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 it contains less oxygen, and it causes knock - which the ECU corrects by retarding the ignition timing, resulting in less 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 factory intercooler lets power collapse on the second lap of a circuit or during the third acceleration run. Beware of the instinct to go for the biggest option: excessive volume takes longer to fill and blunts throttle response. The right intercooler maintains temperature throughout real-world use.