Alfa Romeos are classed separately from non-Alfas. Alfas are classified based on a point system which starts with a point value for the base car model. Points are then added or subtracted for performance modifications which have been made to the car. The approach is fairly complicated but the goal is to have cars with similar performance competing with each other.
Non-Alfas are classified based on a weight-to-power formula which includes adjustments for tire type, suspension mods plus a few other things. It too, is intended to result in cars of similar performance competing with each other.
Both classification schemes are done on an honor basis. We will not be checking to see who, if anyone, is cheating. We may, at some point, ask you some questions about your car, especially if it seems to have an unusual level of performance for the class it has been registered in. Please keep in mind that it feels a lot better to beat someone fairly than to win by cheating.