Air-to-air intercooler kit for BMW 1 Series (E82, E88), 3 Series (E90-E93), Z4 (E89) — 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 knocking - which the ECU corrects by retarding the ignition, in other words reducing power. The intercooler cools it before it enters the engine.

The benefit is therefore not so much top-end power as consistency: a saturated factory intercooler lets power drop off on the second lap of a circuit or during the third acceleration run. Beware of automatically choosing the largest one: excessive volume takes longer to fill and dulls throttle response. The right intercooler keeps temperatures under control throughout actual use.