There are two types of ‘pop-off valves’ – more accurately relief valves and blow by valves.

Relief valves are simple spring loaded valves that relieve excess pressure in order to save the system/engine from over boost. This is what we need but they are sometimes hard to find.

Blow by valves are very common as aftermarket additions in cars are commonly called “pop off valves.”  They use a MAP(downstream) reference and a spring.  With the throttle open, the valve stays closed almost regardless of the MAP.  When the throttle closes, the vacuum on the MAP reference opens the valve and bleeds the air on the upstream side of the throttle.  This protects the turbo from over boost when the throttle is suddenly closed but does not protect the engine when the throttle is open.  The primary purpose in street racing is to keep the turbo spooled up during shifting and to make a cool sound.  Marv, if you have one of these do not connect the MAP reference, or better yet, get a true relief valve.