40 CFR § 205.153
Engine displacement
June 9, 2020
(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) × 3 × number of rotors.