HB series Cosmo

How did it work? Anyone familiar with the operation of a rotary engine would know that during normal driving only two “primary ports” are used for the gas flow to the engine. The size of the ports is proportional to the power/torque the engine produces aswell as it’s fuel economy. Mazda’s solution was to design a pair of very small primary ports which resulted in the rotaries excellent economy.

Now for the first time, the 12A engine was on the better side of 35 mpg (hwy).To maintain the expected performance, the secondary ports wouldn’t have been enough for adequate flow so another pair of ports were cut above them with the gas flow controlled by rotating cylinders within the port attached to vacuum operated diaphragms. At around 4000 rpm these diaphragms would spin the cylinders & open the extra ports to the air-fuel mix providing what’s commonly known (in the rotary industry) as an extend port. The final result of 135 hp was adequate enough to ensure the Cosmo was a reasonable performer for it's era.