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Jaguar XJS Engines

The Jaguar XJS was originally launched with the classic 5.3 litre V12 engine used in the E-Type and XJ saloons.  Although hardly a thing of beauty, this compact engine is acknowledged as one of the all-time greats because it combines quiet refinement with utterly smooth and rapid progress.  But like the Jensen Interceptor it just loves fuel, consuming it at around 14 miles per gallon.

Jaguar's solution was to introduce the High Efficiency (HE) head in 1981, which was designed to rapidly burn lean fuel and air mixtures.  This gave the car a very high compression ratio of 12.5:1, a slight power increase (up 10 bhp to 295 bhp) and better fuel consumption of 22.5 mpg - quite remarkable for a V12 engine. 

In 1976 Jaguar began developing a 6 cylinder engine to replace its aging motors.  Jaguar decided to broaden its customer base by introducing a new V6 'entry level' engine for its cars.  It took seven years to develop - mainly due to cost constraints - and several low cost options were considered, including effectively chopping the V12 in half.  The final result though was a completely new engine, the AJ6, an in line six cylinder 24 valve 3,590 cc motor developing 225 bhp and 240 lb ft of torque.  The awkward problem for Jaguar was that while this car lacked some of the smoothness of the V12 it was just as quick and more fuel efficient. 

The 3.6 litre engine proved popular in Jaguar saloons but never really took off in the XJS, which as a grand tourer was ideally suited to a V12.  But in 1991 the capacity of the AJ6 engine was increased to 3,980 cc to improve low speed torque and increase the sense of effortless power that the V12 was renowned for.  Power rose by 10 bhp to 235 bhp, torque increase by 20% and 0-60 mph acceleration improved by 0.5 seconds.  This option in the XJS proved more popular and in its final years 4.0 litre sales far exceeded those of the V12.

XJS engines at a glance

5.3 litre V12
AJ6 3.6 litre in line 6
AJ6 4.0 litre in line 6