When the air is compressed by the turbocharger the molecules are forced tighter together. This causes friction between the molecules, kind of like what sometimes happens between co-workers. In turn, when the temperature is decreased the pressure will decrease. The air is cooled by going through a heat exchanger also known as an intercooler. The method of cooling is by means of ambient air, ice water or engine coolant.
Ambient air is the most common in automotive use.
The temperature drop through the intercooler will vary according to the temperature of the cooling medium and the available flow rate of the cooling medium. The pressure of the charge will always drop as it goes through the intercooler, but the air should become more dense. This will allow a greater mass of air per minute to flow through the engine at a given intake manifold pressure. The load on the engine is reduced and a higher density charge is supplied. A one degree decrease in intake manifold temperature will result in a one degree decrease in exhaust temperature.