Jaguar XKE V-12 Engine Design
Where the XKE's 4.2-liter six cruised at 100 mph with
little throttle opening, the Jaguar XKE Series 3 V-12
(above) cantered at 120, and still had power
enough to surge when unreined.
The angle between the two banks of six cylinders was 60 degrees, traditional for a V-12 because it gives even crank throws and thus smoother running. Both the heads and the block were cast in aluminum alloy for light weight -- a savings of 116 pounds over the same design in cast iron.
That weight-savings was known exactly because, concerned about noise, the factory did cast one experimental block in iron. But when installed in a test car, it proved not significantly quieter than the aluminum engine.
Within the block were pistons running in wet iron liners. For strength and, again, smoothest running, the skirts of the crankcase extended well down around the crankshaft, which was made of forged steel and whose main bearings were secured by four-bolt cast-iron caps. To be sure of adequate crankshaft support, seven of those bearings were specified.
The prototype "XJ6" twelve had twin-cam heads in the classic mold of the XK six. But Hassan, a former Jaguar man who'd returned to the company after its purchase of Coventry-Climax, was not impressed with the Baily engine's power output.
Having just spent some years building world championship-winning Formula One engines -- tiny 1.5-liter V-8s producing around 200 horsepower (a startling 133 horsepower /liter), Hassan thought that out of 5.0 liters one should have seen much more than 500.
This was not an especially kind judgment, for Baily and his team really hadn't had much chance to develop their racer. Moreover, compared to the racing Coventry-Climax eight, each of the V-12's pistons had to shoulder almost two-and-a-quarter times as much work.
However, the quad-cam layout did create problems with bulk, weight and cost for passenger-car applications. It also made for a lot of trouble in the mounting of accessories necessary on a street car.
Investigating alternatives through a series of single-cylinder test engines, Hassan decided that the most suitable head design was one with a single camshaft and valves parallel to one another and to their cylinder axis.
Because bore was large relative to displacement, there would be ample valve area for good breathing at the moderate rpm this engine would be turning in its everyday traffic duties. Twin cams and hemispherical combustion chambers simply weren't needed. Not incidentally, using single-cam heads rather than a pair of XK-type twin-cams saved about 44 pounds.
In fact, not even combustion chambers were needed. At least, not in the conventional sense. For reasons connected both with smog control and manufacturing ease, Jaguar's new V-12 cylinder heads were machined completely flat on the bottom, flush with the valve heads. Combustion chambers were formed by depressions cast into the piston crowns.
Such a design, not invented by Jaguar but not common, is known as a Heron head. However, Jaguar's engineers took pains to explain that their version was not a true Heron, because the piston "bowls" were broader and shallower.
These dished pistons were designed to give a compression ratio of 9.0:1 to suit 97-octane gas. Although fuel injection was becoming familiar in the industry, the engine couldn't be made to meet smog levels with such a system, so Jaguar fell back on emission-control Zenith-Stromberg carbs. There were four of these mounted at the ends of long, over-the-top intake manifolds designed to boost low- and mid-range torque.
On the ignition side, though, Jaguar did break new ground with the first production application of a Lucas electronic system called OPUS, originally created for racing cars.
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- New Jaguars: Reviews, ratings, prices, and specifications on the current Jaguar lineup from the auto editors of Consumer Guide.
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