pretty much the only thing that you can do to stay in the stock class is exhaust and tires but they must be DOT approved, so racing slicks are a no go but is you put street slicks than you are ok. With me, i would be in h stock but becuase of my intake, i went to d-stock, if you look on the classifications, porsche 944 or maybe 928's were placed in there, tuff to beat.