Barely has the current season ended than two manufacturers are the subject of speculation relating to exploitation of a loophole in the 2026 regulations.

Just days after Mercedes became the second manufacturer to release a teaser soundbite of its new engine, there is speculation that the German company, along with Red Bull, has found a means of increasing the compression ratio in its engines.

It is claimed that rival manufacturers have sought clarification from the FIA in light of the regulation which see the compression ratio reduced from 18.0:1 to 16.0:1 under Article C5.4.3 of the new rules.

The aim of the reduction is to curb peak performance while promoting more efficient combustion, yet months before the engines fire up on track the fireworks have begun.

It is claimed that both manufacturers have found a means of ensuring the ratio is legal during official checks but by means of using special materials for certain components the ratio is increased as the engine temperature rises, leading to Ferrari, Honda and Audi raising their suspicions with the sport's governing body.

By having components expand the piston could be pushed nearer the cylinder head when hot than when officially tested at ambient temperature. This, it is claimed, could see a gain of as much as 15 horsepower, around 0.3s a lap.

There was already concern over what is regarded as a grey area and consequently the wording of the rule has been tweaked on a number of occasions in recent months, and while it is made clear that the test will be "executed at ambient temperature" the subsequent Document C042 demanded that manufacturers fully inform the FIA of their measurement procedures for its approval.

"The topic has been and continues to be discussed in the technical forums with the PUMs (power unit manufacturers), as the new limit naturally raises questions about interpretation and compliance," said the FIA in reaction to the speculation. "The FIA continuously reviews such matters to ensure fairness and clarity and, if necessary, may consider changes to the regulations or measurement procedures in the future.

"The regulations clearly define the maximum compression ratio as well as the method for measuring it, based on static conditions at ambient temperature," added the sport's governing body.

"This procedure has remained unchanged despite the reduction in the permitted ratio for the 2026 season. It is true that thermal expansion can influence dimensions, but the current rules do not provide for measurements to be carried out at elevated temperatures."

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READERS COMMENTS

1. Posted by Superbird70, 23/12/2025 18:34

"@Max Noble- Who has input into the FIA documents?

Any competent technical organisation would have specified a 'standard' temperature and pressure, and then defined it using ISO, NIST or IUPAC criteria. We have dullards trying police the the likes of Adrian Newey."

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2. Posted by Max Noble, 23/12/2025 9:55

"@NS Biker - Yup Miss Physics does it HER way and we have to fall in line. What also cracks me up is the FIA saying they want innovation and “future thinking” and the like… yet if anyone makes an innovation they stomp on it (Mazda’s Rotary engine at Le Mans being a prime example).
And “Ambient” measured where? Zak’s office? The “Lab”. The outdoors in an English Winter…? Lazy rule with poor definition as far as I can see. Something like…. “Compression will be measured once the engine has been run on a test bench for 20 minutes at 60% of maximum revs.” That will warm the darn thing up!

God bless the “Brains Trust” at the FIA."

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3. Posted by alejandro.torres.1991, 22/12/2025 13:38

"Either the FIA is understaffed and can't measure rule compliance properly, or they write down the regulations ignoring how creative F1 teams can be."

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4. Posted by NS Biker, 22/12/2025 3:29

"There are a bunch of misunderstandings and info that just don't make any sense.
The thermal expansion coefficient of aluminium and magnesium alloys is around double that of steel and titanium. This means that as the engine block and internals heat up, the clearance volume in the cylinder head will increase as temperatures rise. Net effect, a reduction in actual compression ratio (CR) at higher temperatures. Not a good thing, but Ms Physics doesn't play fair.
Since the FIA, que up infinite wisdom, specifically mandated detailed measurements and clearance volume, hence CR, be measured at "ambient temperature", it has the effect of increasing the measured CR the lower the temperature. Some seem to think the opposite.
If what you want is a higher actual CR at operating conditions, then design for 16.0 at operating temperature and do all your measurements at an "ambient" of 100 C as an eg.
The teams can claim this is the case as they won't even turn over an engine that hasn't been preheated to something close to 100 C. This becomes the new ambient.
What is it worth in terms of power and efficiency? Doesn't matter, it is positive and relatively cheap.
There is always the anticipation that there are other games being played (an easy money bet) so hold on, could be a bumpy ride. Man, I love this stuff.
"

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5. Posted by Tyrbiter, 21/12/2025 23:21

"Perhaps the FIA should consider rules that prevent the engines from getting hot."

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6. Posted by Endre, 21/12/2025 13:19

"I find this incredibly stupid. Why is there even a need to regulate compression ratio? It made sense before the budget cap to keep costs down, but now it is just a pointless over regulation like may others. This could be an area where the teams could compete and differentiate themselves. "

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7. Posted by Wokingchap, 20/12/2025 16:32

"Well FIA, test the Compression Ratio at working temps then !"

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