Cars and Bikes

Below is a list of external influences on a spark plug's operating temperature. The following symptoms or conditions may have an effect on the actual temperature of the spark plug. The spark plug cannot create these conditions, but it must be able to cope with the levels of heat, if not, the performance will suffer and engine damage can occur.

Air/Fuel Mixtures seriously affect engine performance and spark plug operating temperatures.

• Rich air/fuel mixtures cause tip temperature to drop, causing fouling and poor driveability
• Lean air/fuel mixtures cause plug tip and cylinder temperature to increase, resulting in pre-ignition, detonation, and possibly serious spark plug and engine damage
• It is important to read spark plugs many times during the tuning process to achieve the optimum air/ fuel mixture

Higher Compression Ratios/Forced Induction will elevate spark plug tip and in-cylinder temperatures

• Compression can be increased by performing any one of the following modifications:
• reducing combustion chamber volume (i.e.: domed pistons, smaller chamber heads, mill ing heads, etc.)
• adding forced induction (Nitrous, Turbocharging or Supercharging)
• camshaft change
• As compression increases, a colder heat range plug, higher fuel octane, and careful attention to ignition timing and air/fuel ratios are necessary. Failure to select a colder spark plug can lead to spark plug/engine damage

Advancing Ignition Timing

• Advancing ignition timing by 10° causes tip temperature to increase by approx. 70°-100° C

Engine Speed and Load

• Increases in firing-end temperature are proportional to engine speed and load. When traveling at a consistent high rate of speed, or carrying/pushing very heavy loads, a colder heat range spark plug should be installed

Ambient Air Temperature

• As air temperature falls, air density/air volume becomes greater, resulting in leaner air/fuel mixtures.
• This creates higher cylinder pressures/temperatures and causes an increase in the spark plug's tip temperature. So, fuel delivery should be increased.
• As temperature increases, air density decreases, as does intake volume, fuel delivery should be decreased

Humidity

• As humidity increases, air intake volume decreases
• Result is lower combustion pressures and temperatures, causing a decrease in the spark plug's temperature and a reduction in available power.
• Air/fuel mixture should be leaner, depending upon ambient temperature.

Barometric Pressure/Altitude

• Also affects the spark plug's tip temperature
• The higher the altitude, the lower cylinder pressure becomes. As the cylinder temperature decreases, so does the plugs tip temperature
• Many mechanics attempt to "chase" tuning by changing spark plug heat ranges
• The real answer is to adjust air/fuel mixtures by rejetting in an effort to put more air back into the engine