Displacement is: Piston area multiplied by the stroke,for small engines usually expressed in cubic centimeters or cc.,one cc equals 0.061 cubic inches,or one cubic inch equals 16.4 cc. One cc. is one thousands of a liter.For instance a 50 cc. engine has close to 3 cubic inch displacement (50/16.4 ) or 0.05 liter.Typically an engine produces around 0.05 HP per cc.