Compression ratios
It involves the engine's compression ratio, which compares the volume of the cylinder when the piston is at top dead center to the volume when the piston is closest to the crank. Under the 2014-2025 rules this was set at 18:1, but from 2026 this has been reduced to 16:1.
However, this is measured at ambient temperature, not while the engine is running. A running engine is hotter – much hotter – than one at ambient temperature, and as metals heat up, they expand. The engines have a very short stroke, so it does not take much expansion to increase the compression ratio by reducing the distance between the piston and the cylinder head at the top of the stroke. The advantage can be up to 15 hp (11 kW), which translates into a lead of several tenths of a second per lap.
Unfortunately for the other teams, the FIA stated that its rules indeed only specify that the compression ratio should be 16:1, based on static conditions and at ambient temperatures. “This procedure has remained unchanged despite the reduction of the permitted ratio for the 2026 season. It is true that thermal expansion can affect dimensions, but the current rules do not provide for measurements at elevated temperatures,” the FIA said.
So if Mercedes and Red Bull have a horsepower lead, it will likely be baked into the 2026 season.
The compression ratio explanation was not the only one issued by the FIA. F1 has long used ultrasonic fuel flow meters as a way to monitor power output. Under current regulations this was limited to 100 kg/hour, but with the switch to fully sustainable synthetic fuels this will instead change to an energy ceiling of 3,000 MJ/hour.
In the past it has been theorized that teams could try to game the fuel flow meters – the most impressive idea I heard involved pulsing more fuel between the sensor's sample inputs to increase power, although I don't believe this was ever implemented.
Don't even think about being so smart this time, says the FIA. “Any device, system or procedure whose purpose is to alter the temperature of the fuel flow meter is prohibited,” it says, updating regulations that previously prohibited “intentional heating or cooling” of the fuel flow meter.
