Points type and transistor high-energy ignition (HEI) ignition systems are both inductive discharge ignition systems. Points type ignition systems relay on contact breaker points for spark timing and distribution while transistor HEI ignition systems use magnetic pulses and electronic circuitry instead of contact breaker points. For this reason, transistor HEI ignition systems are often referred to as electronic ignition systems. Both points type and transistor HEI ignition systems use timing advance mechanisms.
Points Type Ignition
The contact breaker points and the distributor cam in points type ignition systems provide the switching trigger for the primary, low tension circuit. When the points are closed, the current from the primary windings in the coil flow through the points and is earthed. This causes a magnetic field to be formed around the coil's secondary windings. When the points open, the current can no longer flow to earth and the magnetic field around the coil's secondary windings collapses. This causes a high voltage current of 30,000 to 40,000 volts to be induced in the coil's secondary windings. This voltage is strong enough to jump the spark plug gap and is directed to the appropriate spark plug via the rotor in the distributor.
This system works well is stock engines but are not optimal for modified and tuned engines as at higher RPM there is less time for the current to fully saturate the coil and form the magnetic field around the coil's secondary windings. Generally, the points type ignition system is good for up to 18,000 quality sparks per minute. This is sufficient for engine speeds of up to 9,000 RPM on a 4-cylinder engine; 6,000 RPM on a 6-cylinder engine; and 4,500 RPM on a V8 engine.
Transistor HEI Ignition
The distributor in a transistor HEI ignition system has a pulse generator rotor that turns inside a permanent magnet. The pulse generator induces an electric current in a pickup coil that flows through an ignition module. The ignition module is located in the distributor and functions much like a contact points breaker; when a current is induced in the pickup coil, the current from the primary windings in the coil flow through the ignition module and is earthed. This in turn causes a magnetic field to be formed around the coil's secondary windings. When the current in the pickup coil is disrupted, the magnetic field around the coil's secondary windings collapses and the high voltage current required to jump the spark plug gap is induced in the coil's secondary windings.
Transistor HEI ignition systems are superior to points type ignition systems because they do not have tungsten points that can pit or burn. This makes it possible to use a stronger current in the primary circuit to saturate the coil. As a result, transistor HEI ignitions systems require less time to saturate the coil, allowing a stock HEI ignition system to produce 20,000 quality sparks per minute as opposed to the 18,000 of the points type ignition system. A competition HEI ignition system, on the other hand, is good for up to 30,000 quality sparks per minute. This is sufficient for engine speeds of up to 15,000 RPM on a 4-cylinder engine; 10,000 RPM on a 6-cylinder engine; and 7,500 RPM on a V8 engine!