What are the differences between 6-Port & 4-Port rotaries?
All Normally Aspirated RX-7 13B’s are 6-port. RX-7 turbos, all pre-RX-7 13B’s, and pre-RX-7 emission-controlled production Mazda rotaries were 4-port. 6-port & 4-port refer to the number of intake ports per two-rotor engine. A 4-port 13B can easily be made using 12A end housings in place of the 6-port end housings. With engines built this way, aftermarket, Cosmo, REPU or RX-4 intake systems are used. The 6-port configuration is somewhat analogous to a boinger having a two stage camshaft arrangement, where different timing and longer duration are used when operating at higher RPM. To accomplish this, the 5th & 6th ports have a longer open timing , which is good for high RPM power. Since the longer timing is bad for low end power, sleeve valves are incorporated to close off the 5th & 6th ports except under high load and RPM. The opening and closing of the sleeve valves is accomplished through sprung diaphragm actuators similar to vacuum operated carburetor secondary barrels, with pressur
