In Part 1 we looked at the origins of the V12 – in Part 2 we look at the development of the V12 in a little more detail.
Before the V12, Jaguar’s racing and practically all road cars were powered by the powerful and renowned XK straight-six double overhead-cam unit. This engine had its origins in pencilled sketches drawn during the London blitz by Sir William Lyons and his engine designers; William Heynes (Chief Engineer), Walter Hassan and Claude Baily. These sketches and original designs were followed by working prototypes as early as 1943. The first 3,442cc production unit saw the light of day in the beautiful XK120 of 1947. The same basic engine continued production into the 1990s.
1947/1948 Jaguar XK120
XK engine cross-section
This engine went on to power Jaguar to a number of famous Le Mans wins. However, as early as the mid 1950s, the pace of international racing engine development led Jaguar to believe they needed to develop a successor to the XK engine to keep them at the forefront of racing.
The Le Mans Sports Car Regulations at the time dictated the maximum capacity of the new engine – up to 5 litres. The Jaguar engineers agreed that the maximum power and tuning potential could be achieved with either a 8-cylinder or 12-cylinder “Vee” formation. The current XK engine had a relatively long stroke and the ability to achieve greater power by running at higher revs was compromised by this design. The XK's cylinder bore of 83mm and stroke of l06mm resulted in a piston speed of 3,820 ft./minute at 5500 r.p.m. – much faster and the engine integrity could not be guaranteed. It was decided that the future engine would have a reduced stroke of 70mm which would allow the engines to run safely up to 8,500rpm.
In around 1965, the project became reality when a number of V12 cylinder blocks and associated components were commissioned. These blocks were used to build up two types of engine – one with internal modifications made to allow a specially-modified crankshaft, lacking two “throws”, so the unit could function as a V8; the second was a full V12. One of the original “V8” engines has survived. The following picture shows this engine as it appeared in the collection of the late Jaguar collector Walter Hill in the 1980s:
V12 prototype block internally modified to run as a V8
The previous article, “Part 1 : Origins” made reference to the fact that, of the originally-planned eight V12 engines, only four are known to survive today. Two (Nos.1 & 7) remain with the Jaguar XJ13 Le Mans prototype, one (No.2) has survived as a complete engine and is in the process of a full restoration prior to being placed in an authentic recreation of the original 1966 XJ13. The fourth V12 (No.8) was placed in a rather inaccurate XJ13 copy – made by the well-known and talented car builder Bryan Wingfield and sold to the collector Walter Hill. In Wingfield’s own words (as reported in “Supercar Classic” magazine) – “I got a call from somebody I knew at Jaguar who told me that there were a couple of old prototype engines lying around which were of no use to anybody else, and asked whether I was interested … I had to buy those V12s through another engineering company.”
The Wingfield copy survives today as a running car – albeit with “Ford GT40-inspired” chassis and an approximation of the XJ13 body shape. The engine itself was bought as “a box of bits” and was made up from assorted original and new parts. The most desirable feature of this engine, the only heads with the ultimate development of intake angle (41 degrees), were removed from the “Wingfield” No.8 engine some time before its sale and fitted to the No.2 engine by Jaguar in 1969 and remain with the No.2 engine to this day.
V12 prototype twin-cam engine - stages of inlet port angle development. Greatest power was developed with a 41 degree inlet angle.
V12 prototype twin-cam engine – original inlet port angle (left) and final angle (right)
It was known that a V8 configuration needs a two-plane crankshaft with wide outer crankshaft balance weights to run smoothly. A V8 firing sequence is also not as efficient as a V12 if carburettors are used. In comparison, the V12 engine has equal firing impulses along each bank and can be treated as two sets of 6-cylinder engines as far as carburation is concerned. The V12 engine is inherently smoother than a V8. As well as these technical reasons for favouring a V12 over a V8, the USA car market was very important to Jaguar. It was felt that a V12 would have greater appeal than a V8 in this market.
The first V12 prototype was assembled in 1964. The prototype engine main features were:
- Twin overhead cam per bank
- 87mm bore x 70mm stroke
- 4,991cc capacity
- LM8 (aluminium) sand cast cylinder block
- Sump face on crankshaft centreline
- Top deck with flanged split rim cast iron liners
- Seven main bearings of 3.0” diameter
- Side-by-side connecting rods offset 0.75”
- 2.187” diameter crank pins
- Forged steel crankshaft with eight balance weights
- Crankshaft lubrication end-to-end feed fed from grooves in the main bearings
- Crankpins using sludge trap system used on the XK 6-cylinder engine and transverse feed holes
- EN 4A nitrided crankshaft
The following picture shows the second engine that was assembled in 1964
V12 prototype twin-cam engine with ultimate development of cylinder heads.
The prototype V12 cylinder head design was very similar to the tried-and-tested XK 6-cylinder head design but with a number of important differences.
The depth of the new combustion chamber was shallower (1.03” versus the XK’s 1.30”) and the included valve angle was more narrow (60° versus 70°). The combination of shallow combustion chamber and narrower angle was theoretically more efficient.
The following comparisons were made during development:
|XK 6-cylinder (85mm x 88mm)
|V12 (87mm x 70mm)
|312 @ 6,750 rpm
|502 @ 7,600
|BMEP (Brake mean effect
|205 lbs/sq in
|191 lbs/sq in
Competition and production versions of the twin-cam engine were developed at the same time. They were all basically similar except for things such as valve and port sizes and camshafts.
Ideally, the prototype engine would have employed the use of transverse inlet ports which became the norm for equivalent competition engines being built by Ferrari and B.R.M. This was found by Jaguar to be a more efficient layout for their engines but it would have been impossible to fit a V12 engine with transverse ports such as those on the XK 6-cylinder engine, with an adequate induction tract length, within the confines of an engine bay – even that of the Mk10!
While the competition version was being developed, Lyons and Hassan kept in mind the need to eventually fit a version of the engine in a production car.
During development, Jaguar found that the two stage chain drive was not completely reliable and the noise level was deemed unacceptable in a sophisticated saloon car. For this reason, a partial gear -drive was proposed for the competition engine camshafts as in the following diagram:
Camshaft drive (with gears) proposed for the competition engine.
However, this arrangement was never fully developed. A single engine was completed to this specification and was run on the test-bed. However, it remained in storage after the Le Mans project came to an end and was not fitted to the XJ13 car until 1978 when a missed gear necessitated an engine change – long after the project had ceased and the rebuilt XJ13 was only used for demos etc. For all its development life, the XJ13 ran with duplex chain drive to its camshafts.
The original engines were fitted with twin distributors which were found to be troublesome. One
incorporated two sets of contact breakers plus the centrifugal and vacuum advance mechanisms for both; the other was used simply to distribute the HT current. At high engine speeds difficulty was experienced in matching the timing of the two sets of contacts and the variations were deemed unacceptable.
The following picture shows the original twin distributors as still fitted to the surviving engine number 2:
Twin distributors fitted to surviving V12 prototype No.2
As may be apparent from the picture, it would have been difficult to fit a single 12-cylinder distributor in the “Vee” and so this twin-distributor arrangement continued throughout development. Around 1973, at about the time the crashed XJ13 was rebuilt by Jaguar, they were able to modify and fit a single 12-cylinder distributor and also updated the ignition system to OPUS (Oscillating Pick-Up System). A twelve-cylinder engine running at 6000 rev/min requires a spark rate of 600 sparks/second which is well above the capability of a conventional make and break “points” system (400 sparks/second). OPUS uses an electro-magnetic pick-up and electronic solid-state switching, mechanical delays are eliminated.
The prototype engines were subjected to extensive testing – not only in cars (including the XJ13) but also on the test-bed. My own engine, No.2, was also fitted to two Mk10 Jaguars. These big and heavy cars, one white and one sable, were used for road trials as the original XJ13 may have been rather too conspicuous! There are many stories surrounding these two cars including a road test by “Wilkes” of Motorsport magazine who was allowed a test drive on the understanding he never opened the bonnet to see what was inside! Retired ex-Jaguar employees also tell stories of how the cars were used to surprise and embarrass the Aston Martins being tested on the M1 motorway around Newport Pagnell …
The following picture shows this engine fitted with six carburettors for these road tests. Close examination reveals its origins as the dry sump originally fitted in 1964 (modified to wet sump for fitment in the Mk10s).
Archive photo of No.2 V12 prototype – as fitted to Mk10s
Towards the end of the V12 project the emphasis switched from racing to powering a production saloon. This eventually led, via prototype single-overhead cam V12 engines of 6.4 and 5.3 litre capacity, to the final 6.0 litre HE engine of the mid to late 1990s.
Developments from the quad-cam racing engine to the final single overhead cam engine will be covered in the next blog post.
To be continued ….