flathead ignition



5237 FORD STreet speedway IN 46224 us

The Ignition coil is essentially a step up transformer which converts the low voltage ( primary) circuit into a high voltage circuit.

As current begins to flow in the primary coil winding, a magnetic field surrounds the ignition coil. The strength of the magnetic field is dependent on the amount of primary coil current.  ( approx 4 amps )When the primary current is interrupted, the magnetic field will collapse and induce a voltage into the secondary windings.

As a result a secondary high voltage is produced at the coil output brush and is sent to the high tension button at the rotor.

Two bolt-two bolt diagonal-and three bolt coils.

Some ignition system notes !!!!

There are three kinds of secondary voltage.

Voltage available: Is the max voltage of the ignition coil, typically 20,000-30,000 volts.

Voltage required: Is the voltage the engine needs to fire the air fuel mixture, typically 5,000 volts

Voltage reserve: Is the voltage left in reserve between what you have available and what the engine actually needs.  (20,000 volts) This voltage is used up with acceleration, loads, and wear....A properly tuned engine will never use much over 10,000 volts..

Current waveform of flathead coil showing 3.2 amps !


Ignition Coil Testing:

Typically the coil is tested for ;

  1. Primary Resistance= Low ohms scale should be less than 4 ohms     Ford typically .6-.7 ohms
  2. Secondary Resistance = High ohm scale should be 3000 to 6000 ohms  Ford typically 3500-5000 ohms
  3. Coil maximum output= Bright blue spark jumps 1/2 inch minimum
  4. Scope output test= 20,000 - 30,000 volts

Skip Haney rebuilds these Ford coils and does a great job.  E-mail: skip@fordsrus.com