ADUO: How F1 will unlock engine upgrades so strugglers can catch up

Formula 1 engine upgrades are banned for the next five years to keep costs down but — to avoid locking in the winners and losers for years to come — the rules have a built-in pressure valve: Additional Development and Upgrade Opportunities (ADUO).

Although development on the new-for-2026 hybrid units was frozen at the start of the year, ADUO will allow manufacturers whose power units are significantly underperforming rivals to restart work and upgrade their engines.

The first manufacturers are expected to be granted the ADUO exemption soon, potentially allowing the likes of Honda and Audi to introduce improvements in a bid to catch Mercedes.

But how will underperforming power units be defined? And how much can be changed under ADUO?

Here’s what we know about how the process will work.

How the performance gap is measured for ADUO

From 2026, F1 cars have used radically different hybrid systems, delivering power from an internal combustion engine (ICE) and battery pack in a roughly 50-50 split.

ADUO is designed to avoid large performance differences between the ICE component. Throughout the season, racing’s governing body, the FIA, monitors the output of every power unit on the grid, supplied by five manufacturers — Mercedes, Ferrari, Red Bull, Audi and Honda.

Each engine is ranked in what the regulations call an ICE Performance Index —  a numerical measure of engine output. — and compared to the highest-rated on the grid. The gap determines whether ADUO is triggered and, crucially, to what degree.

How the index itself is created, is opaque even by Formula 1 standards.

The method of calculating performance is contained in an FIA technical document not available to the public; even the teams themselves aren’t fully aware how the FIA arrives at its figures. We do know that it’s not just a simple horsepower number: the way that power is delivered across a lap; how sustainably it can be maintained; and the efficiency of a power unit’s energy recovery system are all taken into account.

The complexity goes further than most would expect because it’s not just the engine itself that affects the power output. For example, the FIA takes into account exhaust back pressure, and that figure is affected by the small aerodynamic vanes that Ferrari and Haas run above their exhaust outlets to redirect airflow. A minor aero detail, in other words, has a direct bearing on how a power unit’s performance is officially judged.

With manufacturers aware that the assessment is coming, there is an incentive to game the system. Mercedes in particular, but also Ferrari, may both have been running their power units below full capacity in the early races of 2026 to hide the real gap to their rivals.

How much can be upgraded with ADUO

The thresholds are as follows:

A manufacturer whose ICE Performance Index falls at least 2% but less than 4% below the leading engine is eligible for one additional homologation upgrade in the current season and one more in the following season.

A manufacturer whose index falls 4% or more below the best qualifies for two additional homologation upgrades in the current season and two in the following season.

The regulations carry a notable caveat, inserted in square brackets: “The proposed 2% threshold and subsequent resolution of the ICE performance index will be validated or adjusted after conclusion of the ongoing activities between PU Manufacturers and F1 Teams related to the on-track ICE performance measurement.”

In other words, even the FIA acknowledges that the precise numbers are still subject to revision.

When ADUO can be triggered

The season is divided into three ADUO assessment periods: rounds 1-6, rounds 7-12, and rounds 13-18.

At the end of each period, the FIA assesses whether any manufacturer qualifies.

A manufacturer that qualifies in the first period can implement its upgrade from the first race following the grant. Upgrades are not cumulative within a season: a manufacturer receives its allocation based on the first occasion it is assessed as eligible, not for every period it qualifies.

A manufacturer that fails to qualify in either of the first two periods of a season is not eligible to receive ADUO in the third period at all.

The timing of the first assessment is not entirely settled.

Originally set after race six, it may shift after both Middle East races scheduled for April were cancelled, while emergency regulatory changes expected for the Miami Grand Prix will alter the performance picture.

The FIA has indicated it wants to work through those decisions before addressing the ADUO timeline.

Any ADUO homologation upgrade that is awarded in one season must be introduced before the final race of that season, or it will be forfeited and can’t be carried forward.

What can actually be changed under ADUO?

ADUO doesn’t grant unlimited development rights.

The components that qualify for ADUO upgrades are specifically identified in a detailed table within Appendix C4 of the technical regulations.

Each item is marked either with a tick (permissible under ADUO) or a cross (not permissible, even with ADUO status).

A manufacturer granted ADUO may only modify those ticked components; everything else remains frozen.

For a manufacturer in the most severe deficit category – over 6% – the allowance could, in theory, extend to re-homologating most of the power unit from scratch, even though that’s not explicitly written in the regulations.