Air-to-air intercooler kit for Audi A4 type B6, Passat 3BG — TA Technix

The turbo compresses air, and compression creates 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 timing, reducing power. The intercooler cools it before it enters the engine.

The gain is therefore not so much in peak power as in consistency: a saturated stock intercooler lets power drop off on the second lap of a circuit or during the third acceleration. Beware of the bigger-is-better reflex: excessive volume takes longer to fill and dulls response. The right intercooler maintains temperature throughout actual use.