The turbo compresses air, and compression heats it: at the compressor outlet, air can exceed 150 °C. It is then less dense, and therefore contains less oxygen, which causes knocking - something the ECU corrects by retarding the ignition, meaning less power. The intercooler cools the air before it enters the engine.

The gain is therefore not so much in top speed as in consistency: a saturated stock intercooler lets power collapse by the second lap of a circuit or the third acceleration run. Beware of automatically choosing the biggest option: excessive volume takes longer to fill and dulls throttle response. The right intercooler keeps temperatures under control throughout actual use.