An unconfirmed theory about how Mercedes and Red Bull have circumvented F1’s power unit compression ratio regulations has emerged in Germany.

Pre-season has been dominated by concerns that a loophole in the FIA, said to be exploited by Mercedes and Red Bull, could have significant competitive consequences through the F1 2026 season.

Mercedes power unit compression ratio theory put forward

The loophole is concerned with the compression ratio which for 2026 has been reduced to 16:1 under the new regulations, down from 18:1.

It’s understood both Red Bull and Mercedes have found a way of producing a 16:1 reading when in the garage – where it is tested at ambient temperature – but increasing to 18:1 out on track.

Much discussions take place behind the scenes, including meetings with the FIA and the power unit advisory committee as recently as Thursday, with Auto Motor und Sport (AMuS) now proposing an unconfirmed theory on how such a system could work.

The German outlet suggests that it is to do with how the individual components expand differently under heat and that Mercedes has used 3D printing to create special pistons for this reason – which raises the compression ratio to 17:1.

According to AMuS, Mercedes has also connected a small pocket with a volume of one cubic centimetre to the combustion chamber via a thin channel near the pre-chamber spark plug at the very top of the cylinder.

During a static test, this volume fills up as the piston moves. When the power unit is at higher temperatures, a critical pressure value must be exceeded so that the volume does not expand into the narrow channel during compression. The impact of that would be to further increase the compression ratio.

AMuS suggests that Red Bull also discovered this trick but has yet to build a reliable solution to exploit it.

During the car’s launch this week, Mercedes team principal and part-owner Toto Wolff insisted that everything his team has produced was legal.

“When it comes to the engine question, I just don’t understand that some teams concentrate more on the others and keep arguing a case that is very clear and transparent,” he said.

More on the engine talk from PlanetF1.com

* FIA moves to shut down F1 2026 engine ‘loophole’ fears ahead of Australian GP
* F1 2026 engine ‘loophole’ claims: What’s really going on?
* Toto Wolff tells F1 rivals ‘get your s*** together’ over PU loophole

“Communication with the FIA was very positive all along. And it’s not only on compression ratio, but on other things too – and specifically in that area, it’s very clear what the regulation says, very clear what the, let’s say, standard procedures are on any motors, even outside of Formula 1.

“So, just get your s*** together and…you know, doing secret meetings and sending secret letters, and keep trying to invent ways of testing that just don’t exist is…

“I can just say at least from us here, we are trying to minimise distractions, and minimising distractions is looking more at us than everybody else.

“It’s pretty clear what the what the regs say, and also pretty clear what the FIA has said to us and has said to them so far.

“But maybe, we’re all different. Maybe you want to find excuses before you even started, why things are not good.

“Everybody needs to do it at the best of their ability, but that is really not how is not how we how we would do things, especially not after you’ve been told a few times that that is fine, it’s legal, and it’s what the regulations say.

“Again, if somebody wants to entertain themselves by distraction, then everybody’s free to do this.”

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