Air-to-air intercooler kit for Ford Focus III ST — TA Technix

The turbo compresses the air, and compressing it heats it up: at the compressor outlet, the air can exceed 150 °C. It is then less dense, therefore contains less oxygen, and 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 collapse 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 response. The right intercooler keeps temperatures stable throughout actual use.