Reason why temperature at the exhaust manifold is higher than of the exit of the Exhaust Valve.

When as an operator, we observe the exhaust temperature is generally higher on the common exhaust manifold when compared to the temperature right after the exhaust valve. A practical example can be that  no:1 exhaust valve temperature maybe 380 deg cel and the common exhaust manifold temperature might be 420deg cel, clear 40 deg increase from the exhaust valve.

The common perception is that the exhaust manifold is operating at a higher pressure hence a higher temperature in comparison to the temperature after exhaust valve is following gay lussac's law which makes up part of the ideal gas might just be partially right or a myth.Gay-Lussac's law states that "the pressure of a given mass of gas varies directly with the absolute temperature of the gas, when the volume is kept constant."

What really happens.

The temperature sensor after the exhaust valve goes through three very different situations:

1. When the exhaust valve open: hot gas is flowing past the sensor with very high kinetic velocities or flow.
2. Exhaust and inlet valves both open: a mixture of hot gases and relatively cool air is flowing past the sensor.
3. Exhaust valve closed: the sensor is immersed in a relatively cool “scavenging pocket”.

Basically, a cycle or pulses of very high temperature combined with high velocities and  then to a relatively cooler temperature .

The temperature sensor after the exhaust valve measures the average of the rapid temperature fluctuation and this value is seen by the operator which in turn is lower than the exhaust manifold temperature.

Hence, as the exhaust manifold is exposed to a constant uniformly hot gas, it has a higher temperature reading in comparison to the temperature reading right after the exhaust valve.