The turbo compresses air, and compression generates heat: at the compressor outlet, the air can exceed 150 °C. It is then less dense, meaning it contains less oxygen, and it causes knocking - which the ECU corrects by retarding the ignition, resulting in lost power. The intercooler cools it before it enters the engine.

The benefit is therefore less about top-end power than consistency: a saturated stock intercooler lets power drop off during the second lap of a circuit or the third acceleration run. Beware of the bigger-is-better reflex: excessive volume takes longer to fill and blunts throttle response. The right intercooler keeps temperatures under control throughout actual use.