Quote:
Base Group = Engine Displacement Range (in Liters rounded to nearest .1)
X1 = 4.7+
X2 = 3.3 – 4.6
X3 = 2.1 – 3.2
X4 = 0.1 – 2.0
After the Base Group number is determined, move up 1 Group (e.g.: X3 to X2) as follows:
- Increase 1 Group if engine is rotary (Wankel engine)
- Increase 1 Group if engine uses forced induction (factory or aftermarket)
- Increase 1 Group if R-Compound tires are used. R-Compound is classified by NCC Autocross as any DOT tire with a tread wear rating of less than 140.
Examples:
- A 2008 Mazda RX-8 (1.3L) would be in X4, but is an X3 car because of its rotary engine. If the same RX-8 competes with R-Compound tires, it moves up 1 more Group to X2.
- A 2010 Subaru WRX STI (2.5L) moves up from X3 to X2 because it has forced induction. Even factory forced induction, such as in this case, causes a car to move up 1 Group.
- X1 is the maximum Group. Example: a 5.0 liter car with R-Compound tires remains in the X1 Group.