Jaguar XK6 engine

The renowned Jaguar XK dual overhead camshaft (DOHC) inline 6-cylinder engine was introduced in 1949 and continued to be manufactured until 1992. It was produced in five displacements for Jaguar passenger cars, with other sizes being made for racing both by Jaguar and privateers.

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 Jaguar factory 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 of the 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. Confusingly, 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 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 I) 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 by then well into its development for the Mark I 2.4-litre of 1954. Readers may draw their own conclusions as to why the paper does mention both the 1,996 cc XJ 4-cylinder (clearly an experimental engine) and the XK 4-cylinder (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″).

Description
Visually the most recognisable aspect of the XK engine is the dual cam covers atop the engine, which were polished alloy until a change to ribbed black and alloy finish soon after the arrival of the 4.2 litre versions in 1966. 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 of 70 degrees between the valves was chosen with quite long valve stems. To efficiently operate valves whose tops were such a long way apart, the dual overhead cam arrangement was ideal.

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. Three 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 cam chain tensioner proved to be too weak and after a couple of years was changed to hydraulic operation.

The cylinder heads were made of 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 Weslake 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, camshaft lift, compression ratio and carburetion were frequently amended from model to model, depending upon whether power or torque were being emphasised. A straight port head was introduced later for use with the SU carburettors.

The block was made of cast iron (with the exception of some made of alloy for racing engines), with the crankcase split on the centreline of the seven-bearing crankshaft. The crankshaft was made of EN.16 steel, heat treated prior to machining. A new innovation was the fitting of a proprietary Metalastik vibration damper on the nose of the crankshaft to ensure there would be no damaging crankshaft resonances. The design and materials of the bottom of the engine hardly changed throughout its life, apart from the respaced cylinders required by 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 first engines to receive fuel injection were some D-Type sports racing cars but fuel injection did not become standard on the road cars until the late 1970s.

Readers should note that during the 1950s and 60s 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. Gross power was flattering as 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 DIN rating system. An objective comparison of the power outputs of the various models of XK6 powered Jaguar is therefore quite problematic.

3.4
The first production use of the XK 6-cylinder was in the 1949 Jaguar XK120, which used the 3.4 L (3,442 cc / 210 cuin) 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) gross at a compression ratio of 8:1, rising to210 bhp (157 kW) 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) 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 (described below) was released, there was pressure on Jaguar to fit the 3.4 litre engine to it. This was duly done in 1957, the car being known as the Jaguar 3.4. The designation “Mark I” for these cars was applied retrospectively, after the release of the Mark II in 1958.

The original 3.4 litre XK6 was used in the following cars:
 * Jaguar XK120
 * Jaguar XK140
 * Jaguar XK150
 * Jaguar XKSS
 * Jaguar Mark VII
 * Jaguar Mark VIII
 * Jaguar Mark 1
 * Jaguar Mark 2
 * Jaguar 340
 * Jaguar S-Type (original)
 * Jaguar C-Type

2.4
In the mid-1950s, Jaguar lacked a compact saloon of the type represented until 1949 by the Standard engine 1-1/2 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 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 2-1/2 litre and link it to the now familiar 3.4 litre XK engine. The first 2.4s produced 112 hp (84 kW) gross using twin Solex downdraft carburettors.

In 1959 the engine was carried over into the new Mark II, in which it produced 120 hp (89 kW) 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.

The 2.4 litre XK6 was used in the following cars:
 * Jaguar 2.4 Mark I
 * Jaguar 2.4 and 240 Mark II

3.8
The 3.8 L 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 cuin). The distance between the cylinder bores was sufficiently small that it was decided to fit dry liners to the cylinder 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) gross and up to 265 hp (198 kW) gross in the XK150SE with the straight port head.

The 3.8 litre XK6 was used in the following cars:
 * Jaguar XK150
 * Jaguar Mark IX
 * Jaguar Mark X
 * Jaguar Mark 2
 * Jaguar E-Type
 * Jaguar S-Type (original)
 * Jaguar D-Type
 * Panther J72

4.2
The 4.2 litre version was officially released in 1965, although racers had been boring the 3.8 out all the way to 4.2 litres for years. The factory 4.2 litre block was an all new Siamesed sleeve design which used respaced cylinders set evenly along the length of the block, new main bearing spacings and a new crankshaft. The middle two cylinders were moved closer together and the outer two moved further apart but the overall length of the block remained unchanged. This enabled the cylinder bores to be taken out to 92.07 mm (3.6 in) whilst retaining the familiar long stroke of 106 mm (4.2 in) for a total displacement of 4,235cc / 258.4cuin.

Despite the re-spacing of the cylinder bores, the cylinder head castings remained unchanged. As a result, the centrelines of the combustion chambers in the head no longer aligned precisely with the centrelines of the cylinders but this did not matter greatly as the combustion chamber diameters in the head were smaller than the cylinder diameters. The 3.8 and 4.2 cylinder heads and cams are therefore interchangeable. The 4.2 litre engine was poorly finished from the factory with the heads not being flowed and the gaskets too small for the gap. These engines therefore make excellent bases for some modifications achieving approx. 220 bhp (164 kW; 223 PS) and much more torque just by flowing the head.

The version of the engine fitted with Zenith-Stromberg carburettors produced 170 PS (125 kW; 168 hp), whereas the triple SU HD8 carburettor version was rated at 265 hp (198 kW) gross (the difference in the measurement systems being significant).

This form of the XK Engine has been praised by many and notably Gunnar Heinrich: "Big, noisy and inefficient; those engines were great", although in contrast to BMW and Mercedes  engines of the same generation the XK engine was relatively unstressed and produced high torque thanks to its more advanced valve and head configuration.

The 1987 Jaguar XJ6 was the last Jaguar car fitted with the XK6 engine. In 1992 the last Daimler DS420 Limousine was produced. Like all other DS420 limousines, it was equipped with a 4.2L XK6 engine. The 4.2 variant was also used in some military vehicles with relatively few modifications. The length of time the XK engine has remained in production means it has the rare accolade of having been in service in cars spanning 6 consecutive decades.

The 4.2 litre XK6 was used in the following cars:
 * Jaguar Mark X
 * Jaguar 420
 * Daimler Sovereign
 * Jaguar 420G
 * Jaguar E-Type
 * Jaguar XJ6
 * Daimler DS420 Limousine
 * Panther J72
 * Panther De Ville

It was also used in the following military vehicles:
 * FV101 Scorpion, a British CVR(T)
 * FV107 Scimitar, a British Armoured car.

2.8
A 2.8 litre version was introduced for the entry-level XJ6 models in 1968. It returned to the old short block of the 2.4, retaining the bore of 83 mm (3.3 in) but lengthening the stroke by 9.5 mm (0.4 in) to 86 mm (3.4 in) to give a displacement of 2,792cc / 170.4cuin. The power output of the 2.8 was listed as 142 PS (104 kW; 140 hp), which cannot be directly compared with the 133bhp (gross) of the last of the 2.4s in the 240 Mark II.

The 2.8 gained a bad reputation early in its career due to a tendency for excessive carbon deposits to accumulate on the piston crowns during low-speed running, which would burn through the crowns when run at higher revs. 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.

The 2.8 litre XK6 was used in the following cars:
 * XJ6 Series I
 * XJ6 Series II
 * Daimler Sovereign

"New 3.4"
Introduced in 1973 to replace the 2.8 as the entry level XJ6, the new 3.4 was not a simple revival of the original XK. Whilst it shared the old 3.4’s bore and stroke of 83 mm (3.3 in) and 106 mm (4.2 in) respectively, it was actually a small bore derivative of the 4.2, using its block, cylinder spacings, main bearing spacings and stiffer bottom end but dispensing with the cylinder liners used by the 3.8 and 4.2. It used a straight port head and delivered 161 PS (118 kW; 159 hp) and far superior torque to the 2.8.

The "New 3.4" litre XK6 was used in the following cars:
 * XJ6 Series II
 * XJ6 Series III