I have searched but cannot find a solid answer
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What does the n75 valve do. Is is worth or even needed toupgrade it.
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Sorry in advance

 

I can give you a very basic answer and hopefully someone will be able to elaborate on it. The n75 valve is what controlls the amount of boost by controlling the wastegate. It works off vacuum pressure and the ecu tells it how much to open or close the waste gate. It is located on the turbo inlet pipe incase you didn't know where it was located. It is not a required or even commonly recomended upgrade with a chip or other mods. Some people get the H or J valve since they allow for a little more boost but some dbw cars have problems with this and they get put in limp mode.

 

^can't really get that much more elaborate. the basic version would be it tells the wastegate to open at a certain psi. when the wastegate is opened it directs the exhaust flow away from the turbine therefore stops building boost. without it the turbo will run off its 5psi spring in the internal wg. so you'll only boost 5psi, but it wont harm anything