Forza uses a classification and “Performance Index” scoring system to fairly balance cars across the game. This PI system refers to a ranking number based on a car’s best lap time around a simulated test track. The slower the lap time, the lower the PI number; the faster the lap time the higher the PI. This test track is mathematically generated to be representative of the all race tracks in a specific game. We call this a “PI test track.”
PI ranking, however, is different for each game. It is relative to each game’s car list and race routes. Each new Forza title gets another revision of this test track to be a good representational average of all tracks in the game. This, in addition to the changing and growing car list with each title, is why class boundaries and PI #s are not the same with each new version of the game, or between Motorsport and Horizon.
Forza Motorsport titles use an entirely different PI test track than Forza Horizon titles, because race tracks (like Silverstone, for example) are not representative of the types of race routes found in Forza Horizon’s open roads. Horizon’s roads tend to have longer straights, faster turns, and less hard braking and cornering in general. For this reason, the actual class letters and PI values for a given car are not the same between Horizon and Motorsport games. Additionally, Forza Motorsport has Formula 1, Indy, and modern prototype race cars at the top of its PI system, while Horizon 2’s top-end cars are “hypercars” like the McLaren P1 and LaFerrari. This means a PI of 998 in Forza Motorsport is a much higher level of performance than a PI of 998 in Forza Horizon. It also means less granularity is needed in Horizon’s car classes than we have in Motorsport. Again, it’s relative to each game’s cars and race routes.
Once we have PI test track lap time data for every car in the game, we analyze it to determine class boundaries. Most of these land exactly where we would expect, based on the fact that all the performance data for each car is based on extensive real world research, including data provided to us directly by car, tire, and aftermarket parts manufacturers as well as race teams.
We rarely hand tune PI, and if so it is to maintain fair game balance across career, multiplayer, and leaderboards, or to keep cars of similar type and performance in the real world in similar PI ranges or classes, and never to a degree that a car’s performance characteristics are significantly compromised compared to its real world counterpart.
Lastly, of course we allow players to upgrade and tune their cars to be competitive with cars that have a much higher PI in stock form, within the bounds of real-world limits. In multiplayer or rivals competition, it’s not uncommon to see cars which, in stock form, would have no chance of winning, but with a great tune (and driver!) can smoke the competition.