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Aluminium Cylinder Blocks.The later Ds and the "light weights" also had aluminium cylinder blocks so were a very substantial departure from the production engine. There were also some aluminium block race engines built to the 3 litre class rules with 85 x 88 mm bore and stroke rather than 83 x 91 (or 92) mm used with equivalent cast iron blocks. Initially head gasket failure was a problem, caused by the cylinder liners sinking and relaxing the gasket "nip", but this was cured by adding flanges to secure the liners at the top. The original aluminium main bearing caps proved to be inadequate and were replaced by steel items but at the time an aluminium cylinder block was quite innovative so some difficulties could be expected.
By the standards of the time these were pretty powerful engines though the project lacked the commitment of earlier days, and they never really came to terms with Ferraris all conquering V12s.
Once the aluminium block had been made reliable there were plans to develop a production version and one or two 4.2 engines were built. They proved to be unacceptably noisy and were abandoned after a short time.
Fuel and SparksA number of carburetter arrangements were used over the years, the twin SU set up being the most common although the carbs themselves varied in type (H, HD, HS, HIF) and size over the years from 1.75 inch on the original XK120 and most early saloon engines to 2 inch in later years as most enthusiasts will know. 24 mm Solex down-drafts were used on most 2.4s although 1.75 inch SUs were an option which became standard on the later 240s and of course the MK10 and E Type had the triple 2 inch SU setup. From the late 1960s emissions legislation in the USA meant that SUs in any configuration had to be abandoned for that market, to be replaced by a pair of CD Strombergs with their greater accuracy and more sophisticated control of warm up fueling. Accurate warm up fueling was never a strong-point for the SU company - the electric choke devices used with their early carbs were barely satisfactory whilst the later AED was really pretty hopeless.
Finally even the Strombergs were superseded by Bosch/Lucas L Jetronic Fuel Injection which made it possible for the XK to meet ever tighter emission regulations in the late 1970s using Lambda feedback of exhaust oxygen content and a three way catalyst. This EFI system relied on air mass flow measurement rather than manifold pressure/speed as used with the V12 and there was much debate about how best to apply the calibration measurements from the test bed. It was not that there were errors in the work, just that it could be interpreted in such a way as to introduce errors so that the overall fueling could be about 4-5% richer than it should have been. All this was resolved however long before reaching production.
A characteristic possibly unique to injected engines using the twin tank system of Jaguar series 2 and 3 saloons was that of weak back-fires indicating that a tank had run dry. On early development cars this could blow the inlet elbow off the throttle thereby immobilising the car. When this was made more secure these back-fires found the next weakness, slamming the airflow meter flap shut with sufficient violence to damage the pivot bearings. A spring relief valve mounted in the flap proved ineffective and the cure was a rubber buffer for the flap to close against.
Incidentally, the 4.2 EFI engine, aided by the largest of all production inlet valves at 1.875", and rated at just 200 b.h.p. DIN (that means certified) was almost certainly the most powerful production XK ever. One might say that there was a degree of optimistic exaggeration about the 265 b.h.p. claimed for the earlier triple SU engine which was never verified under similar conditions.
Less well known are those carburetter arrangements which never made it beyond the experimental department. Various down-draft and side-draft configurations with as many as four carbs were tried but I understand that the most effective by far was a sort of reversed triple SU setup with long ram-stacks extending out to three small plenum chambers from which the carbs pointed inwards between the pairs of stacks. Apparently this even had advantages over the triple Weber setup used for racing but it lacked the tremendous visual appeal of the conventional triple SUs and was abandoned largely for that reason. Another promising long ram system that did not see the light of day was developed with the aim of giving the 2.4 a much needed torque boost.
Ram length has always been crucial to getting the best out of any Jaguar engine but sadly the production versions always seem to be compromised by the available space. This was never the case with the racing XKs, other than very early ones with twin SUs, as would be obvious from a glance at the long trumpets of the triple Weber and Lucas Injection (fig 6) systems mentioned earlier.
For many years ignition was by a conventional contact breaker but in 1978 US emission engines began to use a version of Lucas OPUS, successful enough in top level racing and a little troublesome on the Jaguar V12, yet not far short of disastrous on the poor old XK. The act of cramming everything inside the distributor body resulted in an unexpected sensitivity to heat having the effect, if not of failing permanently, of periodically shutting the engine down for about 20 minutes until the critical components cooled down again. OPUS was hurriedly replaced by the new Lucas Constant Energy system featuring automatic control of coil current to give consistent spark energy over a wide range of engine speeds. This lasted until the end with centrifugal and vacuum advance mechanisms so we never had the chance to see how much the XK might have been improved by full electronic engine management.
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