What makes good ignition?
Ignition occurs in a modern automobile when an arc is struck and current flows between the electrodes of a spark plug, or when current migrates across the conductive medium in a surface gap plug. While that may sound simple at first, the process becomes progressively more complicated as engineers try to optimize the type of spark plug with the ignition system generating the required voltage.
The amount of voltage necessary to arc the electrode gap is set by the following characteristics:
The size of the gap... arc-over voltage is roughly proportional to the gap size
The air/fuel ratio within the gap... the richer the air/fuel ratio (more gasoline vs. Air), the lower the required arc-over voltage
The compression at the moment arc-over is to occur... the higher the compression, the higher the required arc-over voltage
The composition of the electrode... certain metals for all the same conditions stated above will require less arc-over voltage than other metals. For example, platinum requires less arc-over voltage, all other things equal, than does steel
The shape of the electrode... the sharper and more jagged the shape, the easier it is for voltage to jump
The amount of fouling deposits trying to remove the electron flow from the arc... more fouling deposits and lower resistance to ground pulls more energy out of the spark gap.
While it may therefore seem desirable to lower the required arc-over voltage, since without arc-over there is a total misfire and no ignition, low arc-over voltage produces low spark power because spark power is directly proportional to arc-over voltage. That is, by doubling the required arc-over voltage, you double the instantaneous peak spark power, and the higher the spark power, the better the ignition.
All ignition is, therefore, a balance between the requirement to have sufficient arc-over voltage and increasing peak spark power for better, quicker ignition.
What benefits to specialty plugs bring to this mix?
One popular specialty type is platinum plugs. The primary advantage of these plugs, especially when used in an OEM ignition system (especially an older system, which may not be producing as much voltage as when it was new), is that platinum will require less arc-over voltage and therefore, particularly in a weak ignition, allows the gap to be jumped a higher percentage of the time.
For example, if at factory gap and with steel electrode plugs, it requires as much as 18,000 volts five percent of the time to jump the arc... due to the changing engine environment and running conditions... and if the OEM only produces 17,000 volts, then it follows that five percent of the time there would be a misfire.
Now, if one installs platinum plugs, which may only require, say, 13,000 volts to arc, the five percent misfiring with steel plugs would be eliminated. Since the ignition output on OEM ignition rolls off as rpm increases, platinum plugs in this case would allow the motor to reliably turn to higher rpm, thereby giving and increase in performance and possibly gas mileage.
The disadvantage of this method of reducing misfires is that the higher arc-over voltage, the better the spark when it does fire.
Therefore, platinum plugs will show a performance improvement with a weak ignition because the benefit from reducing the percentage of misfires more than outweighs the loss from reduced spark power.