Ignition Coils Removal, Inspection, and Installation on the DOHC 6G72 engine in the Mitsubishi 3000GT and Dodge Stealth
by Jeff Lucius
Automotive ignition coils are step-up transformers that consist of two separate wires wound about a soft-iron core. One wire, called the primary wire, is thicker and has less turns (a turn is one wrap of the wire about the core) than the other wire, called the secondary wire. The primary windings surround the secondary windings. Our engines use an electronic inductive-discharge ignition system (2-ignitionsystem.htm). Energy is stored relatively slowly in the primary windings and then released rapidly into the secondary windings.
While the transistors in our engine control unit (ECU) and power transistor unit ground the coil's primary wire circuit (also called the Low Tension circuit), current and voltage increase through the primary windings and a magnetic field (flux) is produced around the coil and strengthens until the field maximizes. The time (traditionally measured in camshaft degrees) that the ECU holds the coil's circuit grounded is called dwell (analogous to the points being closed in an older distributor system). In our coils, the (dwell) time required to maximize the magnetic field should be around 0.004 seconds (that is, 4 milliseconds or 4 ms). Some modern, high-performance coils need only 2 to 3 ms. Other coils need as much as 8 to 9 ms to maximize the magnetic field. The current in the coil primary windings is limited by the ECU to 6 amps. The dwell time is also limited by the ECU to no more than 75% of the available time, which is one engine revolution because of our wasted spark ignition system. At 7500 rpm there are 8 ms between coil firings (60,000 divided by 7,500). At this engine speed, dwell time is limited to less than 75% of 8 ms or less than 6 ms. In other words, there is plenty of time even at 7500 rpm, which is above the factory redline, for the coils to completely charge.
When it is time to fire the spark plugs, the ECU releases the ground to very quickly stop current in the primary wire. The magnetic field collapses first through the primary windings producing a current in these windings (a process called self induction). This current, typically with a potential in the range of 200 to 400 volts, produces its own magnetic field, which in turn produces an electromagnetic force (emf) in the secondary windings (also called the High Tension circuit) by a process called mutual induction. When there is a complete circuit on the high tension side, that is when both spark plugs fire, then there is also an induced current generated in the secondary windings with a voltage directly proportional to the self-induced voltage in the primary windings times the ratio of the number of turns in the secondary windings to the number of turns in the primary windings, and directly proportional to the resisitance in the circuit. A small bit of power is lost to inefficiency in the coil. Typically there are 65 to 135 secondary turns for every primary turn. The final result of the initial magnetic field collapse is a very-high voltage current (with proportionally lower amperage) through the secondary windings and the spark plug wires. The voltage produced in the secondary ignition system is the minimum amount required to produce sparks across the two spark plug gaps. The wasted spark should require from 3,000 to 6,000 volts. The combustion spark may require from 12,000 to over 40,000 volts. Once a spark is produced, resistance in the circuit drops drastically and the energy is quickly dissipated.
Three ignition coils are used in the distributorless ignition system on the Mitsubishi 3000GT and Dodge Stealth DOHC engine (turbo and non-turbo). Each ignition coil fires a pair of spark plugs simultaneously: one with the piston before TDC on the compression stroke and the other one with the piston before TDC on the exhaust stroke. The spark during the exhaust stroke is wasted and the basically inert exhaust gas does not combust. This is called a wasted spark ignition system (see my web pages 2-sparkplugtech.htm for more information on spark plugs and 2-timing.htm for information on timing events).