Explained: The engine compression row that risks overshadowing F1's new era

The 2026 grand prix season hasn’t even started, yet the paddock is already ablaze with typically technical F1 controversy over engine compression ratios.

F1 power unit suppliers are currently voting on whether to clamp down on a loophole that Mercedes is thought to have exploited, potentially giving its engine an extra 10-13bhp that would be worth between 0.2 and 0.3sec per lap.

As well as its own factory team, Mercedes also supplies McLaren, Williams and Alpine. Rivals fear being left behind, and rumours of Mercedes’ advantage has triggered heated exchanges, secret letters to the FIA, and some choice words from Mercedes team principal Toto Wolff.

With just weeks until the season-opener in Australia, the row continues to rumble on. Racing’s governing body, the FIA, has now called a vote of all power unit manufacturers on whether to impose a new test from August this year, which would eliminate the loophole. If the vote passes — and the rumours are correct — then Mercedes would be forced into an urgent redesign of its engine before the end of the summer break.

What is compression ratio?

Before diving into the controversy, it’s worth understanding what all the fuss is about. For those who don’t live and breathe internal combustion engines, the compression ratio might sound intimidating – but the concept is actually quite straightforward.

The compression ratio is simply the relationship between the largest and smallest volume inside a cylinder.

When the piston is at its lowest point (bottom dead centre) inside the cylinder, the cylinder has its maximum volume. When the piston reaches the top of its stroke (top dead centre), the volume is at its minimum. The compression ratio is the comparison between these two volumes.

For example, an 18:1 compression ratio means the mixture of air and fuel is compressed to one-eighteenth of its original volume before ignition. The higher the ratio, the more the mixture is squeezed – and crucially, the more power can be extracted from the same amount of fuel.

This matters in Formula 1, where extracting every last drop of performance from a limited fuel supply is paramount. A higher compression ratio can deliver two key benefits: either more power from the same fuel quantity, or the same power from less fuel.

In F1’s tightly regulated world, where fuel flow is restricted, that could be the difference between fighting for podiums and languishing in the midfield.

The loophole

The 2026 regulations reduced the maximum compression ratio from 18:1 to 16:1 – a change designed to make the new power units easier for prospective manufacturers to develop and to align with the F1’s shift towards sustainable fuels.

The rules, in theory, are clear: no cylinder may have a geometric compression ratio higher than 16.0.

In practice, however, it’s not so clear-cut.

The controversy revolves around a crucial detail in Article C5.4.3 of the technical regulations: compression ratio measurements are taken “at ambient temperature” – in other words, when the engine is cold and stationary in the garage.

Mercedes has reportedly found a clever way to exploit this wording.

Through the use of materials with specific thermal expansion properties, it has designed components that expand when the engine reaches operating temperature on track.

This thermal expansion effectively increases the compression ratio beyond 16:1 when it matters most — during actual racing conditions.

Reports suggest Mercedes may have achieved compression ratios approaching the old 18:1 limit while running, despite passing the static 16:1 check with flying colours.

This could translate to a power advantage of 10-13bhp, or roughly two to three tenths of a second per lap depending on the circuit.

The implications are obviously significant.

Mercedes supplies not only its own works team but also McLaren, Williams, and new customer Alpine , meaning potentially eight cars on the grid could enjoy this advantage.

Former Mercedes and Ferrari technical director Ross Brawn described the approach as a “clever interpretation of the regulation” rather than anything untoward.

Indeed, the FIA has confirmed that its power units comply with the regulations as written, since the 16:1 limit is only verified under static conditions.

The leak

Details of Mercedes’ engine are understood to have emerged from an employee who transferred from its power unit project to Red Bull, taking knowledge of the loophole with them.

It’s thought that Red Bull then tried to exploit the same trick, but with limited success.

The FIA’s response

Faced with mounting pressure from Ferrari, Honda, and Audi – who have jointly written to the governing body demanding answers – the FIA finds itself in an unenviable position.

On one hand, Mercedes has followed the letter of the law. The regulations explicitly state that measurements are taken at ambient temperature, and both manufacturers pass these checks. Changing the rules mid-stream because other teams failed to exploit the same area would set a troubling precedent.

On the other hand, there’s the question of sporting intent. Did the FIA genuinely intend for compression ratios to reach 18:1 under operating conditions, having explicitly reduced the limit to 16:1?

The FIA’s initial response was measured. A spokesperson emphasised that “the regulations clearly define the maximum compression ratio and the method for measuring it, which is based on static conditions at ambient temperature. This procedure has remained unchanged despite the reduction in the permitted ratio for 2026.”

However, the statement also left the door ajar: “If necessary, adjustments to the regulations or measurement procedures can be considered in the future.”

Following multiple meetings with engine manufacturers to discuss potential solutions, including a new framework for testing compression ratios under running conditions, the FIA’s Power Unit Advisory Committee called a vote on February 18.

“A proposal has been submitted whereby, from August 1, 2026, compliance with the compression ratio limit must be demonstrated not only at ambient conditions, but also at a representative operating temperature of 130C.

“The vote has been submitted to the power unit manufacturers, and its outcome is expected within the next 10 days.”

The delayed introduction is likely to be the earliest realistic opportunity to change the regulations. Engines will be homologated on 1 March, leaving virtually no time for hardware changes. Even meeting the August test could be a push for Mercedes.

However, the alternatives may have been less palatable: both in leaving the regulations as is and risking a split in the gris between Mercedes with a sizeable advantage and its rivals lagging behind, or in outlawing the Mercedes power unit, leaving eight cars unable to score points — an outcome described as “catastrophic” by Motor Sport‘s grand prix editor Mark Hughes in our F1 Show podcast

Wolff’s response: “Get your s*** together”

Against this backdrop of regulatory uncertainty and rival complaints, Mercedes boss Toto Wolff delivered a characteristically forthright defence of his team’s position at the team’s 2026 launch on Monday.

Asked about the controversy, the Austrian didn’t hold back.

“I just don’t understand that some teams concentrate more on the others and keep arguing a case that is very clear and transparent,” Wolff told media.

“Communication with the FIA was very positive all along and it’s not only on compression ratio, but on other things too.”

He continued: “Specifically in that area, it’s very clear what the regulations say. It’s very clear what the standard procedures are on any motors, even outside of Formula 1. So just get your s*** together.”

Wolff’s message to rivals was blunt: focus on your own performance rather than complaining about Mercedes’ ingenuity.

“They are just doing secret meetings and sending secret letters and keep trying to invent ways of testing that just don’t exist,” he said. “I feel like I can just say at least from us here, we are trying to minimise distractions, looking more at ourselves than at everybody else when it’s pretty clear what the regs say.”

The Mercedes boss was adamant about the legality of his team’s power unit.

“The power unit is legal,” Wolff stated emphatically. “The power unit corresponds to how the regulations are written. The power unit corresponds to how the checks are being done. The power unit corresponds to how these things are measured in any other vehicle and everything else.”

Wolff suggested that rivals’ complaints might be more about managing expectations than genuine technical concerns.

“Maybe we’re all different,” he said. “Maybe you want to find excuses before you even started for why things are not good. If somebody wants to entertain themselves by distraction, then everybody’s free to do this.”

When asked whether he expected protests after the first race, Wolff remained confident in Mercedes’ position, backed by FIA assurances.

The team had completed an impressive 500 laps during the Barcelona shakedown – more than any other team – and appears to be heading into the new campaign as early favourite.

The stakes

The controversy echoes previous regulatory disputes in F1 history, most notably the 2009 double-diffuser saga.

In that instance, Brawn, Toyota, and Williams exploited clever wording in the rules to gain a significant aerodynamic advantage. Despite protests from rivals, the FIA deemed the designs legal, and Brawn went on to win both championships.

The compression ratio row differs in one crucial respect: the timescales involved in power unit development. While aerodynamic solutions can be copied relatively quickly, engine modifications require extensive redesign and testing – a process that takes many months.

There’s a potential earlier window through the Additional Development and Upgrade Opportunities (ADUO) system, which allows struggling manufacturers to implement upgrades if they fall significantly behind.

However, it has been questioned whether ADUO truly accounts for the lengthy gestation period of power unit developments.

The 2026 regulations also introduce financial disincentives for bringing additional engines. Unlike previous years, extra power units for performance reasons now come directly out of manufacturers’ cost cap budgets, meaning there’s a significant penalty for attempting to close the gap mid-season.

Audi technical director James Key articulated rivals’ concerns at his team’s launch.

“We have to, as we do, trust the FIA with making the right decisions here,” Key said. “It’s new regs. You’ve got to have a level playing field. If someone came up with a clever diffuser and you said it’s not the right thing to do, no one else can have it, but you can have it for the rest of the year – it doesn’t make sense. We’d never accept that.”

The growing coalition

Adding further drama to the situation are efforts to force a rule change that are now accelerating behind the scenes, with Mercedes’ rivals attempting to pull together a unified proposal in time for the Australian GP.

Ferrari, Audi and Honda, who jointly authored the initial letter to the FIA, have now reportedly secured the support of Red Bull, creating a potential voting group of four manufacturers.

Red Bull’s alignment with the complainants is particularly noteworthy given that the team was understood to have pursued similar thermal expansion benefits to Mercedes.

Reports suggest Red Bull may have been unable to replicate the performance gains it had anticipated, prompting a strategic shift towards closing the loophole rather than exploiting it.

This four-manufacturer coalition is significant because it opens the door to the super-majority vote required under Power Unit Advisory Committee governance procedures to implement an immediate rule modification.

Any such change would need backing from four of the five current manufacturers, plus support from both the FIA and FOM.

The rivals’ proposed solution would mandate compression ratio measurements when engines are running at operating temperature, rather than under ambient conditions.

A sudden FIA U-turn approving last-minute regulatory amendments would represent a significant policy shift and could have dramatic implications for the competitive picture heading into F1’s new era.

What happens next?

With the Australian Grand Prix looming on 8 March, the clock is ticking for a resolution.