To understand what we are talking about, we need to return to the basics of thermodynamics. The compression ratio of an engine is the ratio between the cylinder volume when the piston is at the bottom of its stroke and when it is at the top. The higher this ratio, the more powerful the engine. The FIA set the limit at 16:1 for 2026, down from 18:1 under the old regulations, notably to attract new engine manufacturers and make the formula more accessible.

The problem: the regulations specified that this value had to be measured "at ambient temperature", meaning with a cold engine, at rest. Yet metals expand under the effect of heat. According to rivals' suspicions, Mercedes may have designed an engine architecture that intelligently exploits the properties of its materials to remain under 16:1 during cold checks, while reaching a significantly higher ratio once the engine reaches its actual operating temperature.

In short: the Mercedes engine was legal when cold. But nobody knew what it was doing when hot. And it is precisely this grey area that the new rule has come to close.

What makes this situation even more explosive is that the supposed Mercedes advantage does not only concern the Brackley team. Alpine, Williams and McLaren also run the Mercedes engine in 2026. If the thermal loophole has genuinely delivered a power advantage since the start of the season, these three teams have benefited from it too, whether knowingly or not.

This raises an immediate question: was McLaren's advantage over Ferrari and Red Bull in the early part of the season partly the result of an illegitimate engine benefit? And what will happen in the coming weeks if Mercedes is forced to adapt its architecture?

Mercedes says nothing needs to change

Mercedes' official position is clear: according to the German manufacturer, no engine intervention will be required to comply with the new rules. This is impossible to verify from the outside, and the paddock is therefore treating it with pronounced scepticism. Other sources believe Mercedes will need to modify the cylinder head, possibly by adjusting the combustion chamber volume. A modification that would take time and consume resources under the cost cap.

The irony of the situation is that Monaco is precisely the circuit least sensitive to engine power in the entire calendar. The streets of Monte-Carlo, with their slow corners, short straights and intense braking zones, favour aerodynamics, mechanical balance and driver mastery far more than extra horsepower from an engine.

In concrete terms: if Mercedes loses a few horsepower in Monaco, the impact on the race result will probably be minimal. The circuits where the effect will be most visible are: Barcelona (fast straights), Silverstone and Monza. That is where the real verdict will be delivered.

• Monaco (June 7): Estimated impact very low. Circuit too slow for engine power to be decisive. Leclerc remains the favourite from pole.
• Barcelona (June 14): First real test of the impact. Long pit straight, several fast downforce sections where power counts.
• Silverstone (July): High-speed circuit par excellence. If Mercedes has lost power, it will be measurable here.
• Monza (September): The temple of speed. The definitive verdict on the extent of the advantage Mercedes has lost.

The question is inevitable and will fuel debate until the end of the season. Were Kimi Antonelli's four consecutive wins since Melbourne partly based on an engine advantage that should never have existed? The honest answer is: nobody really knows. The FIA itself never officially quantified the gain linked to the thermal loophole, if it genuinely exists at all.

Nikolas Tombazis, the FIA's head of single-seaters, even declared during the winter that he did not believe the subject deserved "such a level of attention". Which did not prevent the federation from changing the regulation two months earlier than planned.

What is certain: if the loophole had no impact, the FIA would not have brought its implementation forward by two months. The fact that five manufacturers voted unanimously to modify the regulation is in itself a powerful signal. You do not change a rule in a hurry if the problem is cosmetic.