Resistor Plugs

[tom slick]

A resistor is added to the plug to reduce the current flowing through the secondary windings, potentially protecting them.
I wonder if it also "tunes" the RL (resistor-inductor, or resistor-coil) differently. it would have an affect on the older ignition systems with condensors. It would change "resonance" in the circuit (RLC) when the voltage bounces after the initial voltage spike for "fire"

here is a cool .gif I found

 

[BrianWC]

Whoa that graph is too fancy for me!

I dunno. SAAB's lead engineer on the direct ignition pack is always very adamant that people only use the spec'd NGK resistor plug (NGKBCPR6ES-11 for my 9-5) or else they might have knock issues....

 

[tom slick]

-11 in the part number indicates a projected electrode. that helps keep the plug clean but not too hot.

 

[BrianWC]

In our cars, we have a high voltage cycle immediately after the ignition is cut to help burn off any excess carbon that accumulates.