Mercedes F1 compression ratio loophole closed as new FIA rule takes effect in Monaco

The FIA has officially closed Mercedes' power unit compression ratio loophole, with the new technical directive taking effect from the Monaco Grand Prix. The rule change requires the compression ratio to be measured at an engine ambient temperature of 130 degrees Celsius, eliminating the advantage Mercedes gained by running an effective 18:1 ratio against the regulation limit of 16:1.

Why it matters:

This closes a significant technical advantage for Mercedes, who have won all five races in the 2026 season and lead the constructors' championship by 74 points over Ferrari. While the team remains dominant, the rule change could gradually shift the competitive balance as rivals adapt and the FIA tightens oversight.

The details:

• Mercedes HPP exploited a loophole in the regulations that only measured the compression ratio at ambient temperatures while the car was in the pit lane. This allowed the team to use heat-expanding metals in the engine design to achieve an 18:1 ratio on track.
• After protests from rival power unit manufacturers, the FIA agreed to amend the rules. The measurement will now be taken at 130°C, effectively closing the loophole.
• The original deadline for the change was August 1st, but it was brought forward to June 1st, meaning it will be enforced for the first time in Monte Carlo.

What's next:

Ferrari team principal Fred Vasseur believes the closure of the compression ratio loophole alone won't immediately close the gap. He points to the Additional Development and Upgrade Opportunities (ADUO) process as a more impactful mechanism for catching Mercedes. Under ADUO, power unit manufacturers within 2% of the leading unit are allowed one performance upgrade per season, while those more than 2% behind are permitted two upgrades. This could accelerate convergence as teams push to close the performance deficit.