F1 Engine Compression Ratio - What is it and why is it so controversial?

Didn't expect you'd be looking up articles online about engine compression a few months ago, did you? That's what new F1 regulations can do to you. Luckily, we're here to help.

Before we dive into the controversy surrounding Mercedes and Red Bull's engines, let's get a basic grounding in what a compression ratio...well, is.

Simply put, it's the ratio between the largest and smallest volume inside a cylinder (current F1 engines have six cylinders, known as V6 engines).

When the piston inside the cylinder is at its lowest point, it's at its maximum volume. When it's at the highest points of its stroke, it's at its minimum. The compression ratio is simply the ratio between those two volumes.

F1 HEADLINES: Mercedes star in pre-season crash and Lewis Hamilton gets 'goosebumps'

Why do engine compression ratios matter?

Great question, glad you asked!

The higher the ratio (in F1's specific case, 18:1 vs 16:1), the more the fuel and air inside the cylinder is compressed – to 1/18th of its original volume, or 1/16th (other compression ratios are available, but these are the ones germane to F1's current kerfuffle).

The more the fuel and air is compressed, the more power can be extracted from it at ignition point. In other words, the higher your compression rate, the more power you're able to extract from a given amount of fuel. Given that 'more power' is what every engine manufacturer in the sport is looking for...that matters quite a lot.

What are F1's engine compression rules?

Under the previous set of regulations, teams were allowed to run a compression ratio of 18:1. However, under new engine regulations introduced for 2026, that ratio limit is now 16:1.

That seems simple, what's the controversy?

You are just killing it with the questions today! Well, F1's measuring system is, under the regulations, done 'at ambient temperature', when the engine is cooled down and going nowhere in the garage.

The clever little elves at Mercedes (and Red Bull, but apparently less aggressively) have managed to get their brand new power units running at a compression rate of around 18:1 when they're hot and on track – while still passing the 16:1 ambient temperature checks.

That's a theoretical gain of about a dozen horsepower, or in the region of a quarter of a second a lap. That is...not insignificant.

Mercedes insist that their system is entirely legal. After all, it passes the tests laid down in the rules, and that's what counts – and the FIA, for their part, have confirmed that the letter of the law is being obeyed. Rival manufactureres are less impressed by the ingenuity.

Unfortunately for them (and the three other teams using Mercedes engines in 2026), Red Bull's failure to replicate the Silver Arrows' success with the system appears to have shifted them to the 'against' camp along with Honda, Ferrari and Audi. That creates a formidable voting bloc if this issue comes down to a vote.

We're unlikely to have heard the end of this saga, with the FIA clear that they want to resolve the controversy before the first race of the year in Melbourne. There was one important phrase in the statement they released confirming that Mercedes' engine is legal, and it reads: “If necessary, adjustments to the regulations or measurement procedures can be considered in the future.”

The most likely outcome at this point appears to be the power unit being ruled legal for this year, with time running out to decide upon and implement a change before Melbourne, with a move to clamp down on the practice for the 2027 season.

What have they said?

Toto Wolff, Mercedes team principal: “I just don’t understand that some teams concentrate more on the others and keep arguing a case that is very clear and transparent.

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

Mattia Binotto, Head of the Audi F1 project: "If that's true, firstly, it certainly makes a significant difference in terms of performance and lap time. That would definitely make a difference when we get to the track and competition."

FIA single seater director Nikolas Tombazis: "As these engineers are very clever and always pushing for an advantage, some have found ways to potentially increase it [the compression ratio] when the engine is running hot, and that is the discussion we're having now.

"We've spent a lot of time discussing how we solve those issues, and our intention is of course to solve them for the start of the season. We don't want to have controversies. We want people to be competing on the track, not in the courtroom or in the stewards' room, and that's what we try to do."

Audi technical director James Key: “We have to, as we do, trust the FIA with making the right decisions here. 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.”

READ MORE: Adrian Newey's F1 design questioned by team rival

Related