Air-to-air intercooler for Subaru Impreza WRX STI, year 2008 - 2015 — 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, reducing power. The intercooler cools the air before it enters the engine.

The benefit is therefore less about peak power than consistency: an overwhelmed stock intercooler lets power drop off on the second lap of a circuit or during the third acceleration. Beware of automatically choosing the largest option: excessive volume takes longer to fill and dulls throttle response. The right intercooler keeps temperatures under control throughout real-world use.