EGT Explained

EGT (Exhaust Gas Temperature) is a very useful tuning tool when combined with a wideband O2 sensor.  EGT seemed to get a bad rap when used to try and determine actual AFR (Air Fuel Ratio) because it is influenced by many other factors as well.  A rich mixture will see high EGT temps because the air fuel is still burning as it gets pushed out the exhaust valve.  As the AF gets leaned out the EGT will drop, but then around 14.6 it will start to go up again and even faster till the motor gets so lean it will barely run.  Ignition timing also plays a major role in EGT temperatures.  An overly retarded timing at a safe AFR will give incredibly high EGTs, just as overly advanced timing will also do.  Excessively high EGTs can help the exhaust valves smash into the head causing deformations in the valve seat, and or burning holes in the valves.  Usually a temperature of 1800F (982C) is considered to be the max upper limit and is too hot for nearly any combination.  Steel will take on a reddish tint and begin to slightly soften if heated to this point.  Remember that just because the exhaust temperature is that hot does not mean your headers or exhaust valves are that hot, but that they will be quickly approaching those temperatures unless the EGTs are reduced.  You will also begin damage O2 and WO2 sensors at about 1650F (900C) and therefore should use this value as the upper limit when dealing with a street car with O2 and or WO2 sensors.  The melting point of mild steel is about 2730F (1515C) and your headers will be dripping at that point, but you should never see EGTs anywhere near that point.  Typical EGT values should be between about 1250 and 1600 at WOT.  Most engines will make max power at an AF ratio between 12.0 and 13.5:1 but should be lower sometimes if detonation is a problem or high IATs are expected from a forced induction application.  There is no “ideal” EGT.  The optimal EGT values will change with engine combinations (cams, pistons, headers, etc).   Raising the compression with no other changes will drop the EGT at the same AFR.  Many times a decrease in EGT is the result of detonation.  It seems counter intuitive, but all the extra heat from the detonation gets absorbed by the combustion chamber so there is less heat transferred to the exhaust.