The turbo compresses the air, and compression heats it up: 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, meaning less 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 collapse on the second lap of a circuit or during the third acceleration. Beware of the instinct to go for the biggest option: excessive volume takes longer to fill and blunts throttle response. The right intercooler keeps temperatures under control for the actual duration of use.