V12 Engine 1971 – 1997

Although Jaguar had experimented with a V12 design during the early 1950s, it was not until 1971 that the engine became a production reality.  

Quad-cam V12 engine
Note the XK engine cylinder heads and cam covers

With the XK engine in production at Browns Lane from 1951, the engineering department commenced a development programme for the next generation of power units.

In the early 1950s, although the USA had long been an advocate for V8 power, Jaguar did not consider the V8 favourably.  Notably, Walter (Wally) Hassan, who had been part of the XK design team, said: “The bank-to-bank firing sequence [of a V8] imposes severe limitations upon the achievement of an efficient induction system if carburettors are used.  A V8 needs a two-plane crankshaft with wide outer crankshaft balance weights to eliminate its out-of-balance couple.”  That said, Claude Baily (also one of the designers of the XK engine) and Hassan had both investigated V8 configurations but nothing came of this and the V12 was the next logical step.

In 1950, Hassan moved to Coventry Climax leaving Jaguar Chief Engineer, William (Bill) Heynes and Claude Baily to continue work on the V12.

Initial Thoughts

A 1952 list of Jaguar Works Orders for experimental projects includes ZX/ZP505 for an ‘XK 4.9-litre V12 engine’.  Having the excellent XK engine already in production meant the logical approach was to take two engines and mount them in a 60° ‘V’ on a common crankcase and share a single crankshaft.  Though the bore capacity would remain the same, as the crankshaft would be newly designed the most suitable stroke could be chosen by the engineers.  Claude Baily started work on such an engine, taking an original XK 3.4-litre bore capacity of 83 mm and selecting a stroke of 75 mm to yield an engine capacity of 4,870 cc, which kept the unit small enough to fit under the bonnet of the existing Jaguar Mark V and Mark VII saloons.  This was encouraging and Heynes asked Baily to make drawings of the proposed engine, with a view to producing an example for testing.  However, it soon became apparent that among the drawbacks encountered of the proposal was the XK’s double overhead camshaft (DOHC) arrangement which made the engine very wide.  Space was lacking between the ‘Vee’ for an induction system so downdraught inlet ports from outboard-mounted SU carburettors (three per cylinder bank) were adopted; the exhaust emerged from beneath the carburettor bank. The DOHC configuration led the team to rethink the engine and it seemed that the only option was for a single overhead camshaft arrangement per bank of cylinders.

At the time Jaguar did not have the time to develop the engine further and it was sidelined for work to commence on a military 9.25-litre V8 for the Ministry of Supply.  This was intended to power tanks and tank transporters.  Heynes had wanted to develop a V12 race engine but, with limited funds and with Jaguar’s production and sales increasing, his team had to turn to other more pressing issues and the V12 programme was put to one side but not forgotten.  Besides, by this time Jaguar had won Le Mans more than once with the C-type, and its successor the D-type was under construction with the proven XK engine being suitably modified, consequently there was no urgent requirement for another power unit.

It would appear that all through the 1950s and the early 1960s the V12 engine was never out of sight with Heynes and Baily constantly discussing and making notes about how it should proceed.  Certainly, drawings of a V12 engine were in existence in the late 1950s.

When Jaguar purchased Coventry-Climax in 1963, Hassan returned to the experimental department and, together with Baily, was tasked to further develop the V12 engine that already existed on paper.  Harry Mundy, who had designed the Flat 16 engine for Coventry-Climax, also joined the small team.  The plan for a race engine – particularly for Le Mans – and a second version for road-going Jaguar sports and saloon cars was revived and work proceeded with a V12.

Running Engines

When Hassan returned to Jaguar he brought with him some of the engine technology that had been successful for Coventry-Climax in Formula One racing with Jack Brabham, who became World Champion in 1959 and 1960.  Hassan was not happy with the four-cam five-litre V12 engine that had been designed in his absence; it was too heavy, large and complex and would not have fitted the car (XJ) for which it had been intended.  As it stood, the V12 would not have fitted the E-Type either and it would have been too costly for quantity production.  Heynes was still keen to proceed with the unit but asked Hassan and Mundy to work on an alternative with a twin-cam design.  However, the four-cam unit was completed and ready to be tested; two versions of the engine were proposed, one with four SU carburettors for road cars and a second with Lucas mechanical fuel injection for racing.

The first V12 engine, with fuel injection and down-draught inlet pipes, was run on a test bed in August 1964.  Phil Weaver, who ran the Experimental Shop and oversaw the Competition car build, made some calculations to make sure the bulky engine would fit under a Mark X’s bonnet for testing, and later in 1964 the unit was fitted to Mark X saloon No 7.  The ‘12-cylinder and 8-cylinder engine range’ programme was given the Jaguar factory code ‘XJ6’ but later the code was transferred to what had been the XJ4 saloon.

Norman Dewis remembered testing the car at the MIRA track and being surprised at reaching 136 mph without effort and still accelerating.  “Given the right gearing and a flat piece of road I think it would have exceeded 150 mph.”   Three cars were fitted with examples of the five-litre V12 and were tested extensively at MIRA, on the nearby M1 motorway (this was before the 70 mph limit) and also on Welsh mountain roads.

Jaguar XJ13 (with Norman Dewis)
Now on display in the Jaguar section at the British Motor Museum

These trials were vital for the Le Mans prototype car which was officially sanctioned in 1964 and intended to compete in 1965, but it would be a year before building of the car, now coded XJ13, would begin.  An ‘Instruction to Proceed No. ZX 558/04/1’  was issued by Jaguar Development Engineer R J (Bob) Knight on 3 June 1965 for ‘XJ13 Car’. This ‘Project Specification’ called for: ‘Build one prototype competition car for development purposes.’

With an 87 mm bore and 70 mm stroke for a capacity of 4,991 cc, the V12 delivered 502 bhp at 7,600 rpm and after the XJ13 was completed in March 1966 the engine was fired up outside the experimental department.  However, it now looked unlikely that XJ13 would compete at Le Mans (see below).  Sir William did not want time and money wasted with a major test programme but did allow limited trials to be carried out on Sundays.  It is worth noting that it had been just three years from initial concept to an engine that powered XJ13 to a record lap of 160 mph of the high-speed track at MIRA in 1966.

However, the Jaguar-BMC merger in July 1966 further complicated the saga of XJ13 and the V12 engine.  Le Mans looked even more unlikely when the Ford GT won the race that year; then the race rules were changed to a return of a maximum capacity of 3.0-litres for prototypes.  That left Jaguar and the XJ13 with no race to enter but they still had valuable data and an engine design that warranted further development.

Improving the Engine

There was a great deal of work to be done on the engine to make it an efficient road-going version, and with the XJ13 (and its engine) consigned to store, that work could proceed.  Hassan and Mundy were concerned that the chain drive to the four camshafts was complex and noisy, also they were worried about long-term reliability.  Such a unit would be acceptable in a race car but not in a conventional saloon, especially as the XJ6, then in development, was intended to be announced in 1968 powered by the V12.  Another area of concern was the cylinder head layout; several alternatives were investigated by Hassan and Mundy and a ‘Heron’ head was singled out as most suitable.

In a Heron head the valves sit upright, in line in a completely flat head, the combustion chamber being formed in the piston crown.  There were draw-backs to the Heron head, which restricted valve diameter and gas flow but on a production saloon these did not matter, as the restrictions would not affect torque output for normal motoring.  As the V12 was intended to fit into the XJ6 engine bay, this meant that the single cam heads would result in a more compact unit when compared with the twin cam alternative.  Gerry Beddoes, Engines Development Engineer, who had carried out some of the drawing work for the original V12 in 1952, now produced comparison drawings of the two engines superimposed on each other to demonstrate just how space would be saved and the all-important fact that the V12 would fit into the engine compartment as a replacement for the XK engine without the need to redesign the XJ6 or the engine bay.  Gone too was the complicated three-chain drive of the four cam engine which could be replaced with a single, albeit long, chain following a more conventional path.

Heynes still favoured the four-cam engine design but was willing to allow his team to explore the alternative which would be subjected to a head-to-head trial in due course.  Mundy, Hassan and Baily now worked together to refine the design.  To this end simple single-cylinder test engines were used to evaluate and prove design alternatives.  Mundy later wrote: ‘Single-cylinder work proceeded on the twin-cam head in two forms.  On the original design, changes were made to port sizes and form with little effect on performance.  A more compact cylinder head form was achieved by flattening the sides of the hemispherical chamber parallel with the port axis to increase squish and promote more combustion turbulence.’  There were problems with a reduction in maximum power though performance was improved in the low speed range.  ‘The next problem was finding the right type of combustion chamber.  There was much in favour of the so-called wedge-type chamber and our associated company Coventry Climax had experience of it.  They had recently developed a ‘flat head’ design strictly comparable with a wedge head.  The results were superior and design studies were initiated for a single-cylinder programme using the flat-head design.’

V12 Cylinder Heads Compared

Various single-cylinder flat-heads were built and tested for angle of entry for the inlet port and the sparking plug position.  It was found that the more vertical the port on a flat-head, the better the performance as the fuel/air mixture discharged into the cylinder head.  Following these lengthy tests ‘…it was decided to build a full-scale engine based on the results with the flat cylinder head and the results followed very closely the trends observed in the single-cylinder work.’   The racing engine was extensively redesigned and the cylinder heads were replaced with a more conventional two-valve design, employing a single overhead camshaft (SOHC) acting directly on vertically inclined valves through bucket tappets.  These changes greatly reduced complexity, weight, size and noise.

The revised head design had restrictive and long inlet ports which sacrificed top-end power but which, along with an increase in displacement to 5.3-litres, greatly improved performance at low-mid-engine speeds, which was desirable in a Jaguar luxury car.  The chain-driven SOHC heads and the soft valve springs fitted to reduce valvetrain noise resulted in the redline being lowered to 6,500 rpm from the 8,000 rpm of the original four-cam design.  Once the team were free of the five-litre race engine they could increase the bore to 90 mm and make the big-end bearings wider.  As Walter Hassan later recalled: “We established right at the beginning at 90 mm bore and 70 mm stroke in contrast to the 87 mm bore used on the racing engine.  We decided on the 90 mm bore primarily to increase the torque at the bottom end and to provide for bigger valves should they be required in the future.”

When a prototype of the new engine had been built it was fitted to one of the department’s Mark Xs and tested alongside the four-cam-equipped Mark X.  It proved much smoother and more efficient and was obviously the engine that should be developed for production cars.  What should be remembered is that both DOHC and SOHC engines were being developed in parallel before tests confirmed the SOHC as the way forward.  The trials were also useful in monitoring the emission requirements for the US Federal and Canadian standards, some of which were already in place and others, more stringent, to be introduced in the future.  Jaguar adopted a system with air being injected into each exhaust port close to the back of the valve heads to burn off unburnt gas particles.

There is an interesting 19 minute video on YouTube of Raymond Baxter interviewing Walter Hassan
and Harry Mundy about the V12 engine.

Reaching Production

Jaguar wanted to make use of fuel injection for the V12 but there was no working British system available at the time.  Four proposals were considered in 1966 from Tecalemit, AE Electronics (Brico), Schebler (in the USA) and Lucas but they were all ‘under development’ and the only working fuel injection system ready to use came from Bosch in Germany.  Trials continued and in 1968 AE-Brico supplied units for tests; results were promising and it looked likely that Jaguar would proceed with the installation of AE-Brico fuel injection on the V12.

Jaguar V12 engine fitted with four Strombergs

For the tests of the AE-Brico and also of Zenith-Stromberg carburettors one of the Mark Xs was converted by the experimental department and given the internal Jaguar number XJ10.  However, by 1970 when the engine was due to enter production at the Jaguar Radford factory, AE-Brico took the decision not to continue with their fuel injection system.

This caused problems for Jaguar who were ready to build the V12 in quantity, they had no alternative and installed four Zenith-Stromberg 175 CDSE carburettors in place of fuel injection.  At a Model Progress Meeting on 5 February 1971 Mr England stated that the XJ12 [saloon] would ‘…go into production with carburettors’.

At a cost of nearly £3 million a new machine shop was equipped at the Radford factory, in Coventry, to manufacture the V12.

The assembly line for the V12 was quite separate from the existing XK engine production line.  New machines from Archdale, Huller, Weatherley, Cincinnati and others were installed; the Huller transfer equipment for cylinder heads cost £700,000 alone.

While the engine building work was going on so too was the testing of the V12.  Several units were running at Radford on dedicated engine test beds to complement the continuing road trials.  Engine assembly took place on a 52-stage mechanised track alongside the machine shop.  On completion of an engine it was bench-tested at Radford before being transported by road to Browns Lane for installation on the assembly line.

The Jaguar V12 engine had become a reality and there was nothing else like it, anywhere in the motoring world.  Jaguar took a bold and brave step with the design and build of a V12 for quantity production.  The original idea was for the V12 to power the 1968 XJ6 saloon but in May 1968 it was recorded at a Directors’ Meeting that the engine would not be ready for the launch of the XJ6 in September.  By 1971, with the V12 in production, Jaguar had no new car to introduce their remarkable engine.

V12 Engine Goes On Sale

1971 V12 engined E-Type Series 3 WHP 205J
used as the Press Car

From the foregoing, it would seem that once the news that the V12 would not be ready for the XJ6 there would have been a concentrated effort to design or find a Jaguar that would benefit from the V12.  However, by this stage (1970) Jaguar did not have the freedom it once had to produce a model at short notice.  Unfortunately, there was no new car ready for the V12, so a plan was devised to adapt the existing E-Type 2+2.  Consequently the existing longer wheelbase E-Type 2+2 was revised to take the new engine.

After he had retired from Jaguar, Lofty England said: “The V12 was never supposed to go into the E-Type in the first place, but because the future project, the XJ-S, was some way off, it got into the E-Type.  The E-Type was never designed for the V12, it was good luck that we managed to get it in.”

Mr England made it sound an easy fix but in reality the entire front frame of the E-Type had to be modified to take the new engine.  When an owner lifted the bonnet of his Series 1 E-Type he was greeted with the sight of an attractive XK engine neatly housed and with easy access to all parts of the unit.  However, with the V12 engine, due to its size and complexity, the view is special when one opens the bonnet of a Series 3 E-Type.  Though the V12 was only slightly heavier than the XK the front subframe was wider and was considerably strengthened, with gusset plates at the junction of the tubes.  An additional tubular member was fitted below the engine sump.  The upper front suspension cross member was now detachable, to make installation of the V12 easier during production and, of course, necessary to allow removal of the engine for maintenance.  It would appear that this cross member was bolted in place after the engine had been fitted.

As both the front and rear tracks were wider, the wings were also wider and the wheelarches were given slight flares.  The front wheel arch was also cut slightly higher into the wing.  All this work was carried out under the guidance of Bob Knight and his department, who turned out the modifications and drawings in a very short space of time and kept re-tooling costs to a minimum.  Records are unclear about costs involved but with British Leyland breathing down Jaguar’s neck they would have been subjected to close scrutiny.

Production of the E-Type Series 3 V12 had in fact started in August 1970 (Coupé) but it was not until 29 March 1971 that the Jaguar 5.3-litre V12 engine was announced by the Company.

Much was made of the world’s first quantity production V12 engine.  Although, before World War Two, and for a few years after, Lincoln had a V12 unit on offer.  Also, in more recent times, Ferrari, Lamborghini and others had V12 engines, but these were virtually hand-built and then only in small quantities.  Further, Jaguar pioneered the use of light aluminium alloys for the cylinder block (in place of the usual cast iron), cylinder heads and pistons, all of which reduced the overall weight of the large engine, which weighed in at 680 lbs (308 kgs) complete (less gearbox).

When the engine was first revealed both Hassan and Mundy commented on future potential.  Mundy: “In this engine we have provided quite a substantial reserve for capacity increases should the need arise.  It is [also] relatively easy to derive a substantially smaller engine from the design.”  Hassan added: “We should stress that derivatives of the V12 are merely options open to us should the need arise.  We have no applications in mind at the moment.”  Naturally the engineers were asked about the XJ13 engine and Mundy commented: “Except for its 60° V angle, and the basic architecture of the assembly, the V12 engine that eventually went into production at Radford is completely different from the racing unit seen in the mid-engined XJ13.  Not only does it have single-cam cylinder heads instead of the twin-cam heads but we also enlarged it to 5,343 cc by a bore increase.”  Hassan noted: “We wanted to produce an engine that was outstanding.  The twelve-cylinder was obviously a good choice; it is technically excellent and is extremely smooth-running.”

Less than a year after the Series 3 E-Type had been launched, Jaguar introduced the XJ12 saloon to critical acclaim for its refinement and performance, but it was a thirsty beast and one magazine that tested the model noted a figure of 11.4mpg.   Carburettors gave way to a Bosch D-Jetronic fuel-injection system in 1973, that was adapted for Jaguar by Lucas and later, after further development, termed Bosch-Bendix-Lucas, but fuel consumption was not greatly enhanced.  However, it improved engine efficiency and when the XJ-S was introduced in 1975 better fuel figures were commented on by the motoring press.

Flat Head v May Fireball Head

In 1981, to boost falling sales of the V12 and to improve fuel economy, Jaguar announced a revised engine which saw the flat head replaced with a new design from Swiss racing driver and engineer Michael May.  His company, Antipollution Industrial Research SA, had used Volkswagen engines to test their engine design theories and their findings were published in 1976.  Jaguar were interested and signed a development contract with May.

May’s design had the combustion chambers formed around recessed valve housings with the pistons being flat-crowned to give a high swirl rate within the combustion chamber.  This allowed the engine to run at an unusually high compression ratio for the time (10.5:1 to 12.5:1, depending on market and year) with a relatively lean fuel mixture.  Power levels remained similar to the previous model, but fuel economy was improved by some 50 per cent.

Known as the May Fireball, the changes were introduced in the XJS HE (High Efficiency) range and 22 mpg became usual with the model.

Sectioned V12 engine
with fuel injection

There was scope to increase the power of the V12 further and the engineers, led by Harry Mundy, investigated changes which involved modifying the tooling to make the block taller, taking the stroke to 84 mm but costs were too high and it was not proceeded with.

Experimental V12 engines of 6.4 litres were made and tested in XJ12 prototypes but it wasn’t until investment was available in the 1980s that the V12 was enlarged to 5,994 cc, launched in the 6.0 litre XJR-S in 1989.

In the late 1980s Tom Walkinshaw Racing produced 7.0 and 7.4 litre versions of the engine which were used to power the XJR9 and XJR12 respectively.

The Jaguar V12 continued to power models in the range during the 1990s and with all the early problems long sorted out the V12 became one of the best engines to have been produced anywhere.

Interestingly, it was only Jaguar’s second engine to enter production in over twenty years (the XK being the first) and the final example was built in 1997.

1997 The final V12 XJ12 P60 SOV
with the last production V12 engine

The last production V12 engine was installed in the final 6.0 litre XJ6 built in 1997 – P60 SOV now on display in the Collections Centre at Gaydon.

Owners of Jaguar V12s are always reluctant to part with their cars and anyone who has driven a V12-powered model can easily understand why they enjoy this smooth and quiet engine.  Credit has to go to the exceptional team of engineers who conceived and designed the engine; a few names have been noted but there are many more who assisted Heynes and his team, all of whom must have been (and the survivors still are) immensely proud of this exceptional power unit.


Authors: François Prins and Tony Merrygold

© Text and Images – Jaguar Daimler Heritage Trust