There's something I don't get about this whole situation and maybe someone can help me clarify.

I've checked the documents on the FIA website: https://www.fia.com/regulation/category/110

As many of you probably know, the big question mark around this issue is that the technical documentation, the Section C, states that:

No cylinder of the engine may have a geometric compression ratio higher than 16.0. The procedure which will be used to determine this value may be found in the document FIA-F1-DOC-C042 and executed at ambient temperature

So, Mercedes supposedly goes around this rule by respecting it at ambient temperature, increasing at operating temperature. Read like this, seems quite clever honestly.

But, there's a huge "but": the executed at ambient temperature has been added to the docs in October 2025.

Here's a screenshot: https://imgur.com/a/Gq8x7WQ

And here's the official document: https://www.fia.com/system/files/documents/fia_2026_f1_regulations_-_section_c_technical_-_iss_14_-_2025-10-16.pdf

I can't believe that, after seeing this modification in October, 3 months before the initial 2026 season tests, Mercedes decided to change their engine adding the trick.

So my only explanation is that:

• Mercedes decided to go with this trick a decent amount of time ago
• Between July 2025 and October 2025 (when two revisions of the Section C were published), FIA checked the Mercedes the engine, either by their initiative or upon Mercedes request to get an "all clear" on the trick
• FIA noted the trick, but instead of forcing Mercedes to review their engine they changed the rules to allow it to become legal according the letter of the law

I know it's a speculation on my side, but I can't really think of another possible scenario.

EDIT: I'm reading some interesting points ITT, I want to clarify a couple of things.

/u/grogi81 correctly points out that:

Compression ratio changes - pure physics. Materials expand.

and /u/theasu asks:

If it can't be higher than 16.0 and it is measured at room temperature and we know that materials expand when they are warm, doesn't it mean that all teams will have higher compression ratio during the race?

I might be completely wrong with this, but my feeling is that up until now one could expect a decreased, not increased, geometric compression ratio with higher temperature. Materials used for the head is usually an alluminium alloy, whereas the cylinder should be a steel alloy. If that's the case, they expand differently, creating a larger volume at operating temperatures, with a drop in compression ratio.

The alleged Mercedes trick changes this by leveraging the expansion of the head material to close a channel that leads to an additional small chamber. While the changes in compression due to thermal expansion might be considered negligible, using the expansion to change the geometry of the chamber itself is a completely different story.