Many airplanes are equipped with an exhaust gas temperature (EGT) gauge. If properly used, this engine instrument can reduce fuel consumption by 10 percent because of its accuracy in indicating to the pilot the exact amount of fuel that should be metered to the engine.
An EGT gauge measures, in degrees Celsius or Fahrenheit, the temperature of the exhaust gases at the exhaust manifold. This temperature measurement varies with ratio of fuel to air entering the cylinders, and therefore can be used as a basis for regulating the fuel/air mixture. This is possible because this instrument is very sensitive to temperature changes.
Although the manufacturer’s recommendation for leaning the mixture should be adhered to, the usual procedure for leaning the mixture on lower horsepower engines when an EGT is available is as follows:
The mixture is leaned slowly while observing the increase in exhaust gas temperature on the gauge. When the EGT reaches a peak, the mixture should be enriched until the EGT gauge indicates a decrease in temperature. The number of degrees drop is recommended by the engine manufacturer, usually approximately 25° to 75°. Engines equipped with carburetors will run rough when leaned to the peak EGT reading, but will run smooth after the mixture is enriched slightly.