Electronic Ignition

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The ignition system is one of the most common sources of problems on a classic car, but there is a simple solution

The vast majority of the cars we write about are fitted with a coil (or coils), and a distributor (or distributors) containing contact breaker points, condenser, weights and springs for centrifugal advance, and often a vacuum advance system too. This system was developed over decades as the most efficient and cost-effective way of providing a reliable spark at each spark plug at the right time: it was perfected by Charles Kettering for Delco in 1908 and first available on the 1910 Cadillac.

You can’t get a decent spark from just 12 volts, especially inside a highly compressed cylinder of damp fuel/air mix, many times a second. That’s where the coil and the contact breaker come in. In simple terms, an ignition coil contains a primary winding and a secondary winding around a laminated magnetic core. The secondary winding is of much finer wire with substantially more turns around the core, providing a step-up in voltage from 12 volts to 20,000-40,000 volts (half that for six-volt systems). One end of both windings is connected together and to the live side of the battery; the other end of the primary winding is connected via the points to earth, while the other end of the secondary winding is connected via the distributor cap and rotor arm to the plugs.

The huge jump in voltage is matched by a corresponding drop in current, but it is the voltage that is needed, to create a spark strong enough to jump the gap in the spark plug despite the adverse conditions inside the cylinder, thereby igniting the fuel/air mixture. The condenser, or capacitor, absorbs the back-shock from the electromagnetic field, adds to the duration of the spark and reduces the tendency for the points to arc and burn.

When the contact breaker points are closed, the primary winding is charged with 12 volts – when that current is removed by the points opening, a large electromagnetic field is generated in the secondary winding, creating enough volts for the spark.

It all happens incredibly quickly at high engine revs: on a six-cylinder engine at 6000rpm, the whole process is happening 300 times a second. V8 or V12 engines may have to use two distributors and coils, or more than one set of points per distributor, to achieve adequate sparks, but this introduces more scope for timing inconsistencies.

No wonder coils get hot and no wonder components wear out. Contact breaker heels wear, closing the gap and adversely affecting the charging time for the coil (the ‘dwell angle’); points get burnt and pitted, reducing the charge available to the coil; distributor bearings and advance mechanisms wear and the ideal timing and advance settings are lost. At high revs, points are opening and closing so fast that they start to bounce, again dramatically reducing the charge to the coil and causing misfiring.

All these factors reduce the efficiency of the engine so it feels less powerful, uses more fuel than it should and produces more harmful emissions. To make things worse, even some big-name points and condensers sold now are poor-quality Chinese-made items that aren’t up to the job – and finding NOS isn’t necessarily better, as condensers have a limited shelf life.

Electronic ignition solves all these problems and is now used on all new cars, often in conjunction with engine management systems. Electronic ignition has been widely used since the mid-1980s; Chrysler introduced it as early as 1973 and conversion of classic cars to electronic ignition is popular and relatively simple. The only practical downside is that it’s harder to fault-find if something goes wrong.

There are several different types of electronic ignition. In its simplest form, it still uses the mechanical points, vacuum and centrifugal advance systems, but it greatly increases the life of the points by using them as a low-current trigger for a solid-state switching system handling the high-ignition current. Though inexpensive (you can buy a DIY version made by Velleman from Maplins for under £10) and particularly beneficial at low and high revs, this ‘transistor-assisted’ system remains subject to all the inconsistencies caused by mechanical wear in the distributor, points bounce and the like.

More sophisticated set-ups dispense with the points altogether, utilising instead a sensor. Systems such as Lumenition used a light beam broken by a vaned rotor, though this system has largely been superseded by the more reliable magnetic trigger known as a Hall Effect Sensor. Electronic circuitry then switches a large current through the coil.

Initially this type of electronic ignition still relied on the centrifugal and vacuum advance units to vary the timing according to engine revs and load, but now a small electronic ‘black box’ provides consistently ideal spark power and timing. Without the limiting factors of the points, a much more powerful or longer spark can be generated for better combustion.

On some cars, swapping parts to a distributor from a later model fitted with electronic ignition is the most cost-effective solution. However, further development has dispensed with distributors altogether, relying instead on individual small coils located very close to each spark plug and triggered in sequence. Sensors on the crank or camshaft detect engine position, while manifold pressure sensors and throttle position sensors vary the timing according to engine revs and load.

Electronic ignition systems that rely on a coil (or coils) in this way are known as inductive discharge systems: the alternative is the capacitive discharge system, which stores charged energy for the spark in a capacitor, so it can then be released via the coil to the spark plug at any time on command from the engine management system. The advantage of the inductive discharge system is that it produces just enough energy for the spark to bridge the gap, surplus energy being used to maintain it, thereby achieving more complete combustion, especially in lean-burn and turbocharged engines. The capacitive discharge ignition system delivers a burst of energy five to 10 times higher than the inductive system, ensuring a powerful spark even in engines that have excessive oil in the cylinder or a rich mixture, so it’s better for older engines.

One point to bear in mind when converting is your rev counter: not all will work with electronic ignition. Smiths RVI distributors may have to be rebuilt with modern electronic internals by, for example, JDO Instruments. If you have a mechanical rev counter driven from the distributor, and you want to fit a distributor with electronic ignition that does not have the mechanical drive, your mechanical rev counter can also be given electronic internals.

You will find a huge range of prices for electronic ignition conversions: use the information above to establish exactly what you are getting for your money and what other components you will need to get the best out of the system.

Particularly interesting is the 123ignition system (available from several agents, such as MG Owners Club Spares – see the 123ignition website), which is supplied as an exchange distributor with switchable mapping: a choice of 16 advance curves to suit different states of engine tune. It’s available for two-, four-, six- and eight-cylinder engines for a range of European marques.


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