Exhaust gas temperature gauge
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An exhaust gas temperature gauge (EGT gauge) is a meter used to monitor the exhaust gas temperature of an internal combustion engine in conjunction with a thermocouple-type pyrometer. EGT gauges are found in certain cars and airplanes. By monitoring EGT, the driver or pilot can get an idea of the vehicle's air-fuel ratio.
At a stoichiometric air-fuel ratio, the exhaust gas temperature is different from that in a lean or rich air-fuel ratio. At rich air-fuel ratio, the exhaust gas temperature decreases due to the cooling effect of the fuel. High temperatures (typically above 1,600 °F or 900 °C) can be an indicator of dangerous conditions that can lead to catastrophic engine failure.
This meter is most used in tuning turbo-equipped cars. If the sensor is installed at the manifold collector before the turbo, the turbine inlet temperature can be monitored. If the sensor is installed after the turbo, the exhaust temperature can be monitored. Because EGT will typically drop 200–300 °F (93–150 °C) across the turbine, installers in general will try to put the thermocouple as close to the cylinder head as possible to give a true reading that will require less mental math to read properly, and a reading that will react faster to the engine's condition compared to an installation after the turbo.
Using an EGT meter alone is considered an older technique for getting the most out of petrol and diesel engines, as a gauge-type wideband digital oxygen sensor can be purchased for about the same price, or for a little more. However, some advanced racers will use EGT gauges in combination with a wideband oxygen sensor to 'lean' the fuel ratio a bit to safely raise the temperature for more power.
Though by tuning primarily by EGT and air fuel ratio values, EGT is still to this day a used data output for engine tuning. When fine tuning an engine, if possible with the ECU manipulation with the cylinders timing can be made. By adjusting the timing, the resultant cylinder temperature can be used to improve cylinder efficiency. Though this is still widely done EGT values should be used as a safe guard sensor measure and as a tuning guide. 
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