No matter how sophisticated your car is, or not, you still need three things to make it run; Spark: Fuel: Timing.

In a previous article we explored the spark plug itself. Let's now look at how the plug gets the power to fire.

For the longest while there was basically one type of ignition system for a gasoline burning engine. Peugeot played around with candles in the late 1800's, but for the most part we can just talk about one system until 1972. The traditional distributor ignition system (DI) used a set of points, a condenser and ignition coil. It was housed in a distributor body that had the ignition wires connected to it through a distributor cap that used an ignition rotor to transfer the electrical pulse generated from the coil to the cap, through the wire, through the plug, back to ground in the cylinder head and hopefully lighting the correct amount of fuel at the correct time.

 The system was pretty fail safe and fool proof. Most problems with it came from lack of maintenance. There were some severe limitations as to the amount of power this system could produce but it is still the longest running ignition system in use today.

To work correctly the DI system needs a steady supply of voltage, your battery; A source of high voltage, your ignition coil; A switching device, your points; A way to control the supply voltage to your points, ballast resistor, wire or resisted coil; Something to cut down on the arcing of the points as they open and close, your condenser.

 In the DI system the vehicle's system voltage, 13-15 volts running, is reduced to between 6 and 7.5 volts. This is accomplished either pre-ignition coli through the use of a ballast resistor or resistor wire. You may also have what is called a resisted coil where direct system voltage is applied to the coil and it is reduced internally to the required voltage before heading for the points. One of these ways must be used to reduce voltage or you will burn up your points. A combination of these systems will reduce the voltage supply to the points by to great a factor.

Once we have the proper voltage flowing into and out of our coil we are now ready to convert, or transform (the coil is really nothing more than a transformer) from our low primary voltage to our high secondary voltage. The primary ignition coil winding has a low resistance - typically 1 to 3 ohms - that drops the 6 volt primary supply voltage to approximately zero volts (or as an electrical engineer would say, "consumes all of the voltage") The secondary windings which are 100-200 times the primary windings, transform the current in the primary windings into the secondary windings when the primary switch is opened (your points). Opening the switch (points) cause the magnetic field in the primary side of the coil to collapse and cut across, or fall into, the secondary windings, inducing our secondary current of between 5,000 and 30,000 volts. This is the high voltage that is sent to your plugs.

 Not all ignition coils are the same. Each is designed with what is known as a turn ratio, the amount of turns in the primary side windings to the amount of turns in the secondary windings. By changing these ratios a coil can be designed to be better in starting or at peak rpms. Most coils are a balance between the two. As in all in the automotive field to gain one, more power at higher rpm, you lose at another, harder starting.

 Some of the drawbacks to a DI system are just the amount of moving parts and connections. Also at best you can only hope for 17,000 to 20,000 miles between point and condenser changes. Plug life is not much better. DI systems typically had a primary resistance of 2.6 ohms. This gave us a coil charge time around 10 ms (milliseconds) While an electronic system typically uses .5 ohm resistance and a charge time around 3.4 ms. What that means is we can produce over 35,000 volts at rpm above 3000! A typical DI system current actually falls off at engine speeds above 1000 rpm. In practical terms this means we have to keep the plug gaps quite small, typically .024 to .028 so the current can jump the electrode to ground and give us our spark to ignite the fuel. Just to get to your plug the current flows from the coil's secondary output, through the coil secondary wire into the distributor cap where it travels through a carbon brush to the ignition rotor. Here it travels along a brass arm and then jumps an air gap to the appropriate electrode in the distributor cap. It then travels down an ignition wire to the spark plug and then jumps from the electrode to ground. Every bit of resistance between the start of the path and the end causes a power loss which means an improper fuel burn is more likely. Anything you do make the path better, clean and tight connections, good parts, will help. Adding big high performance 60,000 volt coils does nothing. The system is incapable of producing that type of current regardless.

 As engine temperatures go up and air/fuel mixtures go down (lean) it gets harder and harder to ignite the fuel. Couple that with the fact that as rpm go up in a conventional system we loose ignition power you can understand why older cars used such a rich mixture across the full range of engine demands.

When the federal government began to demand that cars run cleaner the demise of the traditional DI system was a forgone conclusion. When the government mandated 50,000 mile maintenance free ignition systems it was dead. Morn the king, long life to the king. With the advent of the first electronic ignition in 1972 a whole new world in ignition systems was born.

 Unless you must use a standard DI system for concurs shows or your vintage racing/rally club demands it I would urge you to switch to some type of electronic ignition system NOW