Code of Federal Regulations Title 40. Protection of Environment § 40.205.153 Engine displacement

(a) Engine displacement must be calculated using nominal engine values and rounded to the nearest whole cubic centimeter, in accordance with American Society for Testing Materials (ASTM) E 29–67.

(b) For rotary engines, displacement means the maximum volume of a combustion chamber between two rotor tip seals minus the minimum volume of that combustion chamber between those two rotor seals times three times the number of rotors.

cc=(Maximum chamber volume—minimum chamber volume) x 3 x number of rotors.