The FIA has revealed that manufacturers last year rejected its offer for a different process to establish Formula 1's ranking of engines, as intrigue increases about the factors that will influence its final call on development allowances.

Under the Additional Development and Upgrade Opportunities (ADUO) system, manufacturers that are determined as being adrift of the benchmark F1 engine will be granted chances to improve.

The FIA is set to declare the result of its analysis about the pecking order - and therefore who is allowed to upgrade - over the next few weeks.

With teams having conducted detailed analysis of where they stack up and what their rivals are up to, questions have been asked about what will and will not count when it comes to the FIA's decision on who qualifies for ADUO and who does not.

In particular, some have queried whether design choices like Ferrari's smaller turbo, which results in less power for driveability gains, or its exhaust wing that can have consequences in terms of back pressure with the flow of hot gases, should be taken into account.

One source said: "It is not the engine that is down. It is the car installation that is compromising the engine for a better lap time overall."

The suggestion from some is that the final ranking of engines needs to take into account any such elements that compromise the engine to ensure a level playing field.

But the FIA has made clear that, for now, the metric it is chasing is a simple one based entirely on the power output of the internal combustion engine.

While aware that the other factors mentioned can have an influence on the strength of an engine, it says that consideration to take such elements into account was rejected when offered to manufacturers at a meeting last year.

The FIA's single seater director Nikolas Tombazis said: "The fact that engine power is not just a single number has been obviously known.

"We had approximately, I would say, in the spring of 2025, quite long discussions with them [the manufacturers].

"We offered whether we wanted to consider certain things like the turbo pressures, or the turbo diameters, or the operating of the plenum temperature, for example, and such like.

"The universal position by the PU manufacturers back then was that we should keep it simple. So the fact that it is the current horsepower measurement of the internal combustion has been appreciated right from the start."

Open to change

Last week, Red Bull team principal Laurent Mekies suggested that the FIA had a difficult task when it came to taking into account all the elements that contributed to the final power figure.

"The objective difficulties of evaluating who is where, including for the FIA, is high - is very high," he said.

"The objective complexity of trying to get it right is big, for reasons like the ICE [versus] battery, and fundamental choices: small turbo, big turbo, exhaust blowing, no exhaust blowing. So back pressure, no back pressure. It's not a nice job to have to do that."

But Tombazis has said that rather than it being too complex, he would actually be in favour of making the process of establishing the ADUO ranking more detailed to take into account wider elements than simple power output.

"I would be personally quite open to the idea of complicating the parameter bit," he said. "But that discussion was had back more than a year ago, and it was quite clear what it concluded."

While ADUO is a big talking point among manufacturers because of the opportunities it offers to improve, Tombazis is also clear that he does not think it will shake up the order much.

"Please don't forget that ADUO is not like, as some people may say, a balance of performance," he said.

"It's not like suddenly you get more fuel flow rate or more ballast, or less ballast or anything like that.

"We are looking at just a bit more opportunity to develop your  engine in terms of cash, which, of course, is important. I'm not trying to underestimate it, but you still need to make the best engine in order to win.

"It's not like we give three brownie points to somebody who's behind."