Thermal efficiency is the ability of an intercooler to remove heat. This is measured in relation to the ambient air temperature. For example, if the ambient air temp is 70 °F and you are running at 5500 RPM at 14.7 psi of boost pressure, the air entering the turbocharger or supercharger will be in the range of 75-85 °F (accounting for heat absorbed from engine compartment).
During compression in the turbocharger or supercharger the air is heated (adiabatic process) to the range of 230-260 °F. This hot intake air then passes through the intercooler where heat, if the intercooler is properly designed, is removed. The closer the post-intercooler intake air temperature gets to the ambient air temperature the more thermally efficient the intercooler.
Thermal inertia, also known as thermal momentum, is the heat sink capacity of the intercooler or the amount of heat the intercooler can absorb based on the mass of the intercooler. An intercooler with a high thermal inertia is able to absorb more intake air heat before intercooler outlet temperatures rise than an intercooler with low thermal inertia.
To see it another way, consider two different masses of aluminum, a torch and your hand. If we place a 6” x 6” piece of aluminum foil in the palm of your hand and then heat the exposed side with a torch, you will notice the heat on your palm almost immediately. Now try it again with a 6” x 6” plate of one inch thick aluminum. The timeframe before you feel any heat getting through will be much longer. The aluminum foil has very little mass with which to absorb heat, the thick plate, however, has considerable mass. In much the same way a larger intercooler will be able to absorb more intake air heat than a smaller intercooler with the same base temperature.
Thermal inertia is the combination of the mass of the intercooler (a larger mass being able to absorb more heat than a smaller mass), the base temperature of the intercooler (usually a steady temperature of the intercooler at idle or cruise) and the time required for the temperature at the intercooler outlet to increase with the introduction of hotter intake air for the turbocharger or supercharger. It is important to note that even with high thermal inertia the intercooler outlet temps will begin to rise slightly immediately after introduction of higher intake air temps. These outlet temps will continue to rise slowly over the duration of the increased intake air temperature event until the thermal inertia of the intercooler is overcome, at which point the outlet temps will rise sharply to the a stabilized temperature. This new stabilized temperature reflects the thermal efficiency of the intercooler. In short, thermal inertia is the amount of time required between an increase in the intercooler inlet temperature (from increased boost pressure and/or engine rpm) and the subsequent increase in intercooler outlet temperature. If the increased intake air temperature is removed prior to overcoming the thermal inertia of the intercooler the outlet temperatures will never reach the thermal efficiency temperature.
Pressure Efficiency, more commonly known as Pressure Drop, is the measure of the internal resistance of the intercooler or how much energy or boost pressure is lost as the intake air passes through the intercooler system. While this pressure drop is most often seen at the intercooler core, there is also opportunity for boost robbing resistance in the tubes leading to and from the intercooler, as well as the in the intercooler tanks. Every inch of tubing provides some drag resistance to the intake air, as do bends and turns in the tube system. Also smaller diameter tubes increase the drag resistance compared to larger tubes. The optimal intercooler system uses the most direct tube routing available with tube diameters properly sized for the air flow associated with the target power level.
Resistance at the intercooler core is directly related to two elements: internal flow area and internal resistance. Internal flow area is the amount of space available for the air to move through the core. The larger the internal flow area, the lower the resistance and the lower the pressure drop. The smaller the area, the higher the resistance and higher the pressure drop. Internal resistance, as you might reason, is the resistance encountered within the intercooler core. This is primarily the result of the internal cooling fin design and density with a secondary source being potential restrictions at the entry point to the core.