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All naturally aspirated second generation RX-7s have a 6 port induction system. The 5th and 6th ports stay closed until approximately 3800 RPM, at which time they open. This is similar to the VTec cam in a Honda engine. The resulting port allows the intake to form a natural supercharging effect, and changes port timing slightly. This results in more high end horsepower. Because keeping these ports open at low RPMs degrades performance, they are only open when required.
Unfortunately, these ports have a tendency to stick and freeze up if not used (and shame on you for not using them). A surprising amount of RX-7s are running around with non-working ports, which will result in a loss of about 25HP above 4000 RPM. There is a simple you can do to test your own ports. If you find them inoperable, the actuators will have to be removed and cleaned.
Now, put a dab of grease onto each actuator rod.
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