horsepower calculation. This was superseded by the more realistic SAE net horsepower system around 1972, although Jaguar power ratings of that era may also be expressed using the very similar DIN net rating system. An objective comparison of the power outputs of the various models of XK6 powered Jaguar is therefore quite problematic.
The 1961 XK fitted to the E-Type had a claimed output of 265 hp SAE; the final fuel injected XK which was significantly more powerful had a measured output of 205 hp DIN.
The first production use of the XK 6-cylinder was in the 1948 Jaguar XK120, which used the 3.4 L (3,442 cc / 210 cu-in) version with an 83 mm (3.3 in) bore and 106 mm (4.2 in) stroke. It had an iron block and aluminium cylinder head with no bore liners. It had a wider gap between cylinders 3 and 4 than between the other cylinders. The 3.4 was first rated at 160 bhp (119 kW) SAE gross at a compression ratio of 8:1, rising to 210 bhp (157 kW) SAE gross with the C-type cylinder head (confusingly not the head from the C-Type as raced at Le Mans) all the way to 250 bhp (186 kW) SAE gross with the "straight port" head at a compression ratio of 9:1 as fitted to the XK150SE.
Almost as soon as the new compact Jaguar 2.4-litre (described below) was released, there was pressure on Jaguar to fit the 3.4-litre engine to it. This was duly done in February 1957, the car being known as the Jaguar 3.4-litre. The designation "Mark 1" for these cars was applied retrospectively, after the release of the Mark 2 in 1959.
The original 3.4-litre XK6 was used in the following road cars:
- Jaguar XK120- 1948-53, "Standard (or "A type")" cylinder head, bare aluminium finish, 2 × SU H6 carburettors. The XK 120 C came with the C-type head (red) as an option from 1951 to 1952 with 2 × SU H8 carburettors. In 1953, the XK 120 C switched to a triple Weber 40 DCOE carburettor setup. The XK 120 SE and M came with the C-type head as an option with 2 × SU H6 carburettors (occasionally SU H8)
- Jaguar XK140- 1954-57, "Standard (or "A type")" cylinder head, 2 × SU H6 carburettors. The XK 140 SE or M came with the C-type head as an option, painted red with 2 × SU H6 or H8 carburettors
- Jaguar XK 150 3.4- 1957-61, "B type" cylinder head, painted light blue-green, 2 × SU HD6 carburettors
- Jaguar XK 150 3.4S- 1957-61, "Straight port" cylinder head, painted orange, 3 × SU HD8 carburettors
- Jaguar Mark VII- 1950-54, "A type" then "C type" (as option from April 1953) cylinder head, unpainted aluminium, 2 × SU H6 carburettors
- Jaguar MkVIIM- 1954-57, "C type" cylinder head, 2 × SU H6 carburettors, some had high-lift cams
- Jaguar Mark VIII- 1956–1958, "B type" cylinder head, painted light blue-green, 2 × SU HD6 carburettors
- Jaguar 3.4-litre 'Mark 1'- 1957-59, "B type" cylinder head, painted light blue-green, 2 × SU HD6 carburettors
- Jaguar 3.4-litre Mark 2- 1959-67, "B type" cylinder head, painted light blue-green, 2 × SU HD6 carburettors
- Jaguar 340 Mark 2- 1967-68, "B type" cylinder head, ribbed cam covers, 2 × SU HD6 carburettors
- Jaguar S-Type 3.4- 1963-68, "B type" cylinder head painted light blue-green, ribbed cam covers, 2 × SU HD6 carburettors
In the mid-1950s, Jaguar lacked a compact saloon of the type represented until 1949 by the Standard-engined 1½ litre. In choosing a power unit for its all new compact saloon, Jaguar could choose between the 1,995 cc four-cylinder XK prototype and a downsized version of the 3.4-litre six-cylinder XK. The 4-cylinder was considered too low powered and unrefined. The 3.4 was already well "undersquare", which means that its bore was much less than its stroke, so a smaller bore version of the 3.4 was not seen as a realistic proposition. Jaguar was therefore obliged to create a short-stroke version of the 6-cylinder XK with a shorter cylinder block, reducing it in height from 11.5 in (292 mm) to 8.85 in (225 mm).
Introduced in the Jaguar 2.4-litre in 1955, the engine had a stroke of 76.5 mm (3.0 in) while retaining the bore of 83 mm (3.3 in) to give a capacity of 2,483 cc (151.5 cu in). Despite having a displacement of almost 2.5 litres, the new car was called the "Jaguar 2.4" to create an obvious separation from the old Standard 2½ litre and link it to the now familiar 3.4-litre XK engine. The 2.4s produced were rated at 112 bhp (nett) by the factory, using twin Solex downdraft carburettors.
In 1959 the engine was carried over into the new Mark 2, in which it produced 120 hp (89 kW) SAE gross, still with Solex carburettors. 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.
The 2.4-litre XK6 was used in the following cars:
- Jaguar 2.4-litre 'Mark 1'- 1955-59, "B type" cylinder head, unpainted, 2 × Solex B32PB15S carburettors, valve angles 30 degrees inlet, 45 degrees exhaust
- Jaguar 2.4-litre Mark 2- 1959-67, "B type" cylinder head, unpainted, 2 × Solex B32PB15S carburettors, valve angles 45 degrees inlet and exhaust
- Jaguar 240 Mark 2- 1967-69, "Straight port" cylinder head, unpainted, 2 × SU HD6 carburettors, valve angles 45 degrees inlet and exhaust
The 3.8-litre version was released in 1958, initially for the last of the XK150s and the Mark IX saloon. It retained the 3.4’s bore centres and 106 mm (4.2 in) stroke but was bored out to 87 mm (3.4 in) for a total displacement of (3,781 cc / 230.7 cu-in). The distance between the cylinder bores was sufficiently small that it was decided to fit dry liners to the bores. The 3.8 had a number of detail differences from the 3.4, particularly in its valve gear and carburetion. The standard 3.8 produced 220 hp (164 kW) SAE gross and up to 265 hp (198 kW) SAE gross in the XK150SE with the straight port head.
The 3.8-litre XK6 was used in the following road cars:
- Jaguar XK 150 3.8- 1958-61, "B type" cylinder head, painted dark blue metallic, 2 × SU HD6 carburettors
- Jaguar XK150 3.8S- 1958-61, "Straight port" cylinder head, painted pumpkin orange, 3 × SU HD8 carburettors
- Jaguar Mark IX- 1958-61, "B type" cylinder head, painted dark blue metallic, 2 × SU HD6 carburettors
- Jaguar Mark X 3.8- 1961-64, "Straight port" cylinder head, painted gold, 3 × SU HD8 carburettors
- Jaguar Mark 2- 1959-67, "B type" cylinder head, painted dark blue metallic, 2 × SU HD6 carburettors
- Jaguar E-Type Series 1 3.8- 1961-64, "Straight port" cylinder head, painted orange then gold, 3 × SU HD8 carburettors
- Jaguar S-Type 3.8- 1963-68, "B type" cylinder head, dark blue metallic or later with ribbed cam covers and unpainted, 2 × SU HD6 carburettors
- Panther J.72
3-litre versions of the XK engine were built from 1959 onwards for FIA sports car racing. It was also popular with small race car manufacturers such as Lister Cars who could not afford to produce their own engines. As well as endurance racing, the engine was also used in Formula Libre racing.
A 3.0-litre XK6 was used in the following road cars:
- Jaguar E2A prototype sports racer (1960)
- Jaguar E-typelightweight sports racer (1961)
- Jaguar D-type(non-works) sports racer (1959)
- Lister Carsendurance and formula racers
- Hersham and Walton Motors(HWM) endurance and formula racers
- Cooper Car Company endurance and formula racers
- John Tojeiro endurance and formula racers