The questions at the centre of the latest F1 rules controversy

A story that gathered attention last year concerned a loophole in the 2026 power unit regulations. It was alleged that some manufacturers had found, and were planning to exploit, a way of achieving, during running, a higher compression ratio than the regulations intended.

Compression ratio, or to be more precise expansion ratio, is a fundamental factor in engine performance. The thermodynamic cycle that engines operate under shows that compression ratio has a first order effect on the efficiency and hence power that can be obtained. Prior to the 2026 regulations the maximum compression ratio permitted was 18:1 and in the latter years of the last engine generation this was being achieved. Running an engine at this ratio is not easy as high compression ratios can lead to abnormal combustion. Sometimes known as ‘knock’, this is a condition whereby the ignition of the fuel takes place earlier than intended and can be extremely damaging to the engine. However, having mastered the art of taming this complex combustion existing, manufacturers were keen to keep exploiting it.

For 2026, several of the new power unit regulations were aimed at making it easier for a newcomer to compete at a performant level. Part of the way of doing this was to eliminate some of the complexities such as the MGU-H, the electric motor that harvested energy from the exhaust turbocharger, and mandating a lower compression ratio, which would lead to easier control of the combustion process. 

To regulate this, the FIA devised a comprehensive test procedure to measure the compression ratio. Unfortunately, it was dictated that this would be done at ambient temperature with, in this case, ambient meaning ‘workshop temperature’.

Now when an engine is hot it obviously expands and different parts will expand at different rates. Conventionally a cylinder block would be made of aluminium and a connecting rod of titanium. Approximating, the block temperature would be at around cooling water temperature, say 125C and the rod at around oil temperature, say 145C. At these temperatures the block may grow around 0.3mm and the rod around 0.13mm, meaning that the cylinder head moves away from the piston thereby dropping the compression ratio. With these materials and temperatures, the compression ratio, designed to be 16:1 at room temperature would drop to around 15.2:1, with a consequential drop in theoretical performance. 

F1’s 2026 rules have dropped the MGU-H and mandated a lower compression ratio

Photo by: Honda

Now, if one was to make the block out of a material such as Invar, a steel designed to have low thermal expansion, and the rod was made from an austenitic stainless steel, we find that the block now grows by around 0.05mm and the rod by 0.3mm.  A reversal of the situation with conventional materials and consequentially an increase in compression ratio. If the ambient compression ratio was set at 16:1, such a design could lead to an operating compression ratio of over 17:1. Of course, the design could be such that the compression ratio at ambient temperature was set below 16:1 and only achieved 16:1 at temperature thereby mitigating the loss while maintaining legality.

Two questions arise from this theory. The first is whether it is feasible to use these materials and the answer is yes. While one would not want to make a complete block from steel due to the weight, for many years now cylinder blocks have not had liners. The piston runs straight on the specially treated aluminium bore. If one wanted to exploit this loophole one could revert to steel liners using perhaps this very low expansion steel. The liners would have to stand proud of the block with the cylinder head sealing on to them. This would not be a difficult design and the added mass to the engine would be relatively low. The connecting rod, having a regulated minimum mass of 320 grammes could equally be made of steel with minimal penalty. 

An interesting and telling parallel could be drawn with ‘Dieselgate’. Manufacturers have been castigated for devising a procedure that ensured legality compliance during a very specific test regime

The second question is whether such a scheme is legal, and this is a more complicated subject. The regulations governing the measuring of compression ratio state that the measuring will be done at ambient temperature and, while one might argue over the definition of ambient, in this case the FIA say it means room temperature rather than the local ambient operating temperature. Equally Article C1.5 imposes a general obligation that cars comply with the Technical Regulations in their entirety at all times during a competition. 

The reality is that the latter is not always possible or adhered to. To take a trivial example, during a pitstop the car will be under weight for a fraction of a second when the wheels are removed. This aside, the intent of the regulation is clear. 

I think an interesting and telling parallel could be drawn with ‘Dieselgate’. Manufacturers have been castigated for devising a procedure that ensured legality compliance during a very specific test regime. The regulators, and society, do not accept this as honest or complying with the intent of the regulation. Is a contrived design that passes a specific inspection of compression ratio but operates outside agreed limits in service any different? I think not. If, however, the design ensures that 16:1 is always achieved then that is clever engineering. 

The FIA will hold a meeting with technical experts to discuss the compression ratio saga

Photo by: Audi