The turbo compresses air, and compression heats it: 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 peak power than consistency: a saturated stock intercooler lets power fall off on the second lap of a circuit or during the third acceleration. Beware the instinct to go bigger: excessive volume takes longer to fill and blunts throttle response. The right intercooler maintains temperature throughout real-world use.