For thermodynamic analysis the specific heats of air can be treated as functions of temperature, which they are, or they can be treated as constants, which simplify calculations at a slight loss of accuracy. Because of the high temperatures and large temperature range experienced during an engine cycle, the specific heats and ratio of specific heats k do vary by a fair amount At the low-temperature end of a cycle during intake and start of compression, a value of k = 1.4 is correct. However, at the end of combustion the temperature has risen such that k = 1.3 would be more accurate. A constant average value between these extremes is found to give better results than a standard condition (25°C) value.

With Variable specific heats, the temperature at the end of compression will be less the magnitude of drop in temperature is proportional to the drop in the value of ratio of specific heats.

As the temperature is changing efficiency of air standard cycle changes.