After cooling is the term used for cooling the charge (boost) between the discharge of a turbo or a supercharger and the engine intake manifold. This is achieved with the use of one of two types of heat exchangers in-line with the path of the boost.

Cooling the charge is desireable as cooler air delivered in to combustion chamber generally means that oxygen concentration will be higher per volume, and charge temperatures will be lower. More oxygen means a more power combustion cyles therefore more power and a cooler combustion cylce will help prolong engine life. An Intercooler is one type of heat exchanger which is simple to design and implement in production. It is also known as an "air to air intercooler" and commonly consists of a "bar and plate" aluminum constructed matrix core. This is similar to a water radiator except charged air (boost) runs through it rather than water. The boost is cooled when cooler ambient air travels through the core (when the car is traveling). The intercooler is limited by the temperature of the ambient air, and its potential to flow air in order to minimise boost restriction, whilst an intercooler is very simple to install and cost effective.

Charge Cooling is the other method for aftercooling. Also called "air to water" cooling, a charge cooler system comprises of a heat exchanger in line with the boost. The heat exchanger is sealed in its own case and water is pumped through it. A reservoir, small water cooler radiator and electric pump are required to circulate the water in this sealed independent system.

The water is constantly pump whilst the ignition is on and is cooled as it travels through the water ratiator. The cool water travels into the charge cooler and cools the boost by absorbing heat energy. The hot water exits the cooler and back to the water radiator via the reservoir. This method of cooling is regarded as more efficient as the cooling action of the water is more consistent than air to air intercooling. The water drops to temperatures lower than ambient and therefore cools the boost with greater efficiency. However charge cooler systems require the installation of more components with a slightly increased cost. Charge cooling is commonly used for high compression engines where efficiency and temperature consistency are key requirements.

Why the need to aftercool

The action of compressing air (forced induction) inherently adds heat into the charge from the compression action. A turbocharger further adds heat to the charge by conduction as it sits on the exhaust manifold. Typically low boost systems (up to 6 psi) are deemed not to benefit from aftercooling. But due to the location of the turbo an aftercooler for most turbocharger systems is a requirement. For supercharger systems aftercooling is generally not effecient below 6 psi as charge temperatures are tolerable. High compression engines are the exception.