2026 Drama: Alleged engine loophole

[Hoffman900]

Racing engine builders from amatuer to oem pro builders have been running a net zero piston to head clearance forever. This always makes the most power. Drag racers, NASCAR, F1 builders, etc run the P-H so they just sliggghtly kiss. This is due to thermal expansion and rod stretching. Anything less is just for a safety margin, at the expense of some power.

F1 builders run very small, compact combustion chambers. Any growth here will effect valve sealing, which is very critical.

Temperarures across the piston, bore, wrist pins, valves, valve seats, chamber walls are not the same. So controlled growth here is very hard to manage. Also materials and dimensions are highly regulated. Managing piston blowby and and valve sealing / reliability matters immensly here and there is big power to give up in letting them slip.

To me, it seems it’s just a rules snafu where teams are actually probably seeing a change from 16:1 to 16.1ish:1 geometric compression in a running engine, which is not what the rules allow.

Most of this is a silly season distraction, people who don’t know race engine building / design, and generative AI / fantasy slop.

Geee, thanks for reminding what we learnt in kindergarten

Which kindergarten did you go to??

He’s being facetious. If some of us posted that we would have gotten our posts deleted for being off topic, but rules are not for everyone…

[Martin Keene]

I have a question. Where did 18:1 effective compression ratio figure came from? Who did report it? I only saw some media speculation about that value.

18:1 was in the old regulation set.
And if you are a newspaper looking for drama, it makes a nice villain and sounds optimal and hence dooms-day-y. IMHO.
The real target may also be 16.3, and the real gain might be 2-3 hp, if any.
Or not, of course.
AFAICT there is zero solid sources for restoring 18:1.

This is my thinking too. I suspect that Merc and Red Bull just asked for clarification because they've realised that if they use the materials they intended then if they start with 16:1 at room temperature the compression ratio may increase a bit when engine gets hot.

I don't remember exactly the number but I did some calculations when this drama started and a jump to 16.3:1 may be the result by conrod expanding linearly by few tens of microns more than the crankshaft - head distance, which is nothing unexpected from normal materials thermal expansion ratios.

This is what I strongly suspect is happening. Every engine ever made will have a different CR at operating temperature to ambient temperature. The question this rule set opens up is how much of that change is allowable.

Which given it is unlikely to be measurable is a bit of a moot point.

[Farnborough]

It's possible they've made use of the piston being smaller than the bore (see statement above) to "argue" that the swept volume is that of the piston, and not the bore diameter. Which would produce a lower recognised geometric compression ratio ----- at ambient temperature------ which brought the geometric calculations to inside the 16:1 ratio specified within the rules. An interpretation of the words published by the FIA in other words.

The piston rings are part of the piston so ultimately it is the bore that defines the swept volume and the cubic capacity of the engine. If the piston could expand by 1mm in diameter (generous) within a bore the expands only by 0.2mm (non-interference is assumed at both ambient and operating temperature), the compression ratio and cubic capacity will remain the same within the orders of magnitude that we’re concerned about with here. Even if you had extreme, science-fiction piston diametrical changes, the CR and cu capacity would still be defined by the bore, because of the piston rings. That said, I don’t think alteration of the CR is the trick at play here, whether achieved by TE or not.

Is that an assumption, or is there record in definitive determination for this ?

A loophole is often, in F1 experience, the reading and application of specific description that can limit or define their reach in regard to component attribute, inclusion or omissions in meaning.

IF the rings aren't included (by definition or not classified specifically) as part of the piston, then the rules could be read and determined to offer contrary outcome.
The rings are, in effect, not part of the piston structure, are in function a simple "seal" and could be argued to be outside the wording for volume calculations.
Just a normal clearance for high durability piston construction in this sphere of operation could be 0.1mm average (that at the noted "Ambient" point of reference) leading to something close to 1.5cc volume shift in swept capacity. That can give the shift that is sensationally (press reporting is always going to give the dramatic and simple headline) claimed at running 18:0 to 1 implementation with rings then in place.
There's some smoke here, if the claims of reporting has any substance. The opposition from non MB & RB PU manufacturers could be recognised as that, if there's any real basis.

Can anyone quote formal wording that defines if rings are in play or not.

The FIA directive appears to take the route of manufacturer PU dossier as indicative of how the CR is calculated/measured by the manufacturer as key in determination of design compliance.
I can't immediately find this published, it seems to be private between PU manufacturer and FIA interactions.
I can find wording that describes it may sometimes be quantified by liquid, but doesn't necessarily apply that as absolute method.

Can anyone here shed light on method, wording and authority any method holds with regard to this area.

It would seem logical if MB were using this method before, it would then travel as knowledge to RB with the personnel movements. But still be "privileged" information between those two and FIA in rules interpretation.

Which could also mean that MB was running up near 20:0 to 1 in the notional 18:0 to 1 era. The PU has been a pretty potent benchmark all through the last rules iteration.

If it were to be classified in this fashion, sans rings, then its a very neat way of stacking the necessitated tolerance for expansion critically underneath the 18:0 to 1 target, while deploying the tolerance to good effect all above the target 16:0 to 1 notional demand of the rules. It would be a very "cute" way of placing the unavoidable tolerance losses into the rules interpretation for gain instead.

[sucof]

Someone correct me if I am wrong:
Most of the methods I read here and elsewhere does not make sense imho, as to increase compression ratio you need to change the travel of the piston not its volume.
The methods I saw all change the volume of the chamber in a constant way. Those methods would only change the compression if the volume changes back to the lower state when the compression begins and the volume shrinks at the time the compression is the highest.

[hollus]

A change in piston length would affect the “small” volume at max compression more than the “large” volume at the opposite end of travel.
But a very interesting point, the most basic geometry extension trick would also result in less engine capacity when hot???!!!
Then again, maybe slightly lower engine capacity is irrelevant in a fuel restricted formula with turbos.

[Hoffman900]

Someone correct me if I am wrong:
Most of the methods I read here and elsewhere does not make sense imho, as to increase compression ratio you need to change the travel of the piston not its volume.
The methods I saw all change the volume of the chamber in a constant way. Those methods would only change the compression if the volume changes back to the lower state when the compression begins and the volume shrinks at the time the compression is the highest.

Correct.

And rod stretch isn’t going to do it either, everyone has been running net zero piston-head clearances for decades in all forms of racing. It makes the most power (we’re talking 300-500rpm ceilings before the stretch actually causes damage). They were already at the limit.

From cold to hot, most racing engines see .1-.3 or so growth in compression ratio by gaps closing up from rod stretch and thermal expansion.

F1’s rules called for measurement at cold. So of course everyone is higher than this in practice.

[dren]

Expansion in the crank would work in both directions. Also, something expanding into the CC, like a sensor, as mentioned before.

[vorticism]

It does but the effect is greatly asymmetric. 1/1000th the CC avg height at BDC vs 1/50th the CC avg height at TDC i.e. the loss of displacement at the bottom of the stroke can be ignored for the sake of conversation.

[vorticism]

It's possible they've made use of the piston being smaller than the bore (see statement above) to "argue" that the swept volume is that of the piston, and not the bore diameter. Which would produce a lower recognised geometric compression ratio ----- at ambient temperature------ which brought the geometric calculations to inside the 16:1 ratio specified within the rules. An interpretation of the words published by the FIA in other words.

The piston rings are part of the piston so ultimately it is the bore that defines the swept volume and the cubic capacity of the engine. If the piston could expand by 1mm in diameter (generous) within a bore the expands only by 0.2mm (non-interference is assumed at both ambient and operating temperature), the compression ratio and cubic capacity will remain the same within the orders of magnitude that we’re concerned about with here. Even if you had extreme, science-fiction piston diametrical changes, the CR and cu capacity would still be defined by the bore, because of the piston rings. That said, I don’t think alteration of the CR is the trick at play here, whether achieved by TE or not.

Is that an assumption, or is there record in definitive determination for this ?

A loophole is often, in F1 experience, the reading and application of specific description that can limit or define their reach in regard to component attribute, inclusion or omissions in meaning.

IF the rings aren't included (by definition or not classified specifically) as part of the piston, then the rules could be read and determined to offer contrary outcome.
The rings are, in effect, not part of the piston structure, are in function a simple "seal" and could be argued to be outside the wording for volume calculations.
Just a normal clearance for high durability piston construction in this sphere of operation could be 0.1mm average (that at the noted "Ambient" point of reference) leading to something close to 1.5cc volume shift in swept capacity. That can give the shift that is sensationally (press reporting is always going to give the dramatic and simple headline) claimed at running 18:0 to 1 implementation with rings then in place.
There's some smoke here, if the claims of reporting has any substance. The opposition from non MB & RB PU manufacturers could be recognised as that, if there's any real basis.

Can anyone quote formal wording that defines if rings are in play or not.

The FIA directive appears to take the route of manufacturer PU dossier as indicative of how the CR is calculated/measured by the manufacturer as key in determination of design compliance.
I can't immediately find this published, it seems to be private between PU manufacturer and FIA interactions.
I can find wording that describes it may sometimes be quantified by liquid, but doesn't necessarily apply that as absolute method.

Can anyone here shed light on method, wording and authority any method holds with regard to this area.

It would seem logical if MB were using this method before, it would then travel as knowledge to RB with the personnel movements. But still be "privileged" information between those two and FIA in rules interpretation.

Which could also mean that MB was running up near 20:0 to 1 in the notional 18:0 to 1 era. The PU has been a pretty potent benchmark all through the last rules iteration.

If it were to be classified in this fashion, sans rings, then its a very neat way of stacking the necessitated tolerance for expansion critically underneath the 18:0 to 1 target, while deploying the tolerance to good effect all above the target 16:0 to 1 notional demand of the rules. It would be a very "cute" way of placing the unavoidable tolerance losses into the rules interpretation for gain instead.

I was speaking in terms of function. Is a piston a piston without its rings? It needs them for the engine to operate. The piston mass C5.5 includes the piston and its rings, inserts, and clips. That's the only mention of the piston being necessarily an assembly. The piston being considered only as a monolith without inserts, clips, and rings, is not mentioned. No formal definition for it, and the only informal definition is for that as an assembly mass.

"Cubic capacity" is defined in the regs as "the motion of the pistons within the cylinders." If that doesn't mean "piston as assembly" but "piston as monolith before assembly" then you might have something. Make the unassembled piston of a smaller diameter, increase the stroke, and increase the width of the piston rings. The piston skirts could (and would need to) remain at typical spec, so the plan area calculation would be "novel reduced piston diameter" + "unavoidable minimum skirt area." From there, increase the stroke, which is not limited, to meet the cu capacity limit. Once the rings are added to the piston, you will have a greater compression ratio due to the increase in stroke.

However C5.4.3 the geometric compression ratio is limited to 16:1 but GCR is not formally defined like cubic capacity is. The testing document is not available to us but it would tell us what they mean by GCR, and of course, how they test for it. It does not necessarily derive from their definition of cubic capacity as the swept motion of pistons, and either way, they don't define what a piston is.

[AR3-GP]

The piston rings are part of the piston so ultimately it is the bore that defines the swept volume and the cubic capacity of the engine.

Is that true? If the piston crown has a large divot in the center, then the cubic capacity of the engine depends on the shape of the piston. If the cubic capacity were to only depend on the bore, then any team could effectively bypass the displacement restrictions by constructing a volume inside the piston crown.

[vorticism]

The crown shape doesn't alter the swept volume. The crown is the same shape at both the top and the bottom of the travel envelope. A piston bowl adds volume to the bottom of the envelope and subtracts volume from the top of the envelope, so it can be ignored. Thus the it's only the bore and piston diameter that determine various definitions of the SV. Functionally speaking, it's the bore diameter that matters more, because of the piston rings.

[AR3-GP]

The crown shape doesn't alter the swept volume. The crown is the same shape at both the top and the bottom of the travel envelope. A piston bowl adds volume to the bottom of the envelope and subtracts volume from the top of the envelope, so it can be ignored. Thus the it's only the bore and piston diameter that determine various definitions of the SV. Functionally speaking, it's the bore diameter that matters more, because of the piston rings.

Okay I see. It is the distinction between the volume and the swept volume.

[Richard C]

The crown shape doesn't alter the swept volume. The crown is the same shape at both the top and the bottom of the travel envelope. A piston bowl adds volume to the bottom of the envelope and subtracts volume from the top of the envelope, so it can be ignored. Thus the it's only the bore and piston diameter that determine various definitions of the SV. Functionally speaking, it's the bore diameter that matters more, because of the piston rings.

Okay I see. It is the distinction between the volume and the swept volume.

I don't think there is a distinction with respect to the measurement of "displacement". Now the shape of the piston top (as well as the chamber in the head) will impact the volume of the combustion chamber.

Richard

[BassVirolla]

Someone correct me if I am wrong:
Most of the methods I read here and elsewhere does not make sense imho, as to increase compression ratio you need to change the travel of the piston not its volume.
The methods I saw all change the volume of the chamber in a constant way. Those methods would only change the compression if the volume changes back to the lower state when the compression begins and the volume shrinks at the time the compression is the highest.

Correct.

And rod stretch isn’t going to do it either, everyone has been running net zero piston-head clearances for decades in all forms of racing. It makes the most power (we’re talking 300-500rpm ceilings before the stretch actually causes damage). They were already at the limit.

From cold to hot, most racing engines see .1-.3 or so growth in compression ratio by gaps closing up from rod stretch and thermal expansion.

F1’s rules called for measurement at cold. So of course everyone is higher than this in practice.

Not correct, because displacement is the swept volume, only affected by crankshaft dimensions, not for piston shape or height nor rod length.

The swept volume that you "loose" at TDC is not lost. Is in fact increased as swept volume, and the increase in piston height / diameter reduces swept volume at BDC, keeping the total swept volume constant, as a function of bore (constant) and stroke (constant; a.k.a. only dependant of crankshaft).

CR = (SV+CCV) / CCV.

Only changing combustion chamber volume (crown height / diameter dependant).

Not changing swept volume (crankshaft and bore dependant).

[BassVirolla]

The crown shape doesn't alter the swept volume. The crown is the same shape at both the top and the bottom of the travel envelope. A piston bowl adds volume to the bottom of the envelope and subtracts volume from the top of the envelope, so it can be ignored. Thus the it's only the bore and piston diameter that determine various definitions of the SV. Functionally speaking, it's the bore diameter that matters more, because of the piston rings.

I missed this answer. Shortly and concisely explained, not like myself.