Mercedes F1 trick now illegal as new FIA rule officially in effect

The Monaco Grand Prix is to be the first F1 race under the new technical regulations, which closed Mercedes' loophole with its power unit compression ratio.

During pre-season, it was reported that Mercedes HPP had found a way to increase the compression ratio of its new engine to 18:1, with the limit in the rules being 16:1. 

The wording of the rules meant that the ratio would only be measured at ambient temperatures whilst the car was in the pit-lane, therefore creating a possibility of it running higher when the car was on-track. 

After an outcry from rival power unit manufacturers, the FIA agreed to change the regulations from June 1st, with the compression ratio now set to be measured when the engine ambient temperature is 130 degrees.

It is believed that through the use of heat-expanding metals in the engine design, Mercedes HPP was able to create the conditions for the 18:1 compression ratio.

The closing of the loophole was originally planned for August 1st, thus after the Hungarian GP, but this was brought forward to the start of the European leg of the season, and will be in force for the trip to Monte Carlo. 

Mercedes has won all five grands prix to take place in 2026 thus far, and enjoys a commanding 74-point lead over Ferrari in the constructors' championship - although its boss Fred Vasseur believes the Additional Development and Upgrade Opportunities (ADOU) process will allow teams to catch Mercedes quicker than simply through the compression ratio loophole being closed.

ADUO allows power unit manufacturers the ability to bring performance upgrades to their engines, with those deemed to be within 2% of the leading unit allowed one per season and those more than 2% behind permitted two.