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

The 2026 Formula 1 season hasn’t even started, yet the paddock is already ablaze with controversy.

At the centre of the storm lies a technical dispute that could determine who dominates the new era: the compression ratio furore.

Mercedes and Red Bull stand accused by their rivals of exploiting a loophole in the power unit regulations – a claim that 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 threatens to overshadow what should be an exciting new chapter for the series.

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 and, to a lesser extent, Red Bull Powertrains have reportedly found a clever way to exploit this wording.

Through the use of materials with specific thermal expansion properties, these manufacturers have 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 10-13 horsepower, 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.

Red Bull, meanwhile, powers both its own team and Racing Bulls.

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 both manufacturers’ power units comply with the regulations as written, since the 16:1 limit is only verified under static conditions.

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 and Red Bull have 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 has been 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.”

FIA single-seater director Nikolas Tombazis stressed that the FIA wants to resolve the matter before the Australian Grand Prix.

The FIA has convened multiple meetings with engine manufacturers to discuss potential solutions.

Technical experts gathered in late January to explore whether a new framework for testing compression ratios under running conditions could be developed.

A further meeting of the Power Unit Advisory Committee followed, with the topic high on the agenda.

Yet any immediate fix appears unlikely.

The engines will be homologated on 1 March, leaving virtually no time for hardware changes. Even if the FIA were to mandate new testing procedures, modifying power units to comply would take months – realistically not feasible until the 2027 season.

One compromise reportedly under consideration would allow Mercedes and Red Bull to retain their advantage for 2026, on the condition they abandon it for 2027.

Other suggestions include reverting to the 18:1 limit or removing the compression ratio ceiling altogether, allowing all manufacturers to compete on equal terms.

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.

Any serious attempt to replicate Mercedes’ approach wouldn’t be feasible until the 2027 season at the earliest.

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.”

What happens next?

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