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 knock - which the ECU corrects by retarding the ignition, reducing power. The intercooler cools the air before it enters the engine.

The benefit is therefore less about peak power than consistency: a saturated stock intercooler lets power collapse on the second lap of a circuit or during the third acceleration run. Beware of the bigger-is-better reflex: excessive volume takes longer to fill and dulls response. The right intercooler maintains temperature throughout your actual usage.