The turbo compresses the 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, reducing power. The intercooler cools the air before it enters the engine.

The benefit is therefore not so much at peak power as in consistency: a saturated stock intercooler lets power collapse on the second lap of a circuit or during the third acceleration run. Beware of automatically choosing the largest one: excessive volume takes longer to fill and blunts throttle response. The right intercooler keeps temperatures under control throughout your actual use.