Some basic stuff from my notes that I copied from the net somewhere.
Operational factors affecting the required voltage
The required voltage decreases with the increase in richness of the air-fuel ratio. However, if the air-fuel ratio is too rich, the required voltage increases due to the cooling down of the electrode. The ideal air-fuel ratio is considered to be 14.7:1.
The required voltage increases proportionally to the increase of cylinder pressure. Higher voltages are required to cause sparking under higher cylinder pressure. It may be wise to consider a colder plug when the compression ratio is increased. The gap, too, needs to be adjusted to a smaller size to accommodate higher cylinder pressures.
As the compression pressure reaches a maximum in what is known as top-dead-center (T.D.C.), the required voltage also reaches a maximum. The required voltage decreases in accordance with the advancing of the sparks timing. This occurs when the compression pressure is lowered and the spark plugs firing-end temperature rises.
Different types of fuel affect the required voltage. The bonding of the gas particles differs from one fuel to the next and each fuel has a different required voltage. When nitrous is introduced into the cylinder, typically lower spark plug gap is recommended to reduce the possibility of blowing out the spark itself. This lowered gap is relative to other specifications of the engine and is not an exact figure.
SPARK GAP AND TEMPERATURE
The required voltage is reduced in direct relationship to a narrow spark plug gap and higher temperatures as well. In a four-stroke engine the normal rate of gap growth (created by erosion of metals) is 0.01mm to 0.02mm per 1,000 miles. In a two-stroke engine the normal rate of gap growth is 0.02mm to 0.04mm per 1,000 miles. In newer spark plug designs, the more exotic metals assist in a significantly reduced rate.
POLARITY OF AVAILABLE VOLTAGE
The required voltage is lower when a NEGATIVE polarity is used. On some newer engine models, the coil fires a cylinder from both the positive side and alternatively the negative side of the coil. This required voltage irregularity seen on a scope should NOT be construed as a defective ignition system or defective spark plug.
In a standard center electrode spark plug, worn and/or rounded center and ground electrodes become increasingly more difficult to fire and require higher voltages to produce a spark compared to new sharp-edged center electrode. In newer vehicle engines, the trend is to have a smaller, more pointed center electrode which require more exotic metals (higher resistant to erosion over time) such as Platinum, Platinum alloy, Iridium or Iridium alloy). This pointed center electrode reduces the required firing voltages thereby reducing the overall demand from the entire ignition system.