Mercedes loophole closed as Monaco rule starts

From 1 June at the Monaco Grand Prix, the FIA will police Formula 1’s 2026 engine compression-ratio limit under hot running conditions as well as cold ones, closing the loophole Mercedes was accused of using to run close to 18:1 despite the regulation’s stated 16:1 cap.

The change had originally been scheduled for 1 August after the Hungarian Grand Prix, but the FIA brought it forward to the start of the European leg after complaints from rival manufacturers including Ferrari, Honda and Audi. Until now, compliance was checked with the engine cold and the car stationary in the pit lane, a wording rivals argued left room for a different effective ratio once the power unit was up to full temperature on track.

Reports through winter testing said Mercedes HPP had exploited that gray area by using thermally expanding materials inside the engine. The principle was that the unit could present the legal 16:1 compression ratio in ambient-temperature scrutineering, then move toward 18:1 in real operating conditions once heat changed the internal geometry.

In a statement published on 28 February, the FIA said, “A significant effort has been invested in finding a solution to the topic of compression ratio.” It added that compression ratio “will be controlled in both hot and cold conditions from 1 June 2026, and subsequently only in the operating conditions (130deg C) from 2027 onwards.”

The timing matters because the rule arrives in the middle of a Mercedes sweep at the start of the new engine era. Mercedes has won all five grands prix so far in 2026 and leads Ferrari by 72 to 74 points in the constructors’ championship, turning a technical clarification into one of the first real tests of whether the field can slow the team’s early momentum.

The size of the suspected advantage has never been settled publicly. Some estimates in the paddock put it at up to three tenths of a second per lap, but Mercedes has consistently played that down. Toto Wolff, Mercedes team principal, called the controversy “a storm in a teacup” and later said, “It doesn't change anything for us,” adding that the team did not oppose the revised measurement protocol.

That leaves Monaco as an important regulatory marker more than a clean competitive benchmark. Several accounts around the paddock have cautioned that the street circuit is one of the least power-sensitive tracks on the calendar, so even if the new 130°C test does trim any Mercedes advantage, the effect may be difficult to isolate immediately. A clearer answer may only emerge once Formula 1 reaches a more representative power-sensitive venue, with Mercedes’ unbeaten start now set against a rule change designed specifically to test whether that edge was built only on interpretation or on a more durable advantage.