F1 engine loophole? Why the FIA is banning 2026 plenum designs for the 2027 season. The FIA clarifies the rules on engine plenum structures.

The FIA has clarified the rules regarding engine plenum structures, but a particular solution will be banned starting from 2027.

Plenum rules clarified amid ongoing Mercedes compression ratio issue

While the decision on the alleged higher compression ratio of the Mercedes power units is still pending, the FIA has already banned another technical solution. What draws attention is the timing of these developments: last December, the Federation amended the regulations on engine plenums, simultaneously forbidding certain elements for 2027. Looking at past precedents, there is suspicion that a power unit manufacturer may have devised a special solution that the FIA could only accept for the upcoming season.

The case

In December, the FIA made several changes to the technical regulations, with two particularly notable amendments concerning power units. The first was a clause specifying that compression ratio measurements must be taken at ambient temperature – an addition that was clearly not coincidental, given that the Mercedes case emerged a few days later. This suggests that the other amendment, concerning engine plenums, may also have a specific purpose. Plenums are the intake boxes where air compressed by the turbo is collected before entering the cylinders for combustion.

The FIA sought to clarify the rule to avoid multiple interpretations. Specifically, it stated that any component of the intake system with a certain minimum volume must be classified as a plenum. Additionally, the Federation clarified that plenums are not limited to two – one per bank – but could potentially be one per cylinder.

However, the amendments did not stop there. The FIA also expanded the list of allowed materials for plenums, permitting steel, aluminum, and titanium alloys, whereas previously only composite materials were allowed, with the exception of inserts. Yet, the Federation has already ratified the removal of this metallic material allowance from the 2027 regulations, effectively making metal plenums legal only for the 2026 season.

The scenario

If three clues make a pattern, these three regulatory amendments reinforce the suspicion that a manufacturer developed a unique plenum solution. The FIA’s approach recalls the way the Mercedes DAS system was handled in 2020: fully compliant with the letter of the regulations, but arguably against the spirit of the rules. The Federation had no choice but to acknowledge its legality while explicitly banning it for the following year.

In the current case, it is difficult to speculate exactly what the solution might be. One possibility is that a manufacturer created one or more air-collection chambers near or within the cylinder head, hence the need for a clear plenum classification and the temporary allowance of metallic materials. It is important to remember that managing air pressure this year is particularly complex, given the ban on variable-length intake trumpets and the turbo lag issues following the removal of the MGU-H. We will have to wait for more details, as this situation could reveal some very interesting behind-the-scenes developments.

As the 2026 season approaches, the technical battleground between Ferrari, Mercedes, and the other manufacturers is intensifying. This latest regulatory shift regarding engine plenums suggests that the quest for performance in the new era of Formula 1 is pushing engineers to find every possible loophole, forcing the FIA into a delicate balancing act of maintaining technical integrity while managing groundbreaking innovations that push the boundaries of the rulebook.