ACC and Balance of Performance (BOP) explained.

You’ll often find it in the patch notes’ physics section: “Minor balancing tweaks: BMW M6 GT3 at BrGT A and IGT D“. These cryptic little messages say a lot of things, without saying much. This post delves deeper into a mechanic hidden in Assetto Corsa Competizione (ACC): the Balance Of Performance (BOP).

What is ‘Balance Of Performance’ (BOP)?

In short, the Balance Of Performance (BOP) dictates the performance of a car on a track, relative to the other cars. In real life racing, the BOP is dictated by the organizing body of a racing series, to try and give each car an equal chance on any given track.

Other than Formula 1, organizers such as the ASO (WEC, i.e. Le Mans 24h), IMSA (WeatherTech SportsCar Championship) and the SRO (GT World Challenge) rely on the BOP to create competitive racing between classes and cars.

For instance (in SRO’s case) GT3 cars are modified versions of road-going cars. This means they come in very different shapes and sizes. These cars have different engine layouts (front/mid, rear/mid and rear-mounted engines) with different form factors (V6/V8, with turbo or without turbo) which have very different power outputs.

Next to the differences in engines, the cars themselves are shaped differently. Some are big and heavy, some are small and light-footed. Some are very aerodynamic, while others look like they are big lego sets. This means they have varying levels of handling, both in terms of aerodynamic and mechanical grip. For instance, you won’t be surprised to hear that the McLaren 720s GT3 is lighter and has a more aerodynamic design than the Bentley Continental GT3.

The many models of GT3 racing need BOP to create a level playing field. Picture by GTWC Europe

To create 1) a level playing field for manufacturers, and 2) to keep costs of converting a road car to GT3 specifications for homologation as low as possible (making it more attractive to do so), the SRO implements a Balance Of Performance. Things the SRO could change are, among others:

  • The weight of the car, by adding or reducing the extra ballast to it.
  • The power of the car, by changing the (turbo) restrictor to the engine they can restrict the amount of air going into the engine, changing the power output.
  • The aerodynamic properties, by dictating minimum and maximum ride heights among other things.

This BOP is never perfect. There are so many variables to account for in a car’s performance on track (unrelated to driver skill) that there’s always a few cars who are a bit faster on certain tracks. But, taking a season (and all its tracks) as a whole, it does give most cars a fighting chance at glory. Or tries to, at least.

Balance Of Performance in ACC

Kunos mimics this SRO-mandated BOP. While the exact figures Kunos uses are fudged to us simracers (SRO and the manufacturers like to keep them secret), we do know a thing or two, which I’ll write about below.

In ACC there are five different BOPs for the GT3 cars, depending on the year or category you choose. The base game has two different BOPs: for the 2018 and 2019 seasons. The 2020-season DLC has its own BoP, and so do the Intercontinental GT and British GT DLCs.

You select between these different BOPs whenever you are selecting a car to go into a session, as pictured below.

You select different BOPs whenever you select the category you race with
  • As a sidenote, these different BOPs in ACC not only affect the cars’ performance itself, but also change the grip characteristics of some tracks which had resurfacing work done to them for those seasons (such as Silverstone and Paul Ricard). Furthermore, the 2020 season BOP also adds the Pirelli DHE tire which the teams ran in the 2020 GT World Challenge season.

Instead of creating a separate BOP for each track, the tracks are divided into categories. The tracks in each category share their general characteristics in terms of power or aerodynamic performance. Each category has a distinct BOP. The BOP category any given track is in, is visible whenever you enter a session in the top bar, as evidenced in the pictured below.

The specific BOP your session uses shows up like this, EU B2 from the 2020 season, in this case.

The 2018, 2019 and 2020 season use the same categories. GT4 has its own separate BOP, which mirrors the EU (2019) classification for the tracks. This results in the following tables:

MonzaPaul RicardSpa-FranchorchampsBarcelonaBrands Hatch
Base game, GT4 and 2020-DLC BOP
Mount PanoramaSuzukaLaguna SecaKyalami
Oulton ParkDonington ParkBrands HatchSnettertonSilverstoneSpa-Franchorchamps
British GT-DLC

Now, with this information, can we say what happened when a patch note reads “Minor tweaks to [X car] at [Y BOP]”?

No, we can not. We don’t get to see the before and after values. This is most likely because the performance of the cars in ACC is something that is legally obfuscated. If Kunos were to say “[car A] is now 10KG lighter on Monza”, the manufacturer of car B might feel this is unfair.

Since Kunos wants to create a good sim racer where multiple cars are viable to get a race win (while not running astray from reality), opening up these BOP tweaks to the public might mean a race to the bottom because of both manufacturers and fans alike complaining about unfair treatment of their (chosen) brand.

Trust the process. Trust Kunos.

Printable and downloadable resources

Now I get that loading my webpage everytime you want to look up a specific BOP would probably be great for my ad revenue, but isn’t user friendly. That’s why I created both printable and downloadable versions of this overview.

>> Download the BOP cheat sheet pdfs by clicking this link (opens Google Drive) <<

If you rather have a simple picture to use, see the previews below. Their full resolution versions are also included in the Google Drive link above.

regular version
printer-friendly version

If you like this, please consider following me on twitter (@FelixDicit) as it helps me increase my reach. Also, feel free to give this a favourable rating on Steam where I uploaded this as a guide!

Please, let me know what you think!