In 1990, the Eunos Cosmo, with the triple-rotor 20B-REW rotary engine, went on sale after a quarter-century of continuous research and development into the rotary engine. While the dual-rotor rotary engine was as smooth as an inline six-cylinder reciprocating engine, the triple-rotor rotary engine could outdo a V8; and nearly match the smoothness of a V12 engine. However, there were a few technical problems with multi-rotor engines Mazda first had to overcome.
For the rotary engine to be adapted to an in-line multi-rotor configuration, it was necessary to lengthen the eccentric shaft that controlled the motion of the triangular rotors. There were two feasible possibilities: either extending the shaft with couplings, or making one of the fixed gears on the rotors with a split-assembly technique. The split-assembled structure was so complicated it was impractical for mass production. On the other hand, an eccentric shaft joined with a coupling had problems with strength, but since Mazda had been building a body of knowledge on couplings since the early stages of development in the 1960s, it was decided to adopt a structure with tapered joints that had been discovered in the 1980s. During development of the triple-rotor rotary engine, extensive road tests for performance and durability were carried out, including participation in international sports car races such as the famous Le Mans 24 Hour Endurance Race. All of which demonstrated how a mere production car could compete in the world of motor sports.
In 1990, the Sequential Twin-Turbo, a system of two turbochargers operating in sequence depending on the engine speed, was added to the 20B-REW and 13B-REW rotary engines. This type of turbo was an original concept of Mazda’s. At low speeds, only the first turbocharger operates, but at higher speeds, the second turbocharger kicks in. Using both turbochargers enabled high power output.
The rotary engine is naturally suited for use with turbochargers thanks to characteristics that include a stronger exhaust pulse caused by the near-instantaneous opening of the exhaust ports, and a short and smooth exhaust manifold. To take full advantage of these features, the Sequential Twin-Turbo engine featured the uniquely shaped Dynamic Pressure Manifold to guide the exhaust gas into the turbocharger over the shortest possible distance.
Through modifications made to the 13B-REW engine in 1999, the third generation RX-7 achieved 280 PS (206 kW) maximum power, the highest in Japan.