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

The benefit is therefore not so much at the top end as in consistency: a saturated stock intercooler lets power drop sharply on the second lap of a circuit or during the third acceleration. Beware of the bigger-is-better reflex: excessive volume takes longer to fill and blunts throttle response. The right intercooler keeps temperatures stable throughout actual use.