The Jaguar V12 – Part Three - Evolution

In Part Two we looked at the development of the quad-cam V12 – we now look at how this evolved into the first single cam V12 engine. Much has already been published elsewhere – not least of all by renowned authors Skilleter, Whyte, Porter, Viart/Cognet et al. I hope to add to this body of knowledge with aspects that may not have been documented elsewhere.

V12 in 5.3 litre Series 3 E-Type guise

In the mid-1960s, and coinciding with the British Motor Corporation takeover of Jaguar (to become British Leyland), the instruction was given that Jaguar were to withdraw from racing. This, of course, affected the XJ13 Le Mans project and further development of the racing car and its competition engine was curtailed.

As there was now no need for a competition engine, the emphasis switched to developing the V12 as a production engine. In a production engine, maximum power is less important than low and mid-range torque – allowing Jaguar’s saloons to waft along in effortless silence.

The downdraft inlet port arrangement had been found to be sub-optimal for the Jaguar engine. However, the sheer width of the quad-cam unit would have made the addition of a sidedraft arrangement impractical.

There were two other areas of the quad-cam that could be inappropriate for a production engine – the two-stage chain drive and its use of twin distributors. The two-stage chain drive used in the quad-cam engine proved to be rather noisy and, although acceptable in a competition engine, was inappropriate for a production engine. During the development of the twin-cam, a single engine (No.4) was assembled with a cast-iron block. However, the weight penalty was too great. It is not believed that this cast iron block has survived – although it may have found use as a ship’s anchor ….

Comparison of single- and twin-cam chain drive mechanisms.

There were distinct advantages to using a single-cam layout rather than twin-cam:

1. There is a cost saving with the simpler design.
2. There is a significant weight saving (approximately 16lbs per head) if a single chain drive using a single sprocket is used rather than four chains and twelve sprockets – not to mention the smaller head castings and fewer camshafts etc.
3. The overall noise level is reduced.
4. The engine runs more smoothly with only two cams rather than four.
5. The greater width of the “Vee” allows the fitment of a single 12-cylinder distributor and other auxiliaries such as air conditioning compressor.
6. The overall width of the engine is reduced (particularly across the exhaust manifolds) which allows for greater wheel movement and a reduced turning circle.

Comparison of the bulk of a twin-cam vs single-cam arrangement.

Some trials were made with a partial gear-drive to the camshafts but this was unsuccessful and wasn’t adopted.

Changes were also made to the cylinder block. Wheras the twin-cam engine had a solid top deck, the later single-cam engine had an open deck. As well as weight savings, the latter design meant the block could be die cast to save money – an important consideration at this time in Jaguar’s history. Also, wheras the twin-cam engine sump face coincided with the crankshaft centreline, the single-cam engine’s sump face was much lower. This latter feature gave the ability to increase power simply by increasing the stroke – using existing sump and block.

Final cylinder block design showing the “open” deck and receptacles for cuff type push-fit liners. This picture is actually of a block that was used to create a coffee-table!

Perhaps surprisingly, it was found that the assembled non-decked cylinder block, with its associated webbing,  was more rigid than the solid deck design of the twin-cam block.

Single-cam cylinder block from beneath showing main bearings and studs

Just before the end of 1969, it seems that exhaust emissions were compared between the most highly-developed twin-cam competition engine (No.2) and the new single-cam production prototypes. It may have been found that the competition engine was more efficient in emission terms but other considerations will have weighted the decision towards a single-cam arrangement. 

A number of single-cam prototypes were assembled and trialled between 1971 and 1977 – development continuing long after the introduction of the new single-cam engine in the Series 3 E-Type of 1973. As well as being developed on the test bed, these prototype engines were fitted to saloon cars for road trials.

Details of a selection of these prototype V12 single-cam engines are as follows:

 “Single-Cam Prototype No.23” (6.4 litre; 8.5:1 compression)

“Single-Cam Prototype No.24” (6.4 litre; 8.5:1 compression)

“Single-Cam Prototype No.25” (5.3 litre; 90mm x 70mm; 4-valve cylinder heads)

“Single-Cam Prototype No.28” (6.4 litre; 8.5:1 compression)

“Single-Cam Prototype No.29” (Die-Cast Cylinder Block)

Other projects – at least three with a 4-valve twin-cam arrangement – were also trialled between 1974 and 1978. The findings and data from these trials will have been fed into the ongoing V12 project.  These engines were “slant six” twin-cam engines of between 3.6 and 3.8 litres capacity.

The following chart gives a comparison of performance of the two engines (5 litre twin-cam competition and 5.3 litre single-cam production).

Power curves of single and twin-cam engines

In the above analysis, each engine used a Lucas mechanical fuel injection system with individual intakes and open exhausts – must have made quite an impressive noise at peak revs …

The single cam engine gave a much better performance up to 5000 rpm where its power reached a healthy 360bhp . The twin-cam competition engine, however, really came into its own above 5000 rpm and achieved over 500bhp @ 7,500rpm.

It is interesting to note that while looking for cost-cutting measures, Jaguar decided to inset the valves by 0.050” in the flat single-cam cylinder head. They could save money by doing this because there was then a higher permissible speed for the cutter used to dress the heads. This meant they could speed up this part of the process. Their static air tests indicated this wouldn’t make any difference to power. However, this cost-saving measure did reduce power in practice as can be seen from the following chart.

Curves showing effect of valve-seat insert depth.

Jaguar looked at various ways of fuelling their new engine – ranging from Lucas mechanical injection as used on the competition engine to carburettors. The first production engine, as fitted to the Series 3 E-Type used carburettors although this quickly gave way to a fully-electronic system to meet the demands of emission control and performance. The system used on the first single-cam injected engines was a fully-electronic fuel-injection system as jointly developed by Lucas and Bosch. The fuelling system continued to evolve throughout the V12’s history.

Single-cam engine showing carburettors (left) and petrol injection (right)

Although originally conceived as a competition engine to keep Jaguar at the forefront of sportscar racing, the possibility of eventually fitting the engine into production cars was always at the back of Lyons’ mind. However, up until the mid 1960s, the emphasis was primarily on competition and racing. This emphasis changed in the mid 1960s towards a “production engine” and the result was the long-lived single-overhead-cam V12. Although many compromises were made along the way, the first production V12 became a commercial success and went on to power a range of Jaguar saloons and sports cars.

I am proud to own an engine that was developed at a time when racing was its main reason for existence.