Heat-soak

From Celica GT-Four / Alltrac - ST165 ST185 and ST205

Heatsoak is the process by which something becomes totally or partially heated above ambient temperature, generally in the context of something that shouldnt be hot. In this case we mean intercoolers - a top mounted intercooler should, in an ideal world, remain cool at all times. During slow movement the heat rising and radiating from the engine will overcome the slow air-flow that cools the IC and will start to heat up the intercooler. Eventually it will be at a temperature that is a lot higher than ambient - this essentially reverses its function and makes it act like a radiator - heating the intake air. This means that for any given manifold pressure you have a lower quantity of oxygen to burn, the ECU realises this and reduces the fuel on purpose but this creates a situation where the engine is producing less power than it could. Hot intake are also promotes detonation (knock) which can destroy a highly tuned turbocharged engine in seconds. Even non-top-mounted intercoolers can suffer heatsoak by sustained boost heating the material, however a front mounted intercooler tends to shed that heat much faster as the car cruises off-boost.