Q: How does the Greddy valve work?
A: The Greddy valve has a rubber diaphragm, steel valve and vacuum assisted movement. The boost valve that is normally closed is held shut via spring tension at the top of the valve. A rubber ring at the bottom of the valve flange helps prevent boost leakage. A screw/nut at the top of the bov adjusts the tension of the spring and thus the sealing power of the valve. Normally under boost, the boost pressure on the top nipple helps keep the valve shut along with spring tension against the boost that is underneath the valve. When the throttle plate is closed, your manifold now has vacuum in it while the intercooler pipes are still pressurized. The vacuum, when transmitted to the BOV, helps assist in bringing the valve up, while the escaping boost pressure pushes further to fight its way out. The higher the vacuum level and the higher the boost, the more the valve will move up. Once the boost is evacuated, the spring tension will push the valve shut.
Q: What is the 2nd bottom port for?
A: The 2nd port can be used to help lift up the diaphragm under low vacuum conditions. If you are experiencing surge before your pressure release, hook up the 2nd port to your turbo pressure line. The vaccum at the top of the diaphragm and the boost pressure under the diaphragm will help it open faster. Keep in mind that the valve will not stay open as long using this method, so the sound will not be as loud.
Q: How do you adjust the Greddy BOV?
A: The BOV has an adjusting screw on the top. Use a 10MM wrench to loosen the nut that keeps the screw in place. Then use a 3MM allen key to turn the screw back and forth. Tighten the 10MM nut (snug but not too tight) when you are finished. First, do a rev test with the BOV if you will vent to air. After a few aggressive revs, the valve should close quickly on it's own power. If it does not close or takes too long, then the valve is set too softly. Then do a test drive to listen and be sure that you are getting proper pressure release. If you boost highly, you may want to consider a Type R Greddy instead.