Mercedes’ rivals have succeeded in securing a rule change following a pre-season technical row over engine performance.
Audi, Ferrari, Red Bull and Honda had been pushing for a change in the regulation governing compression ratios on the basis they believed Mercedes had secured an advantage through clever use of materials technology.
Formula 1’s governing body the FIA said on Saturday that a change to the way the compression ratio was measured would be introduced on 1 June with a further revision for the 2027 season.
A statement said the rule change had been approved unanimously by all power-unit manufacturers.
The compression ratio is a measurement of the cylinder displacement between the two extremes of the piston stroke.
The original rule stated the ratio would be limited to 16:1 in the new engine rules introduced this year – a change from 18:1 under the previous regulations – and this was measured at ambient temperature.
Mercedes’ rivals were pushing for the compression ratio to be measured at operating temperature.
The Mercedes engine has been measured by the FIA at both ambient and operating temperature and complied with the limit of 16:1 at all times, senior F1 insiders have told BBC Sport.
Compression ratios tend to decrease as an engine rises in temperature because of thermal expansion of the materials involved. The belief was Mercedes had found a more effective way to limit this loss than other manufacturers.
From 1 June 2026, the compression ratio will be measured at 130C as well as ambient temperature, and from next year only at 130C.
That means that from next year, manufacturers will be able to have engines that exceed a compression ratio of 16:1 at ambient temperature, even though this reduction from the previous limit was introduced into the 2026 regulations to make the rules more appealing to new manufacturers.
Audi and Ford both entered F1 because they were attracted by the new power-unit rules, which introduce a near 50-50 split between internal combustion and electrical power, and Honda reversed a decision to quit.
A statement from the FIA said: “A significant effort has been invested in finding a solution to the topic of compression ratio.
“The regulations introduced for 2026 represent one of the biggest changes in recent memory.
“All parties acknowledge that with the introduction of such significant regulatory changes, there are collective learnings to be taken from pre-season testing and the initial rounds of the 2026 championship.
“Further evaluation and technical checks on energy management matters are ongoing.”
This last sentence is a reference to discussions over energy deployment and recovery.
Drivers have complained that because cars are energy starved this has led to unusual and counter-intuitive driving techniques – and there are various proposals to change the rules to make energy management easier.