The FIA has officially closed a controversial technical loophole that allowed Mercedes to exploit engine compression ratio regulations, with the new rules taking effect for this weekend’s Monaco Grand Prix. The German manufacturer had found a way to increase their power unit’s compression ratio to 18:1 during racing conditions, despite the official regulatory limit of 16:1.
Mercedes HPP’s clever interpretation of the technical regulations centered on the measurement conditions for compression ratios. The rules previously specified that ratios would only be measured at ambient temperatures while cars were stationary in the pit lane, creating an opportunity for higher ratios during on-track running. Through the use of heat-expanding metals in their engine design, Mercedes was reportedly able to achieve the advantageous 18:1 compression ratio when the engine reached operating temperature.
Following protests from rival power unit manufacturers, the FIA accelerated the implementation of new regulations that were originally scheduled to take effect on August 1st after the Hungarian Grand Prix. Under the revised rules, compression ratios will now be measured when the engine ambient temperature reaches 130 degrees Celsius, effectively eliminating Mercedes’ advantage.
The timing change brings the new regulations into force at the start of the European racing calendar, coinciding with Monaco. Mercedes has dominated the 2026 season thus far, securing victories in all five races and building a commanding 74-point lead over Ferrari in the constructors’ championship. However, Ferrari team principal Fred Vasseur believes the Additional Development and Upgrade Opportunities (ADUO) process will provide a more significant pathway for competitors to close the gap than simply eliminating the compression ratio loophole.
The ADUO system permits power unit manufacturers to introduce performance upgrades during the season, with those within 2% of the leading unit allowed one upgrade and those further behind permitted two upgrades. This mechanism is designed to maintain competitive balance throughout the championship as teams work to match Mercedes’ early-season dominance.