For best optimisation of add-on performance parts such as a different exhaust system, cold air induction etc, and to get the best power for that particular engine (they all came off Jaguar’s production line in very different states…) one needs a re-programmable fueling and ignition computer, aka Digital Engine Management. As a stand-alone upgrade this will achieve increases in power of around 10%. The earlier the car, the more DEM well benefit it. This is especially true for the pre-HE systems for which new replacement parts are impossible to find.

Central to a DEM system is the central processor unit or ECU (engine control unit). This is an electronics unit containing microprocessors, input and output circuits and programmable memory. Also part of the system are sensors to detect engine revs, timing, temperature, engine load, and throttle position.

For a petrol engine to work properly it must receive the right mixture of fuel and air at the right time in the engine’s cycle for each cylinder, and it must receive a spark in each cylinder also at the right time to ignite the fuel which explodes and pushes down the piston in each cylinder, all of which are connected to their own crank (which is effectively a lever) which rotates the crankshaft and ultimately drives the wheels round.

These explosions vary in intensity according to how much fuel/air mixture is received by each cylinder – and just as importantly the point at which in the pistons’ travels up and down each cylinder the explosion occurs. Too late and power is wasted, too soon and the explosion damages the poor old piston as it’s coming up the bore. This is known as pre-ignition and produces a characteristic high pitched note called pinking (or pinging in American)!

In the days before electronics, the control of when the spark arrived to ignite the fuel/air (otherwise known as Ignition Timing) was controlled by mechanical means. To make life more complicated the point in the engine’s cycle that a spark is best produced needs to vary with the engine’s speed, and the power being asked of it. In very early cars this was so difficult to achieve that there was a mechanical lever on the steering wheel for the driver to adjust for maximum power and least pinking. The driver’s brain was the best and cheapest computer around!

As the years went by various quite sophisticated mechanical systems were developed to give good control. But with the advent of electronics and the demand for very high efficiencies from engines (and the resultant low carbon emissions) electro-mechanical and eventually fully electronic systems began to arrive – in the 1970s.

At the same time, the method of controlling the amount of fuel delivered to the cylinders was being developed from carburettors which were purely mechanical to electrically operated high pressure fuel injectors which gave designers the ability to meter the fuel extremely accurately and repeatably. As you can imagine, the mechanical systems for fuel control and ignition timing were rather unreliable and crude, leading to a whole industry devoted to maintaining and improving these systems.

Provided the engine has been well serviced and is under 80,000 miles for a 6 cylinder or 100,000 miles for a 12 cylinder this is usually all that is necessary to restore full performance and reliability to the engine.