HISTORY
Key to the renewed appeal of the XJ-S and the very existence of the XJ40 was the all-new AJ6 engine. We trace its history
AS WE celebrate 40 years of the six-cylinder option which gave the XJ-S such a massive boost in the market, it’s easy to forget what a massive undertaking an all-new powerplant was for a company the size of Jaguar at the time.
The brand’s postwar success had been built on the legendary XK engine, but by the late ’70s it was starting to show its age in some areas. Famously worked up during fire watching sessions at Jaguar’s Browns Lane plant as Luftwaffe bombs rained down on Coventry, the XK had served Jaguar well and in fact would serve even longer in parallel with the AJ6, the final production units being built in 1992 for the DS420 limousine.
As the 1980s appeared on the horizon however, efficiency and emissions became ever more important and it was clear that something more modern was needed – especially when the thorny issue of fuel consumption was raised. The XK’s age meant it was never going to win any economy prizes but the V12 was in a different league, even in HE form.
Work began on what would be only the third new engine in Jaguar’s history as early as the 1960s. Early thinking centred around creating production cost efficiencies by creating a V8 engine by effectively losing four cylinders from the V12 engine – the idea being that much of the machining equipment and process could be retained. The result was a 60-degree 3.5-litre unit, yet although that may be the ideal angle for a six or 12-cylinder engine the ideal angle for a V8 is 90 degrees, meaning that the unit required additional rotating balancer shafts in order to achieve acceptable smoothness. This meant the V12 engine’s production facilities couldn’t be used and also impacted on the proposed engine’s efficiency.
A second alternative, again with production efficiencies in mind, was to create a straight-six engine derived from a single bank of the V12. Simple maths tells us that this worked out at 2.65 litres and since it was felt that this was insufficient capacity to provide the required power, the stroke was lengthened from the 70mm of the V12 to 90mm, making the engine a ‘square’ design at the traditional Jaguar capacity of 3.4 litres. The ‘slant six’ engine was eventually developed to 3590cc and prototype examples were run in 1972, producing around 170bhp.
Once again though, the idea of using the existing production facilities was torpedoed by practicality when it was realised that the longer stroke required a taller block, but the idea of a modern six-cylinder engine was retained.
Work on the project given the name Advanced Jaguar Six – hence the AJ6 designation – began properly in 1976, the unit now slanted at 22 degrees
