The controversy surrounding the alleged Mercedes engine trick has almost completely disappeared from Formula 1 discussion, despite dominating the months before the 2026 season. Rival manufacturers had questioned whether the Mercedes power unit could raise its compression ratio under operating conditions, but the introduction of new FIA checks has brought no public verdict on whether the suspected advantage ever existed.
During the winter, as Formula 1 prepared for one of the most radical regulatory changes in its history, Mercedes faced accusations that its new power unit potentially exploited a grey area in the technical regulations. The fierce debate that accompanied the beginning of the new hybrid era has now vanished, turning “Power Unit Gate” into one of the mysteries of the summer.
How the Mercedes compression ratio controversy began
For months, discussions focused on a possible violation of Article C5.4.3 of the Formula 1 Technical Regulations. Mercedes was effectively accused of developing a system capable of increasing the geometric compression ratio from the permitted 16:1 to as much as 18:1 under certain operating conditions.
A higher compression ratio could theoretically provide a significant improvement in thermal efficiency and power output. The allegation was therefore that Mercedes had found a way to exploit the difference between the conditions in which the engine was initially inspected and those experienced when it reached its normal operating temperature.
Whether the accusation was ever correct remains uncertain. Several theories suggested that the supposed trick involved the controlled and deliberate thermal expansion of specific internal components. Many possible explanations were proposed, although some could be dismissed through relatively straightforward engineering reasoning.
The debate intensified when the FIA acknowledged that certain manufacturers may have found ways to alter the compression ratio as the engine became hot. The wider background to the Formula 1 compression ratio controversy centred on whether a power unit that complied during a cold inspection also had to remain below the limit under every operating condition.
FIA changes Mercedes engine compression ratio checks
Shortly before the start of the championship, the FIA revised the wording and enforcement of Article C5.4.3. The regulation established that no engine cylinder could have a geometric compression ratio greater than 16.0 when measured under the following conditions:
“Until 31 May 2026: when the engine is at ambient temperature.
“From 1 June 2026 to 31 December 2026: when the engine is at ambient temperature and when the engine is at 130°C.
“Any component, assembly, mechanism or integrated arrangement of components designed or functioning to increase the compression ratio under operating conditions beyond 16.0 is prohibited.”
The FIA therefore began applying hot-condition checks from the Monaco Grand Prix, the first race held after the 1 June deadline. The cylinders of every power unit manufacturer could be examined to ensure that the engines remained within the prescribed compression ratio under both ambient and elevated-temperature conditions.
However, it remains difficult for observers outside the governing body and the manufacturers to understand precisely how those measurements are performed. Each power unit manufacturer is responsible for defining the procedure used to demonstrate compliance, provided it follows the instructions contained in FIA-F1-DOC-042.
That document is not publicly available. Each manufacturer’s procedure must nevertheless be approved by the FIA Technical Department and included in its power unit homologation dossier.
Could thermal expansion really produce an 18:1 ratio?
The practical impact of measuring an engine at 130°C is another important part of the debate. Compared with a typical ambient temperature, the difference is approximately 100°C.
The materials permitted within the power unit are strictly regulated. Based on their known properties, a temperature increase of that magnitude would cause the relevant components to expand, potentially changing the combustion-chamber volume by less than one per cent.
Determining whether the final effect would increase or reduce the compression ratio is not straightforward. It would depend on the interaction between several components expanding simultaneously, with the precise behaviour known only to the manufacturers that designed them.
What appears clear is that ordinary thermal expansion alone would not seem sufficient to prove that an engine could move from a compression ratio of 16:1 to 18:1. Such a significant increase would probably require a different mechanism rather than the natural expansion of conventional components.
The FIA’s revised regulation addressed that possibility by prohibiting any component, assembly or mechanical arrangement designed to increase the compression ratio beyond 16.0 under operating conditions. The change therefore covered not only thermal behaviour but also potential mechanical systems intended to alter the combustion-chamber geometry.
Mercedes power unit advantage was not only about combustion
Mercedes’ strength in 2026 has not depended exclusively on the internal combustion engine. Its power unit has also excelled in electrical energy recovery, system integration and software management.
The Stuttgart manufacturer’s hybrid package has demonstrated particularly effective energy deployment, helping the Mercedes-powered cars extract strong performance across a range of circuits. The internal combustion engine itself has not necessarily been regarded as the most powerful unit on the grid in isolation.
It is therefore far from certain that the FIA’s restrictions on the thermal side of the power unit harmed Mercedes. In fact, the results on the circuit appear to suggest the opposite, with the W17 and the Mercedes engine remaining highly competitive after the hot-condition checks were introduced.
That leads to the central questions. What happened after the FIA intervention? Why has discussion of Mercedes’ alleged compression ratio trick stopped? And why has there been no suggestion that Mercedes was forced to abandon the concept once the new checks began?
Did Mercedes simply bring its engine into compliance?
The first possible explanation is also the simplest. Whichever manufacturer was exploiting the compression-ratio grey area may have modified its power unit or operating procedures to ensure full compliance with the revised regulation.
In that scenario, the matter would effectively have been resolved. The engine would have passed the FIA’s hot and cold measurements, ending any grounds for further controversy.
However, such a straightforward outcome would normally be expected to generate some information within the paddock. If a manufacturer had been required to remove a meaningful technical advantage, rival teams would probably have noticed a change in performance or discussed the consequences more openly.
No obvious evidence of such a development has emerged. Mercedes has continued to perform strongly, while public discussion of the controversy has faded rather than reaching a definitive conclusion.
Did Formula 1 reach a political compromise?
A second scenario is more intriguing, although it remains speculative. Mercedes may have defended its interpretation of the original grey area while pointing out that any attempt to outlaw or exclude its homologated power unit could affect four teams—Mercedes, McLaren, Williams and Alpine—representing a substantial part of the grid.
The FIA would then have found itself under pressure from rival manufacturers demanding stricter enforcement while also facing the potentially enormous sporting consequences of taking action against one engine supplier and all its customer teams.
A political solution may therefore have been considered. The governing body introduced additional checks whose exact methodology remains confidential, allowing every manufacturer to demonstrate compliance through an FIA-approved procedure.
Such an outcome could have satisfied all parties without immediately damaging any competitor. The manufacturers that demanded hot-condition measurements obtained a regulatory change, while Mercedes and its customers could continue racing provided their power units passed the approved checks.
The theory cannot be presented as established fact. There has been no public evidence that the FIA deliberately designed an ineffective test or negotiated a settlement intended to protect Mercedes. However, the lack of a clear technical verdict has allowed questions to remain.
The FIA’s pre-season intervention was initially portrayed as the conclusion of the dispute, with Ferrari and Red Bull among the manufacturers pushing for stricter control of the alleged Mercedes power unit grey area. Yet the championship continued without confirmation that the suspected 18:1 compression ratio had been used, removed or conclusively disproved.
With one regulatory amendment, the controversy that shook the beginning of Formula 1’s new era disappeared almost without trace. Mercedes continued winning, rival manufacturers stopped discussing the issue publicly and the so-called engine trick was gradually forgotten—apparently to everyone’s satisfaction.
Leave a Reply