Service Bay
The third engine in Jaguar’s history didn’t appear until 1983 but had a fascinating background.
Inside the Jaguar AJ6 engine which replaced the iconic XK.
D escended in equal measure from the long-serving XK straight-six engine and the legendary V12, Jaguar’s AJ6 engine ended up replacing both of them but was destined to have a shorter life than either of its forebears: after just 10 years it would be replaced by the V8 configuration which the designers of both XK and AJ6 had originally dismissed as insufficiently refined.
The AJ – for Advanced Jaguar – powerplant didn’t appear until 1983, but the idea had emerged as early as the mid 1970s, when fuel economy had suddenly become a hot potato. It was at this point that Jaguar found itself increasingly being pushed into a corner, with its V12’s crippling thirst suddenly unfashionable and the alternative XK powerplant really showing its age as rivals were becoming increasingly refined. It had served honourably but was after all rooted in the 1940s.
Original thoughts on replacing the XK had revolved around a V8 design, something which Jaguar had always been reluctant to embrace on account of its inherent lack of refinement. Prototypes were even built up and run in XJ6s, but being effectively a cut-down version of the V12, they shared its 60-degree angle which created vibration issues engineers could cure only by using a separate balance shaft. A V6 was created in a similar way but also rejected on refinement grounds.
More promising was the option of creating a slant six which could be manufactured using the existing tooling for the V12. The rather fundamental problem with this idea though was that half of the V12 engine worked out at a maximum 2.65 litres, clearly too small for the job in hand. A longer stroke would have successfully increased the capacity, but the production line designed for the V12 simply couldn’t accommodate a bigger block height.
All this explains why attention then turned to improving the XK engine, which would have to soldier on for a few more years. The biggest change was the
development of a four-valve head for the XK, which although a twin-cam design had only ever been manufactured with a two-valve layout.
Ultimately the 24-valve XK engine wasn’t developed to the production stage but in interviews with our sister title Jaguar World, former engineers involved with the project admitted that they were so pleased with the results that the new cylinder head design was carried over to the subsequent all-new AJ powerplant. Indeed, they point out that the rest of the engine was designed around the cylinder head, but despite sharing its basic concept the new engine was very different from the XK.
The rest of the engine was heavily influenced by the V12, with the AJ effectively using one bank of the bigger engine for the basic design and size of the block. This time though, the six-cylinder engine would enjoy its own production line and so the block height could be extended sufficiently to accommodate a suitably long stroke. This did
however dictate the dimensions of the seven main bearings, which is why the AJ is a relatively long powerplant for a six-cylinder.
Both the head and the block were cast in aluminium alloy, with the block using centrifugally-cast shrink-fit dry liners and avoiding the crankcase split along the bearing line which had given the XK engine its reputation for oil leaks. The camshafts were driven by a two-stage design using a duplex chain, with the primary chain also driving an auxiliary shaft which in turn operated the distributor, power steering and air pump. Meanwhile, to reduce the engine’s length, the oil pump was moved from the crankshaft nose to the sump, where it was operated using a chain from the crankshaft.
A 91 mm bore and a 92 mm stroke resulted in a capacity of 3590cc. With a compression ratio of 9.6:1 and Lucas-Bosch electronic injection, the new engine was good for 225 bhp at 5300 rpm, backed up with 240 lbf.ft torque at 4000 rpm. At a weight of 430 lb it was some 123 lb lighter than the XK engine and was a great deal more refined. It was also notably more efficient, producing 20 bhp more than the 4.2-litre XK and significantly better than either the V12 or XK when it came to fuel economy.
The AJ6’s primary task was to power the forthcoming all-new XJ40, but with John Egan in charge and focused closely on improving Jaguar’s quality, the decision was made to install the new engine in the XJ-S first. This was sensible for many reasons: firstly, the lower production volumes of the XJ-S (at that time really very low indeed) were less demanding for the production of the new powerplant, which would also make it easier to keep on top of teething problems and warranty issues. There was also the welcome bonus that the more efficient new engine gave the XJ-S a welcome boost in showroom appeal.
Three years later, with the engine’s early issues in theory ironed out, the AJ6 debuted in the XJ40 as intended. Two versions were initially available: the 3.6 as used in the XJ-S, but also an entry-level 2.9-litre option
designed to create an affordable XJ6 which could be priced to compete head-on with the top end of the Rover 800-series range.
The 2.9 was in itself an interesting creation, related more closely to the V12 and using a single-cam head with two valves per cylinder but employing Michael May’s ‘Fireball’ head as found in the HE versions of the V12 engine. Running a high 12.5:1 compression, it was good for a respectable 165 bhp.
It was destined to be a short-lived option though, since in September 1990 the 3.6-litre AJ6 was upgraded to 4 litres and a year later the 2.9 was replaced by a new, smaller version of the 24-valve engine marketed as a 3.2 with 91mm bore and 83mm stroke for a capacity of 3239cc.
The 4-litre was no more powerful than the 3.6 but was able to produce its 223 bhp even when fitted with a catalytic converter and offering a more useful spread of torque.
The AJ6 itself was upgraded in June 1994 to an evolution design codenamed AJ16 which featured significant design changes aimed at improving refinement and meeting tighter emissions standards. These included a new block and head, revised cam profiles, increased compression to 10:1 and a new engine management system. Once again, the new powerplant was designed for the forthcoming new XJ saloon but was introduced first in the lower-volume XJS some months before it was fitted to the XJ6. The AJ16 appeared in 4-litre form in the XJS from June 1994 and then in September was launched in the new X300 generation of XJ saloons, where it was rated at 219 bhp for the 3.2 and 249 bhp for the 4.0.
The AJ engine still hadn’t reached the limit of its development however, since the X300 also debuted a new era of supercharged performance for Jaguar. Whereas the XJR-badged cars of the XJ40 generation had only been mildly tweaked by TWR, the X300 XJR boasted 326 bhp courtesy of an intercooled belt-driven Eaton M90 blower bolted to the 4-litre AJ16 and operating at 0.9 bar boost.
Like the 2.9 though, the supercharged AJ16 engine was destined for a short production life, since the XK8 in 1996 would usher in the new V8 era for the company. In September 1997 the X300 became the X308 powered by the same V8 powerplant and the straight-six Jaguar engine was discontinued.
STRENGTHS AND WEAKNESSES.
It’s no mere coincidence that the 3.6-litre engine’s bore and stroke are shared with a contemporary Mercedes diesel engine: serious consideration was already being given to the idea of a diesel-powered Jaguar and the development team considered a tried-and-tested ‘square’ formula a sensible place to start.
This in turn means that despite its aluminium construction, the AJ6 block is actually very strong and if well maintained can be a very reliable unit.
One thing they don’t like is being overheated and a head gasket leak isn’t uncommon, often causing an oil leak near the distributor even if it doesn’t affect compression. The 2.9 is less robust, being known for timing chain issues and in all honesty doesn’t offer much improvement in economy over the larger versions.
Since the engine uses iron liners, there’s no need to worry about the effects of fuel on the Nikasil bore coating of the later V8 engines.