"This is measured at ambient temperature, though, not while the engine is running. A running engine is hotter—much hotter—than one sitting at ambient, and as metals heat up, they expand. The engines have very short throws, so it doesn't take much expansion to increase the compression ratio by reducing the distance between the piston and cylinder head at the top of its travel."
"Unfortunately for the other teams, the FIA stated that its rules indeed specify only that the compression ratio should be 16:1 based on static conditions and at ambient temperatures. "This procedure has remained unchanged despite the reduction in the permitted ratio for the 2026 season. It is true that thermal expansion can influence dimensions, but the current rules do not provide for measurements to be carried out at elevated temperatures," the FIA said."
Compression ratio for regulated engines is reduced from 18:1 (2014–2025) to 16:1 for 2026 onward. The mandated measurement is taken at ambient temperature and in static conditions. Running engines operate at much higher temperatures, causing metal expansion that shortens piston-to-head clearance and raises effective compression. That thermal-induced increase could produce up to 15 hp (11 kW), yielding a few tenths of a second advantage per lap. The FIA clarified that current rules do not require measurements at elevated temperatures, making such an in-operation advantage permissible. Fuel controls shift from a 100 kg/h flow limit to a 3,000 MJ/h energy cap for synthetic fuels.
Read at Ars Technica
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