The engine compression ratio story – initiated by claims that Mercedes' new F1 2026 power unit exceeds the 16:1 ratio limit when hot, after complying when at ambient temperature - becomes more complex almost by the day.
The ‘cold and hot test’ proposed by the Power Unit Advisory Committee (PUAC) is set to be voted on by the five power unit manufacturers within the next 10 days, but may not get the four manufacturer votes required to carry it through to submission to the FIA.
The reason is that some manufacturers were pushing for a ‘hot-only’ test in order to configure their power units to exceed the 16:1 limit when cold and then have them drop down to the regulation limit when up to operating temperature.
The cold and hot test would thwart that plan, essentially because the ‘compliant at all times’ part of the regulations originally used against Mercedes would, in this case, be used against those complaining.
So we now have the somewhat bizarre situation of rival manufacturers voting against their own initiative.
Mercedes’ Toto Wolff outlined the latest situation in Bahrain, saying: "I think the way it's been done now is that it needs to be compliant to the regulations when it's cold and when it's hot. So it doesn't give anybody an advantage.
"Because I think the attempt was from the other guys to have it only measured hot, so they could actually have it cold outside of the regulations. So now it's a fair game for everyone..."
The technical backdrop to this is that most power units (with the apparent exception of the Mercedes) loses compression ratio as it gains temperature.
A former power unit technical boss has explained to The Race that typically an engine with a 16:1 compression ratio when cold would drop to around 15.2-15.2:1 in operation because although the con rod expands, the block expands more.
The proposed ‘hot test’ would involve stripping the cylinder heads from the engine, fitting the measurement devices and heating it in an oven to a typical operating temperature of 130°C.
If the test – proposed to take effect from August – was only done in this way, it would be feasible to have the engine run beyond 16:1 when cold, falling to 16:1 when up to temperature.
This would allow Ferrari, Red Bull, Honda and Audi to potentially have more power than currently, as well as reducing Mercedes’ power.
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However, the cold and hot test being proposed would preclude this.
At least four of the five manufacturers (plus FOM and the FIA) need to vote in favour of the proposal. Otherwise, it will not progress and the regulation will remain exactly as is.
The power unit hardware specifications are locked in place after March 1, if there is no PUAC adjustment.
