The value of the breakdown voltage for the gap on spark plugs in engines is dependent on many factors including:
Shape of the electrodes: For the same size of gap, sharp points will require a lower voltage than will dull points. So, for the same spark plug gap, sharp edges on the electrode will discharge at a lower voltage than on old, worn out spark plugs which have rounded electrodes.
Size of the spark plug gap: The larger the gap, the higher the voltage required. Remember that as the electrodes wear, the gap widens so plug gaps need to be checked and readjusted regularly.
Cylinder pressure:
This is highest at low speed and high load situations (think accelerating from a stand still) and at high RPM operation so under these circumstances a higher voltage is required.
Compression Ratio: The higher the compression ratio, the higher the voltage required.
Air/fuel ratio: The weaker the mixture (less fuel per volume of air), the higher the voltage requirements. If the air-fuel mixture becomes weaker because of problems with the fuel system, misfire occurs more easily.
Overall engine temperature: The lower the overall engine temperature, the higher the required voltage.
Electrode temperature:
As the electrode temperature rises, the required voltage drops. Since electrode temperature rises at higher engine speeds, misfires can occur more readily at low engine speeds.
Humidity:
As the ambient humidity rises, electrode temperature decreases, so the required breakdown voltage increases slightly.