Leaping Jaguar

The fundamentals - combustion chamber and port design
By the time the first V12s were actually built the original concept had evolved somewhat and whilst the cylinder heads were clearly of XK parentage the inlet ports had shifted to a downdraft (Fig.1) position between the camshafts. This layout was not unique to Jaguar and almost seemed to be a fashion in the early 1960s with BRM and Ferrari using it in Formula 1 and also Ford going the same way for their advanced 4 valve, 4 cam, Indy V8. In those days the advantages of the modern, narrow angle, cylinder head design had not been recognised and relatively wide valve angles were common leaving little space between the heads of vee engines to find room for inlet porting. The downdraft layout with direct flow into the cylinder seemed to offer a better solution and whilst it was known that the complex Mercedes M196 engines which earlier did battle with the D Type were far from remarkable for their performance despite their high specification, this was generally considered to have been because the ports and valves were too large, rather than a failing of the downdraft head layout.
Jaguar had to do a lot of work on the inlet port geometry to get the same specific power from the twin cam V12 as had been achieved with relative ease a decade or so before from the XK. A contributory factor was that the pronounced hemispherical combustion chamber which had worked so well on the long stroke XK was not nearly as efficient for a short stroke design like the V12. Only after bringing the inlet ports to an angle barely 40 degrees away from the valve axis instead of the original 60 degrees (Fig. 1) could 500 b.h.p. be exceeded and even then the torque band was unimpressive. Harry Mundys 1972 paper to the Institute of Mechanical Engineers explained all this in detail and concluded that the downdraft layout gave poor flow characteristics and reduced combustion efficiency because of indifferent charge turbulence. Once Keith Duckworth demonstrated the superiority of the narrow angle 4 valve layout when he created the Cosworth DFV F1 engine in 1966-7 the matter was settled and the downdraft port was finally dead and buried. Interestingly, Duckworths well-reasoned and innovative 4 valve design was prompted by his experiences with an earlier cylinder head layout which was destined to appear on the production V12.

Returning for the moment to the twin cam V12, it simultaneously followed 2 development paths, one for the all-out racer and the other towards a more refined and gentle creature for a production car. The noisy partly geared cam drive of the high revving racer became a multiple chain drive: moderate cams and port sizes sacrificed power for drivability: multiple stack Lucas fuel injection was replaced by 6 SU carbs. Yet still it was not good enough, quite apart from being bulky and heavy. Now shortly before this time Coventry Climax, by then a subsidiary of Jaguar, had introduced a range of industrial engines which used a simple flat OHC cylinder head design with bowl in piston combustion chambers. Under the legendary Walter Hassan, Climax had found that this layout provided a very good balance of performance, economy and detonation resistance, and was compact and easy to manufacture. It was not long before it was realised that here could be the answer to the road going V12s problems - simple and compact cylinder heads with single camshafts, a simplified cam drive needing only one chain, and a substantial weight saving. Single cylinder test bed work showed that performance would be more than adequate, in fact, mid-range performance was better than the twin cam so the day came when V12s of both types were fitted into Mk10 saloons and compared on the road. It was no contest really and only the aura of the twin cam remained - but not for long.

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This was a very significant point in the genesis of the V12 and one which is somewhat puzzling. The bad experience which prompted Keith Duckworth to arrive at the DFV design had been with his own Formula 2 SCA engine from 1964 - a flat head, single cam design with bowl in piston combustion chambers. Whilst it performed well enough to win a lot of races it always had a fundamental combustion problem and needed a lot of ignition advance to work properly, just as the twin cam V12 did. Certainly Duckworth was amazed to hear that Jaguar intended to proceed with the V12 as a flat head engine. Perhaps the conclusions reached by Climax were because of a fortunate combination of bore and stroke, valve sizes or whatever, but the usage pattern of an industrial engine may also be significant, spending long periods at about 75% load, rather than full load as in a race engine, or mostly light load as in a road car. It is not a criticism of Walter Hassan, who by now was deeply involved with the V12, or any of his team to point out this quandary. In his 1972 paper to the SAE Hassan admitted that at that time the knowledge of what happened to the charge in the cylinder of the flat head V12 was very much open to conjecture and that charge turbulence may well stagnate in some conditions. Nevertheless there can be no doubt that the flat head V12 (Fig. 2) was a very much more practical proposition than the twin cam version ever was.
In arriving at this point a great deal of experimental work had been done with different port layouts and spark plug locations but gradually the design evolved into the final cross-flow layout with a steeply inclined inlet port which remained to the end. Compression ratio was originally intended to be 10:1 for production, even 10.6:1 being considered for a time, but impending emission legislation and the disappearance of 5 star fuel decreed a change to 9:1 for European markets and a miserable 7.8:1 for 91 octane lead free fuel in the USA. Those who experienced driving them always maintained that the original 10:1 EFI engines were the best by far, but it was not until 1980 that such an engine was actually on sale for about a year prior to the arrival of the HE. The original 3.4 litre version of the XK was always regarded as the best of the line and the same can be said of the rather rare 10:1 V12 rated at 300 b.h.p.

At this point a slight diversion is necessary because in the early 1970s a couple of 4 valve V12s were built, one for road use, the other as a potential race unit. These were modern narrow valve angle designs and the race version was soon developed to produce a very impressive 630 b.h.p. from 5.3 litres using a standard crankshaft. The one and only prototype was loaned to TWR in the 1980s for them to study prior to producing their own 4 valver. Unbelievably, after they had stripped it and looked it over they threw it in a skip. One has to wonder if there is a scrap man somewhere who recognised what it was and saved it from the furnace. If so, perhaps it may yet turn up but what value would be placed on such an engine now?
As the 1970s progressed the thirst of the V12, even with electronic fuel injection, became a matter for serious concern and a number of ways to improve it were tried. Stratified charge pre-combustion chambers were in fashion and some low key experiments took place with devices which screwed into the spark plug hole of a single cylinder test engine. Another experiment which made no headway was a small inlet port concept intended to generate stronger turbulence. Ceramic coatings on heads and pistons to both reduce friction and cut heat loss showed some promise, but of course the real problem had been identified correctly by Keith Duckworth some years before - the flat head combustion chamber design was just not good enough. Only a substantial improvement would be worthwhile yet a major redesign was out of the question, so how could it be achieved? It so happened that around this time a Swiss engineer called Michael May was claiming some impressive results from a high turbulence combustion chamber based on the conventional 2 valve in-line configuration then in widespread use. Most manufacturers looked into it without finding any great advantage but it arrived just in time to save the Jaguar V12.

The rather unimaginatively named May "Fireball" combustion chamber (Fig. 3) consisted of a more or less circular pocket around and under the exhaust valve. The essential feature was that squish action created as the, now flat-topped, piston approached the cylinder head, was directed by a channel from around the inlet valve to impinge tangentially into the chamber to generate a strong swirl effect under the exhaust valve. With the spark plug relocated to the side of this chamber, the entire concept was a clever interpretation of accepted wisdom for burning lean mixtures at high compression ratios. One might easily argue that any combustion chamber design which generated some much needed turbulence in part throttle conditions would have done the trick but there can be no doubt that the May design, aided by a judicious raising of the differential gearing, transformed the fuel efficiency of the V12 and ensured its survival. Some prototype engines ran at diesel-like compression ratios of more than 14:1, but 12.5:1 was decided on for the introduction in 1981 of the "HE V12" as it was christened. The May combustion chamber design continued for the remainder of the production life of the V12 with the compression ratio lowered later to 11.5:1 for catalyst engines and finally to 11:1, lower compression ratios giving a vital benefit of quicker catalyst light up.

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