From 1 June, the FIA will change how engine compression ratios are measured, closing one of Formula 1’s most discussed technical grey areas. This follows speculation in the paddock that Mercedes-powered teams may have benefited from a design that complies with the regulations during official inspections, but operates differently under racing conditions.
With the championship battle intensifying, attention is now turning to whether the rule adjustment will have any meaningful impact on performance.
Why did the compression-ratio debate become such a major issue?
At the heart of the controversy lies the compression ratio used in Formula 1’s turbo-hybrid internal combustion engines.
This measures how much the air-fuel mixture is compressed inside the cylinder before ignition. Under the 2026 regulations, teams are limited to a maximum compression ratio of 16:1. The principle is straightforward: the greater the compression, the more energy can be extracted from the same amount of fuel, increasing efficiency and potentially improving performance.
The FIA introduced the 16:1 limit to control development costs and make the sport more accessible to new manufacturers entering the next engine era of Formula 1.
This was considered particularly important for new manufacturers such as Audi and Cadillac, who joined under regulations designed to reduce barriers to entry compared to previous power unit cycles.
Mercedes’ engine design has come under scrutiny
Questions have emerged after reports suggested that Mercedes may have found a way to operate at a significantly higher compression ratio once the engine reaches racing temperatures, while remaining within the regulations during FIA inspections.
According to speculation in the paddock, the Mercedes power unit could reportedly measure at the required 16:1 ratio when cold, but increase substantially once fully warmed up due to thermal expansion and changes within the combustion chamber design.
The theory centres on a temperature-sensitive pre-chamber arrangement within the cylinder head. While this concept has been widely discussed throughout the paddock, there has been no public confirmation that Mercedes was running at the rumoured 18:1 ratio often cited in reports.
What is known is that all Mercedes power units passed FIA legality checks under the regulations in place at the time. The governing body’s power unit department approved the design, meaning no rules were breached.
Instead, rivals viewed the situation as an example of a team exploiting a gap in the measurement procedure rather than violating the written regulations.
FIA rewrites the regulations to remove uncertainty
In response, the FIA has revised Article C5.4.3 of the technical regulations.
Until 31 May, compliance was assessed using the original cold-engine measurement procedure. From 1 June until the end of the season, engines must remain below the permitted compression ratio limit when cold and when measured at an oil temperature of 130°C.
The revised wording also explicitly prohibits any component, system, or mechanism designed to increase the compression ratio beyond the permitted value under operating conditions.
From 2027 onwards, compliance checks will be based exclusively on operating-temperature measurements, permanently eliminating the possibility of exploiting differences between cold and hot engine conditions.
There are also ongoing discussions regarding measurement tolerances, with some sources in the paddock suggesting that the practical limit may be slightly higher than the nominal figure of 16:1. However, no official FIA clarification has publicly altered the published regulation.
Will the new rule affect the championship battle?
The biggest question remains whether the directive will have any measurable impact on Mercedes-powered teams.
The timing could work in the manufacturer’s favour. Teams are allocated a limited number of power units for the season, and the regulation change comes at a time when many competitors would already be introducing new engines into their rotation.
If modifications to engine hardware are required, Mercedes could potentially incorporate them during a scheduled power-unit change without incurring further penalties.
There is also speculation that fuel development may be a factor. Engine and fuel suppliers work closely together to optimise combustion characteristics, so any change to compression ratios could necessitate adjustments to fuel formulations.
Whether this would result in a noticeable loss of performance remains unclear.
What is certain is that the FIA has acted decisively to remove a regulatory grey area. The upcoming races will reveal whether this change was merely a technical clarification or if it could reshape the balance of power in the 2026 Formula 1 championship.
Thiago Treze is a Brazilian motorsport writer at TJ13 with a background in sports journalism and broadcast media, alongside an academic foundation in engineering with a focus on Computational Fluid Dynamics (CFD). This combination of technical knowledge and editorial experience allows Thiago to approach Formula 1 from both a performance and narrative perspective.
At TJ13, Treze covers driver performance, career developments, and key storylines across the Formula 1 grid, while also analysing the technical factors that influence competitiveness. This includes aerodynamic development trends, simulation-driven design approaches, and the engineering decisions that shape race weekend outcomes.
His reporting bridges the gap between human performance and machine development, helping readers understand how driver execution and technical innovation interact in modern Formula 1. Coverage often connects on-track events with the underlying engineering philosophies that define each team’s approach.
With a global perspective shaped by both journalism and technical study, Thiago also focuses on Formula 1’s international reach and the different ways the sport is experienced across regions.
Treze has a particular interest in how Computational Fluid Dynamics and aerodynamic modelling contribute to car performance, offering accessible explanations of complex technical concepts within Formula 1.
