[Mike Carr]

Shipping weight, divided by NHRA-rated horsepower of the engine, determines the natural class for a car. A car can then adjust weight to:
1) take weight out to make the top of their natural class
2) take weight out to make the next highest class
3) add weight to make the next lowest class

1970 Nova, 350/300 motor. Shipping weight 3,216 pounds. NHRA rating, rated 318 hp with OEM heads, and rated 323 with replacement heads.
3,216/318 = 10.11. Car is a natural SS/IA (cars for a shipping weight-to-horsepower ratio of 10.00 to 10.99 pounds). It can also adjust weight to make SS/HA and SS/JA. 170 pounds must be added, for driver. NHRA horsepower rating of the engine, multiplied by the 'top' factor for the class, plus 170 pounds, equals minimum weight, with driver, the car must cross the scales at. So:

SS/IA 10.00 x 318 + 170 = 3,350 pounds
SS/HA 9.50 x 318 + 170 = 3,191 pounds
SS/JA 11.00 x 318 + 170 = 3,668 pounds