FIA changes F1 engine rules mid-season: New compression ratio checks introduced

Formula 1 engine rules update: The FIA, Formula One’s governing body, has announced a key change to how engine compression ratios will be measured during the 2026 season and ahead of the 2027 campaign.

The decision follows discussions with manufacturers amid scrutiny over how teams interpret the new power unit regulations.

What has changed in the F1 engine rules?

Under the new 2026 regulations, F1 engines are limited to a 16:1 compression ratio — meaning the air-fuel mixture inside the engine cylinders cannot be compressed more than 16 times before ignition. The compression ratio directly affects power output and efficiency.

Previously, compliance checks were conducted at ambient (cold) temperature only.

The rule change:

From June 1, 2026: Compression ratio will be tested in both cold and hot conditions.

From 2027 onward: The ratio will be measured only under hot operating conditions.

This means teams will no longer be able to rely on engine behavior differences between cold and heated states.

There are seven races scheduled before the new testing method comes into effect on June 1.

Why was the rule changed?

Rival teams reportedly questioned whether Mercedes-AMG Petronas Formula One Team had found a way to optimize engine components so that the compression ratio complied during cold inspections but behaved differently when the engine reached racing temperatures.

Mercedes has denied any wrongdoing, stating that its engine is fully legal under existing rules.

The FIA described the move as part of ongoing evaluation following the introduction of the major 2026 regulation overhaul.

Impact on F1 teams and manufacturers

The regulation change affects all engine suppliers, including Mercedes, which also provides power units to customer teams such as McLaren, Alpine and Williams.

The FIA said the 2026 rule set represents one of the most significant technical changes in recent F1 history. 

Why the compression ratio matters in F1

The compression ratio determines how efficiently fuel burns inside the engine. A higher ratio can mean more power and improved thermal efficiency but exceeding regulatory limits could provide an unfair performance advantage.