Air-to-air intercooler kit for Audi A4 B5 RS4 — TA Technix

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 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 drop sharply on the second lap of a circuit or during the third acceleration. Beware of the instinct to go bigger: excessive volume takes longer to fill and blunts throttle response. The right intercooler keeps temperatures under control throughout real-world use.