Hubraum
The chassis generation must match the appearance and modifications of the car. Thus, one can take a chassis from a 2006 Subaru WRX and put a 2003 WRX shell around it, but all parts must be allowable for a 2003 WRX. Engine displacement cannot be more than 2.6 liter with multipliers, and a limiter is placed on any forced induction system. A minimum weight is also enforced. Many other parts can be replaced, as long as they are an option from the manufacturer.