The idea of a V12 engine had been toyed with by Jaguar since as far back as the '50s. Indeed, one or two schemes were drawn up though these were for uses other than automotive. In February, 1951 the Ministry of Supply sent Jaguar a proposed preliminary specification for an 8-litre V8 carburetted petrol engine. 'Engine is primarily for installation in fighting vehicles. Maybe required in other service equipment. Must be of lightweight, compact design but durability of primary importance. Intended the design shall be capable of expansion up to 12 cylinder Vee form or an in-line six cylinder form should a requirement arise. In this event a maximum degree of interchangeability of parts is required. A power output in the V8 for 300 bhp at 4000 rpm is required at the outset.'
When Ferrari so successfully employed a V12 configuration in racing in the 50's and '60s, Jaguar eventually realised the virtues of the V12, one of those virtues being the lower overall height. So, a ‘V’ engine seemed to be the way to proceed.
Unlike the XK engine, which was born a road engine and had racing thrust upon it, as it were, the V12 was conceived, at least in part and initially, with competition in mind. It would, though, make obvious sense to also use such an engine in a future generation of production cars. Making a twin-overhead-cam engine a practical volume proposition in 1948 had been a bold and innovative move that endowed the Jaguar marque with a degree of engineering integrity that it had undoubtedly lacked pre-war and leap-frogged the competition. In the same way, a production V12 – until then the preserve of exotic, hand-built super-cars – would be good for Jaguar’s image as well as making a potentially fine power unit.
A V8 just would not fit the bill in the same way. Apart from inherent technical reasons that favoured the V12, V8 engines were two-a-penny in Jaguar’s main market. Kjell Qvale, one of Jaguar’s most important dealers in that market, recalls: ‘Sir William and I had a lot of chats and I gave him ideas. He asked me if he should make an eight or a twelve and I told him that the world was full of eights.’ No doubt Qvale’s views helped to convince Lyons’s thinking later.
The concept had been implanted in the minds of Heynes and his colleagues for some time. The implementation of that concept was sadly and inevitably delayed by the retirement from racing. There were regular thoughts of returning to the tracks and finally the mid-engined XJ13 was actually built but, for a variety of reasons, this went no further. The power unit created for that car was a twin-cam 5-litre V12.
The first twin-cam V12 was designed by Bill Heynes and Claude Baily with assistance from colleagues such as Mike Kimberley. With the acquisition of Coventry Climax, Wally Hassan was brought into the Jaguar fold once more, and Harry Mundy had also joined the company. Hassan and Mundy masterminded the design of the next engine, a single-cam unit, up to its readiness for production. There were opposing views as to whether each bank of the engine should have had one camshaft or two.
One costing, a hand-written summary, showed that the capital investment needed was £420,000 for a single-cam version and £491,000 for a twin-cam. With parts and labour, this worked out at £24 per unit for the single-cam engine as opposed to nearer £42 for the twin-cam. Unfortunately, neither an amortisation rate nor a date are given on this summary.
In the end, it was decided to adopt the single-cam engine though both designs had been run in saloon cars, starting with the twin-cam, which was certainly running in a Mark X by May 1965. Besides cost, other factors behind the choice of the single-cam unit included the timing gear (less complex, lighter, quieter and cheaper), bonnet height (lower with the single-cam engine), the car’s overall width (owing to installation and steering lock), and the placing of ancillaries (the air conditioning compressor and distributor could be sited between the heads). It was also stated that the single-cam engine was superior at all engine speeds up to 5,000rpm.
The first full-size engines built employed fuel injection and development was done along those lines. However, it was found that the Lucas mechanical type of fuel injection, whilst giving very good performance, would not satisfy emissions requirements without other adverse effects. Thus development was switched to a Brico electronic fuel injection system, which was a great success. There was every intention of introducing the engine in this form until Brico, at the last minute, decided not to go into production. This was a serious blow.