Resistance can be in plug wires (carbon thread core), plug caps, and/or
the spark plugs themselves.
The primary purpose is reduction of stray electromagnetic interference;
mostly with radio-type communications... But, those stray currents can
cause backlash problems for on-board systems; CDI units, computers and
With the simple Kettering system (battery/points/coil) no resistance is needed
except for radio interference. Not so with modern no points, high output systems.
Also, resistance in plug wires depends on length and resistance per foot; change length,
change wire type, change resistance.
So, changing the original amount of ignition system resistance can cause