Why does EGT rise when a spark plug, ignition wire or magneto fails? With a lack of ignition, shouldn it drop?
The combustion in a cylinder is designed to be initiated by both spark plugs. The flame front travels from each plug and the combustion process is largely complete by the time the exhaust valve opens. From the instant of combustion, the temperature in the cylinder rises rapidly to about 4000°F. The expanding combustion gases push the piston toward the bottom of its stroke and the combustion gases cool in the process. By the time the exhaust valve has opened and the piston has risen to expel the combustion gases, the temperature read by the EGT probe is about 1500°F. Since the EGT probe samples the temperature at the completion of the combustion cycle when the exhaust valve opens, any phenomenon that affects the timing relationship between the initiation of combustion and the opening of the exhaust valve will show as a change in exhaust gas temperature. Combustion initiated by a single plug is not as complete or as cool when the exhaust valve opens, so the EGT indicates 75-100°F higher.
