BMW M3 Powered By A Mazda Four Rotor Engine

Wankel engines can be classified by their geometric size in terms radius (rotor center to tip distance, also the median stator radius) and depth (rotor thickness), and fset (crank throw, eccentricity, also 1/4 the difference between stator's major and minor axes). These metrics function similarly to the bore and stroke measurements a piston engine. Displacement is 3√3radius·fset·depth, multiplied with the number rotors (note that this only counts a single face each rotor as the entire rotor's displacement). Nearly all Mazda production Wankel engines share a single rotor radius, 105 mm (4.1 in), with a 15 mm (0.6 in) crankshaft fset. The only engine to diverge from this formula was the rare 13A, which used a 120 mm (4.7 in) rotor radius and 17.5 mm (0.7 in) crankshaft fset.
Mazda rotary engines have a reputation for being relatively small and powerful at the expense poor fuel efficiency. They are starting to become popular with kit car builders, hot rodders and in light aircraft because their light weight, compact size, and tuning potential stemming from their inherently high power to weight ratio.
In auto racing, the displacement a Wankel engine is usually doubled for classing purposes. For calculating taxes in Japan, the displacement Wankel engines is defined as the equivalent 1.5 times the nominal displacement, so the 1300 cc 13B engines are taxed as 1950 cc.