The highly debated decision to measure the compression ratio at ambient temperature was only introduced into the Formula 1 technical regulations with the update approved by the FIA World Motor Sport Council on October 16, 2025. This raises an important question: how could a rule that may benefit Mercedes have passed without objections from the other manufacturers?
At present, no one knows how the compression ratio issue will ultimately be resolved. From June 1, the FIA will introduce an additional verification process, combining the current ambient temperature check with a new test carried out under operating conditions. This “hot” measurement will be performed after bringing the engine oil to a temperature of 130 degrees Celsius, aiming to ensure greater consistency and fairness.
Within Mercedes, there is complete confidence that nothing will change. The team firmly maintains the full legality of its AMG M17 E Performance V6 engine, developed in Brixworth. The FIA itself, through Vincent Pereme—the French engineer responsible for the governing body’s power unit division—had previously given full approval to the solution developed and implemented by the German manufacturer.
According to paddock rumors, the Mercedes power unit had already been homologated before the start of the championship and was capable of passing even the stricter hot test, within the tolerance limit of 16.7:1. Naturally, rival manufacturers who have raised concerns about the team currently dominating the championship with consecutive one-two finishes hold a different view. They argue that the introduction of a dual-chamber concept goes against the intended spirit of the regulations.
The engineers led by Hywel Thomas are believed to have identified a grey area in the rules and fully exploited it. However, there is another aspect that deserves closer analysis. The Formula 1 technical regulations are regularly updated, with changes first discussed within the F1 Commission and then approved by the FIA World Motor Sport Council.
This leads to a particularly intriguing question: who was the “hidden hand” behind the modification of article C5.4.3, approved on October 16, 2025? Until that date, the rule stated: “No engine cylinder may have a geometric compression ratio greater than 16.0:1. The procedure to determine this value is described in document FIA-F1-DOC-C042.” Then, almost unexpectedly, a short addition appeared: “…and must be carried out at ambient temperature.” That clause had not been included before.
This seemingly minor change is what ultimately triggered the compression ratio controversy. Crucially, it was introduced just a few months before the start of the championship, rather than during the initial drafting and approval of the technical regulations. The question remains: who inspired this amendment?
It is easy to assume that the suggestion may have originated from Mercedes. However, it is equally surprising that the measure was approved without any apparent opposition. Where were the representatives of Ferrari, Honda, Audi, and Red Bull Powertrains—teams that later voiced concerns? For now, it remains a mystery.
The 2026 power unit battle is therefore being fought in the fine print of the rulebook as much as on the dyno. While Mercedes appears to have outmaneuvered the field legally, the introduction of the 130-degree “hot test” in June could be the FIA’s way of leveling the playing field. Based on the content of the October amendment, the other manufacturers may have simply been caught napping while the “invisible hand” was at work.
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