The answer is **ambient temperature**. This created the "Expansion Loophole." Teams are reportedly using secret alloys that remain within legal dimensions at room temperature but physically expand when the engine hits 300°C. The engine grows. The compression ratio climbs to an illegal 18:1 while the car is at full tilt. By the time it returns to the pits to be measured, the metal has shrunk back to a legal state. It is a ghost in the machine.