I have had second thoughts about adding this page to my website, questioning wether its such a good idea to encourage people to build their own cannons. However, seeing the dangerous methods of ignition that people use, I figure its in the interest of safety for people to use a proper ignition system and trigger their cannons from a safe distance.
The ratio of secondary to primary turns in an ignition coil is somewhere around 100:1. The ignition coil is operated directly off a 12 volt source. However, the ignition coil does not work like an ordinary transformer. An ordinary transformer will produce output current at the same time that input current is applied. An ignition coil actually does most of its work acting as an inductor. When the ignition coil is connected to the battery, the inductor is 'charged' with current. It takes a few milliseconds for the current to build up the magnetic field - this on account of reverse voltage caused by the increase in magnetic field. During this short charging period, maybe a thousand volts are produced at the high voltage terminal, not enough to actually cause a spark.
The actual spark is generated when the breaker contacts open. For an ideal inductor, the current and voltage relate by:
A less theoretical way of looking at it is that disconnecting the current source will cause the magnetic field in the coil to collapse (because nothing is forcing it magnetic anymore), and the rapid change of magnetic field in turn induces a large voltage in the windings.
In an engine with four or more cylinders, the high voltage terminal of the coil is connected to the distributor, which is just a fancy high voltage rotating switch, for selecting which of the spark plugs needs to be fired. This is much cheaper than having one ignition coil for each cylinder.
Modern cars of course all use electronic ignitions instead of the unreliable mechanical breaker points points for interrupting the current to the ignition coil. However, the ignition coil still works the same way, so you should be able to use an ignition coil even from a modern car. The main thing that has changed is that the 'points' have been replaced with fancy computer gadgetry and power electronics.
One thing that cars usually have that I didn't include in the circuit is a small capacitor across the breaker points, often called the "ignitor". This capacitor in cars before electronic ignition had a tendency to fail over time, so you may have heard people refer to it. The capacitor helps to protect the points by liming voltage across the contacts, so the contacts themselves won't spark. It also forms a bit of a resonant circuit with the coil, and that way enhances the spark, although it probably cuts down on the output voltage a little bit. For the purpose of firing a cannon, it works well enough without one.
Connect the high voltage terminal of your ignition coil to the tip of the spark plug. The high voltage terminal is at the top center of the coil, with the deep socket. Use wire with good insulation - use at least the kind of cable used in a power cord, or something with thicker insulation. Its best to keep the wires a few centimeters apart, and away from any moisture. High voltage will find its way through any defects in the insulation. Keep the wires less than a meter long. Use another wire to connect the body of the spark plug to one of the terminals of the ignition coil (doesn't matter which one).
Next run two wires from the positive and negative terminals of the ignition coil. If you can't tell which is which, don't worry, swapping positive and negative won't make much difference. I recommend you use a long, but fairly thick wire for this, so that you can trigger your cannon from a safe distance. Your ears will thank you.
A pair of screws screwed through the ABS pipe can be used instead of a spark plug. However, I found on my cannon that after just a couple of firings, there was enough moisture buildup in the cannon to prevent this arrangement from sparking. The moisture comes from the water vapour, which, along with carbon dioxide, is a product of the combustion. A proper spark plug will not short out as easily. To mount the plug, just drill a hole slightly smaller than the outside thread of the spark plug, and then screw the plug into the unthreaded hole in the ABS pipe. It should hold quite well that way. But even with a spark plug, moisture buildup from repeated firings can short out the plug. I recommend blowing fresh air through the cannon between shots.
ABS cannon (potato gun) page
Matthias Wandel's home page