The FIA has agreed to stricter compression ratio measurements for Formula 1 power units starting June 1, 2026, amid controversy over Mercedes' engine design. The new rules require checks at both ambient temperature and 130°C oil temperature to ensure no cylinder exceeds 16.0:1. Manufacturers reached unanimous agreement after negotiations in the Power Unit Advisory Committee.
Formula 1's power unit regulations currently permit a geometric compression ratio of 16:1, measured at ambient temperature. All units inspected in Australia complied with the 2026 technical rules. However, rival manufacturers have accused Mercedes of exceeding this limit when hot, potentially reaching up to 18:1 through a pre-chamber in the cylinder head—a value allowed in previous years. While no proof exists of Mercedes hitting 18:1, sources accept that its unit surpasses 16:1. The rumor originated from an individual familiar with the project who joined another team and shared details.
Following negotiations and embarrassment for the FIA—after technical commissioner Vincent Pereme deemed the Mercedes unit legal—the Power Unit Advisory Committee unanimously amended Article C5.4.3. The updated provision states: "No cylinder... may have a geometric compression ratio higher than 16.0, measured in the following conditions: Until 31 May 2026: when the Engine is at ambient temperature. From 1 June 2026 to 31 December 2026: when the Engine is at ambient temperature as well as when the Engine is at 130°C." It also bans any components designed to boost the ratio beyond 16.0 under operating conditions. The 130°C refers to oil temperature in the six-cylinder engine. Reports suggest a possible tolerance up to 16.7:1 at 130°C, though this has not been officially confirmed.
Audi and Porsche advocated for the 16:1 limit during rule drafting. Mercedes insists its engine, developed at Brixworth by Hywel Thomas and the Mercedes High Performance Powertrains team, requires no changes. Opponents argue modifications to the cylinder head—such as enlarging a hole to allow pre-chamber filling at high temperatures—may be necessary.
The June 1 date aligns with the F1 calendar, as teams use three power units across 24 Grands Prix, each intended to last about eight races. The eighth event is the Monaco Grand Prix on June 7, allowing Mercedes potential updates on its second unit if needed. The Petronas sustainable fuel, optimized for higher compression, might see adjustments. Rivals could invoke the Additional Development and Upgrade Opportunities system after the sixth race (Miami Grand Prix), seeking FIA approval for changes if their output lags Mercedes by 2%. Such updates might activate from the Hungarian Grand Prix on July 26.
Mercedes team principal Toto Wolff views Ferrari as the primary challenger, aiming to build an early points lead without the full qualifying advantage of its power unit. McLaren, a Mercedes customer, trails due to suboptimal energy management but is expected to improve over time.
