Mercedes F1 engine controversy explained: The compression ratio loophole

From the very start of the 2026 Formula 1 season, all eyes in the paddock have been fixed on Mercedes AMG and its new power unit. The Brackley based squad has found itself at the centre of a major technical controversy, one that has overshadowed the opening days of pre-season testing and threatens to spill into the season’s very first race.

At the heart of the debate is an alleged loophole in the 2026 engine regulations revolving around how the compression ratio of the internal combustion engine is measured and enforced.

Under the radical new rules, the maximum geometric compression ratio, a key engine parameter that governs power and efficiency, was reduced from 18:1 in 2025 to 16:1 for 2026 with the aim of balancing performance between legacy and new engine manufacturers.

Crucially, the regulations state that this ratio is to be measured at ambient, cold temperature in the garage, not when the engine is hot and running on track.

Mercedes and, reportedly, Red Bull Racing and Red Bull Powertrains have developed power units that comply with the 16:1 limit during these cold measurements but can, by design and thermal expansion of internal components, achieve effective compression ratios closer to 18:1 when at operating temperature. That allows for greater combustion efficiency and potentially more power once the car is out on track. In Formula 1 terms, even a few horsepower can translate into meaningful lap time gains over a race distance.

Word from the Wolff

Mercedes team principal and CEO Toto Wolff has strongly defended his team’s engine, insisting it is fully legal under the current wording of the regulations and that Mercedes has been transparent with the FIA throughout development.

“Communication with the FIA was very positive all along. Specifically in that area, it’s very clear what the regulations say,” Wolff said when addressing the controversy. In a pointed message to rivals, he added: “Just get your s*** together.”

Wolff has also downplayed the scale of any advantage, suggesting the gain amounts to just a few horsepower rather than the dramatic step some competitors fear. He has maintained that Mercedes is operating strictly within the rules as written, and that any dissatisfaction from rivals should be directed at the wording of the regulations rather than at his team.

At the same time, Wolff acknowledged that discussions among manufacturers have grown increasingly political, with lobbying and private conversations taking place behind the scenes as teams push for either clarification or amendment of the rule.

In the same breath, he's also thrown a spanner in the works and called Red Bull's engine the benchmark for this season, which Max Verstappen has called distraction tactics to put the spotlight on other teams.

Rival teams are not convinced

Several top tier teams have made it clear they are unhappy with what they perceive as exploiting a loophole in the spirit of the regulations.

Audi figures have stressed the need for a level playing field, arguing that if the intention of the rule was to cap compression at 16:1, then allowing engines to exceed that figure under race conditions undermines competitive fairness. Honda representatives have similarly indicated that the current wording leaves too much room for interpretation and that clearer enforcement is required to avoid imbalance.

Within Red Bull Racing, the tone has been more measured. While technical leadership has dismissed talk of illegality as noise, there remains recognition across the paddock that any grey area in such a fundamental engine parameter carries significant competitive weight.

The central concern from rivals is that measuring compression only at cold temperatures does not reflect real world operating conditions. They argue that if engines are effectively running at higher ratios during races, the FIA should consider testing procedures that account for hot, on track performance rather than static garage checks.

Fred Vasseur and Ferrari’s position

Scuderia Ferrari team principal Fred Vasseur has taken a notably pragmatic stance. Despite clear dissatisfaction with the interpretation of the rule, Ferrari has decided not to file a formal protest against Mercedes powered teams at the Australian Grand Prix.

“We are not there to make a protest,” Vasseur said. “We are there to have clear regulations and to have everybody with the same understanding of the regulations. The most important thing for me is to get clarity.”

Ferrari’s plan, therefore, is not to escalate the matter through immediate legal channels but to push for unified interpretation and, if necessary, discussion within the F1 Commission to refine how compliance is measured going forward.

With engine homologation deadlines looming and the season about to begin, the Mercedes compression saga has become more than a technical debate. It is now a political and competitive flashpoint that could shape the early trajectory of the 2026 Formula 1 campaign.