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- Compression ratio decreases as temperature grows from my understanding. As it stands every team is at 16 ambient and as temperature grows they hit 15.x.
Mercedes remains closer to 16 or 18 or whatever than the others, potentially giving them an advantage.
Since compression ratio decreases, every engine will anyways pass the test regardless of temperature. So the tests themselves are meaningless. They change nothing.
But the real question that remains is what is Mercedes actually doing? No one seems to understand that. All we have are broken theories.
Unless the FIA know what to look for, it still remains quite meaningless. I have a feeling Mercedes trick might not be that easy to find.
The theory is that Merc has designed their engine to slightly increase the CR at higher temps, but pass them at ambient. The 130C test is designed to target that. So it does change something.
But the real question that remains is what is Mercedes actually doing? No one seems to understand that. All we have are broken theories.
Unless the FIA know what to look for, it still remains quite meaningless. I have a feeling Mercedes trick might not be that easy to find.
The FIA don't need to find it. Toto Wolff said his team would change the engine. They are not going to be dishonest about it and I believe them.
With the latest wording to the rules FIA shows that are firm on this. Nothing stops them to take an engine and completely disassemble it to see with their eyes if for example the cad designs that are submitted to FIA are 100% true.
For me case closed. They have some races only to do whatever they want.
I understand, but in my opinion the variance between the manufacturers will be very small after these revisions. The test are a little bit for show as you say, but realistically these PUs operate very similarly when everyone designs to the same regulatory intention. In my opinion, it's the language that prohibits defeat mechanisms which is the more important amendment. It's significantly limits what can be done beyond menial thermal expansions.
But the real question that remains is what is Mercedes actually doing? No one seems to understand that. All we have are broken theories.
Unless the FIA know what to look for, it still remains quite meaningless. I have a feeling Mercedes trick might not be that easy to find.
The FIA don't need to find it. Toto Wolff said his team would change the engine. They are not going to be dishonest about it and I believe them.
Unless the FIA knows whats up I am very reluctant to trust Wolff. Nothing is stopping Mercedes from running illegal engines as long as the FIA don't know whats illegal about them. I think we all know the FIA well enough.
Mercedes-Benz is one of the largest auto-manufacturers in the world. They don't think like this. It's a risk conscious business. They developed their tricking thinking the FIA approved it. When the FIA said no, they conceded to remove it. That's exactly how a large company like this would operate.
a few days ago , reports said that the MB engine passed a hot test ( some suggest it was performed under 115 C° ) so I don't see much of a change if they test it under 130 C° ; unless they change the test method , or the MB engine will pass the new test as well ...
After 77 pages of this thread, and multiple 'announcements/clarifications' from FIA , we can now say with reasonable confidence :
It's only the Compression Ratio Test (whatever be its definition) that imposes the ICE regulation , not things like 'informing FIA through all development phases' OR 'submission of original CAD drawings' etc ; ie, the test defines the geometric part of the regulation.
Now, the 5 PU manufacturers are going to submit 5 different 'compression ratio testing' methods, just like 5 suspects in court bringing their own personal diary/journal entry page as their alibi for the date of crime. I still haven't come to terms with why there is no 'FIA defined compression ratio test'. It's not rocket science.
Last edited by venkyhere on 28 Feb 2026, 14:12, edited 1 time in total.
a few days ago , reports said that the MB engine passed a hot test ( some suggest it was performed under 115 C° ) so I don't see much of a change if they test it under 130 C° ; unless they change the test method , or the MB engine will pass the new test as well ...
The test is not the major change to the regulations. Engine operating temperature is greater than 130C so that is just for show. This is the most important addition to the regulations:
Any component, assembly, mechanism, or integrated arrangement of components that is designed or functions to increase the compression ratio in operating conditions beyond 16.0 is prohibited.
Anyone who thinks Mercedez-Benz will just outright cheat after Jun 1st, doesn't understand how large auto manufacturers think.
a few days ago , reports said that the MB engine passed a hot test ( some suggest it was performed under 115 C° ) so I don't see much of a change if they test it under 130 C° ; unless they change the test method , or the MB engine will pass the new test as well ...
The test is not the major change to the regulations. Engine operating temperature is greater than 130C so that is just for show. This is the most important addition to the regulations:
Any component, assembly, mechanism, or integrated arrangement of components that is designed or functions to increase the compression ratio in operating conditions beyond 16.0 is prohibited.
Anyone who thinks Mercedez-Benz will just outright cheat after Jun 1st, doesn't understand how large auto manufacturers think.
Not sure about that at all.
There may be small parts of the engine that are above 130degC consistently but the average temperature throughout the entirety of the block, cylinder, piston, rod etc. would be around there. Thermal expansion is a function of the average temperature across the entire length of the material, not the highest temperature measured in one spot. The main contributing factor to increased CR when the engine is hot would be expansion in the rod, but the rod is entirely isolated from combustion behind the piston and would likely not see temperatures above 130degC at any stage.
There may be small parts of the engine that are above 130degC consistently but the average temperature throughout the entirety of the block, cylinder, piston, rod etc. would be around there. Thermal expansion is a function of the average temperature across the entire length of the material, not the highest temperature measured in one spot. The main contributing factor to increased CR when the engine is hot would be expansion in the rod, but the rod is entirely isolated from combustion behind the piston and would likely not see temperatures above 130degC at any stage.
I think that if a manufacturer intended to exploit thermal expansion, they would make sure the rod gets very hot (but not so hot to compromise the stiffness or bearing friction). So static heating to 130C may not reveal much.
That's also if we assume this is about rod stretch and not secondary chambers. In my opinion, the explicitness of the amendment closes the loophole firmly, test or not. Large auto manufacturers are boring. Mercedes is not going to break the rules after June 1st. They do gray areas, not rule breaking.
a few days ago , reports said that the MB engine passed a hot test ( some suggest it was performed under 115 C° ) so I don't see much of a change if they test it under 130 C° ; unless they change the test method , or the MB engine will pass the new test as well ...
The test is not the major change to the regulations. Engine operating temperature is greater than 130C so that is just for show. This is the most important addition to the regulations:
Any component, assembly, mechanism, or integrated arrangement of components that is designed or functions to increase the compression ratio in operating conditions beyond 16.0 is prohibited.
Anyone who thinks Mercedez-Benz will just outright cheat after Jun 1st, doesn't understand how large auto manufacturers think.
Not sure about that at all.
There may be small parts of the engine that are above 130degC consistently but the average temperature throughout the entirety of the block, cylinder, piston, rod etc. would be around there. Thermal expansion is a function of the average temperature across the entire length of the material, not the highest temperature measured in one spot. The main contributing factor to increased CR when the engine is hot would be expansion in the rod, but the rod is entirely isolated from combustion behind the piston and would likely not see temperatures above 130degC at any stage.
You'll need more research in that case. 130C maybe more typical for coolant and oil etc, piston crown could be 200C ~450C as a example.
The surfaces are partly insulated from true combustion peak temperatures by boundary layer dynamics, which if knock occurs will break down to overheat the materials. Ultimately why control of and preventing knock is so important part of combustion control.
So it is somewhat what The Race reported previously, from next year we move to a 130C test only, allowing some / all of the engines to run above 1:16 at cooler temps, but presumably reducing to the target at heat?
So yeah, looking forward to the same fight about that, or fights about what ratio is present at 200C etc etc...
Its the mythical hard line statements that keep showing up here as absolute and controlling that miss the reality of physics involved.
There's tolerance needed and understanding of how they are written into rules etc to sustain any sensible control of mechanical assemblies like this.
You can see all the controls creeping into the E side of it as people realise just how many parts of the hardware and its output can be manipulated and quantified within a performance-based envelope.
There's science, research, development, testing etc that's been in ICE design and performance throughout its evolution, they don't go away just because we call this sport a pinnacle of engineering prowess.
If there's no tolerance above 16:1 CR then it still has to exist SOMEWHERE and that will be below that ratio IF its to not breech (what is a threshold) that limit at all times and temperature. None of the PU manufacturers will want to give away anything of significant value by staying safely below that figure. And so all will chase the ultimate, and closest to the limit by which process was simply not checked when this version of PU spec was formally written in regulation.
This is not coping with a secret device, that's been clearly written out, now. But more the day to day of practical engineering reality in manufacturing anything of this complexity.
What will happen if a engine needs to be re-homologated come the start of June because the design changed?
There is an exemption to homologation requirements for regulatory compliance:
Components may be modified in order to comply with an amendment to the published regulations
or issued FIA guidance. Such modifications must be first approved by the FIA.
There's tolerance needed and understanding of how they are written into rules etc to sustain any sensible control of mechanical assemblies like this.
You can see all the controls creeping into the E side of it as people realise just how many parts of the hardware and its output can be manipulated and quantified within a performance-based envelope.
There's science, research, development, testing etc that's been in ICE design and performance throughout its evolution, they don't go away just because we call this sport a pinnacle of engineering prowess.
If there's no tolerance above 16:1 CR then it still has to exist SOMEWHERE and that will be below that ratio IF its to not breech (what is a threshold) that limit at all times and temperature. None of the PU manufacturers will want to give away anything of significant value by staying safely below that figure. And so all will chase the ultimate, and closest to the limit by which process was simply not checked when this version of PU spec was formally written in regulation.
This is not coping with a secret device, that's been clearly written out, now. But more the day to day of practical engineering reality in manufacturing anything of this complexity.
The inherent tolerances are the same for every competitor. It doesn't produce any meaningful advantages which is why it's not a talking point amongst the manufacturers. The only "issue" here was that 1 manufacturer targeted a higher compression ratio which creates a meaningful advantage. That has now been clarified.
