Here are the Time Trial rules.
http://www.nasa-tt.com/rules
The TT3/TT2/TT1 rules are supposed to be similar enough to the Super Touring (W2W racing) rules that they cross over. Both Terry and I think they are easier to understand the formula and restrictions. Take a look at the ST rules below, look down the list for Super Touring.
http://www.nasaproracing.com/rules