Above is the blow off valve I built.
I started with a standard $6.00 3/4 inch gate type water shut off valve.
I took the handle off, and took the valve apart.  I drilled out the brass gate in the valve and tapped it for a 3/8 inch bolt threads.  I then got a 1 inch bore, 2 inch stroke air cylinder from work.  I threaded the cylinder rod into the gate valve.  I used a 5/8 coarse thread nut to connect the cylinder to the valve, using the threads the valve packing nut threaded to.  I connect a vacuum line to the rear port on the cylinder.  Under vacuum, the piston is sucked up, opening the gate valve. Under pressure, the piston is pushed out, closing the gate valve.  This happens at about 5 psi, and 10 in hg respectively.  I ran a 3/4 inch hose from my factory BOV to this valve.  At anything over 5 psi of boost in the intake, this valve closes, blocking off the factory BOV, when the manifold is under a vacuum, this valve is open.  It should be noted that it "snaps" open and closed, but cannot be over powered by too much boost.

taken apart from right to left:  air cylinder, 5/8 coupling nut, valve top, 3/8 lock not on cyl. shaft,
valve gate, and valve body.

Installed with plastic hose conector and 3/4 inch hose over to factory BOV.
 


Notice pressure switch mounted on the AC connector.  The silver tube connected to the heater valve is actually 1/2" sweated copper tubing painted silver. The factory steel lines rusted through.

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