After-market mapped ignition systems are now quite common, and you may wonder what advantages they offer over a conventional ignition system.  A conventional ignition system is a 2D system that only takes into account engine speed and not load on the engine.  It gives a constant timing that is dependent on engine rpm only.  At full throttle this is acceptable, however on part throttle, economy and driveability are seriously affected.  In another vein with some performance engines, the required advance may not alter in a linear manner.  There may be places in the engine's speed range where required advance can fall even though rpm is rising.

Some 2D systems go part of the way towards varying the ignition timing for load by fitting a vacuum advance device which advances the ignition when vacuum in the inlet manifold is high, e.g. when load on the engine is low, but this will be crude at best.  A mapped system can give precisely the right ignition advance whatever the engine speed or load.  This improves the tractability of the engine dramatically as well as giving far better economy.

To appreciate the difference between a 2D and a 3D mapped ignition system you have to understand a little about combustion within your engine.  When a fuel and air mixture ignites within the combustion chamber, the burning of the charge starts at the sparking plug and spreads throughout the mixture from that point.  It takes a given amount of time for the whole charge in the chamber to burn, expand, and hence force the piston down the bore.  This is why we have to start the ignition process before the piston reaches top-dead-centre.  This lead-time is called "ignition advance".

It follows that as engine revs rise and the engine turns faster there is less time for the charge in the chamber to burn hence the need to increase the ignition advance with increasing engine speed.  Before the age of sophisticated electronics the ignition advance was always controlled mechanically - in the very early days by a lever mounted on the steering wheel or handlebars of the machine.  The driver, or rider, altered the advance according to his best guess, going on the feel of the engine - not always too successfully!

What followed was a mechanical advance system based on a centrifugal system of weights located in the distributor.  As engine speed increased, the centrifugal force acting on the weights increased and caused them to move outwards against the resistance of a couple of clockwork springs, and in doing so advancing the ignition.  The springs pulled the weights back as the engine slowed again reducing the advance.  A series of stops and different tension springs allowed the ignition advance progress to be controlled, or altered from one engine to another, dependent on engine speed.

But there is another factor affecting advance that needs to be taken into account - cylinder filling.  The speed at which the mixture in the combustion chamber burns varies with the amount of compression that the charge is under.  This in turn depends on how full the cylinder is before compression takes place.  For example: on a small throttle opening at higher rpm, the cylinder will only partially fill, compared to wide-open throttle at the same engine speed.  It follows that you need different ignition timings for the same engine speed, but dependent on throttle position or engine load.

With the centrifugal distributor advance systems, manufacturers often fit a vacuum advance unit.  This pulls the timing to more advance when there is a high vacuum present in the inlet manifold (throttle closed or nearly so).  The problem with these mechanical systems is that they are crude in operation and movement of the distributor base plate at high rpm causes timing scatter.  For this reason most performance engines have the vacuum advance removed and the base plates welded up.

Example 4 cyl coil packAn EMS can control the ignition with very few moving parts; all it needs is a trigger and a load sensor of some kind.  The EMS knows the load on the engine as well as the engine rpm.  Since the ignition timing is mapped for each engine speed and load, the timing is at the optimum for the engine for each load condition including part throttle.  This gives the best possible performance and economy whatever the throttle position.  In addition since the triggering systems invariably have no physical wear points the timing stays set correctly more or less indefinitely and is maintenance free.  There are other spin-offs such as rev limiting, shift light, accurate tacho driving and tell-tale as well as the certainty that the timing is never likely to ‘go off’.

The benefits from a mapped system have to be experienced to be appreciated.  Throttle response is razor sharp, economy is improved and tractability (especially with more radical cams) is amazing.  In my own experience an engine converted from a centrifugal advance type of system to a mapped system undergoes a radical transformation.