Air-to-air intercooler kit for Audi A4 B5 S4 — TA Technix

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, i.e. reducing power. The intercooler cools it before it enters the engine.

The benefit is therefore not so much at peak power as in consistency: a saturated stock intercooler lets power collapse on the second lap of a circuit or during the third acceleration. Beware of automatically choosing the largest one: excessive volume takes longer to fill and blunts throttle response. The right intercooler keeps temperatures under control throughout actual use.