Blowoff valve operation questions…

I need some help in understanding the way a blowoff valve functions on a centrifugally-supercharged (or, turbocharged) motor.

Attached is a picture of the blowoff valve in my setup, and as you can see, it has the required vacuum line (from the intake manifold) running to the top of it.

Inside it is a fairly strong spring which is adjustable, and keeps the ports closed until needed.

What I fail to understand is, if excessive manifold pressure overcomes this internal spring poressure to open the valve when needed, why is there manifold vacuum introduced to the backside of this valve? Is it simply to aid in the opening, helping the built-up pressure in the discharge delivery tube to open the valve?

That was the only scenario I could imagine, but I am so ignorant of these things, never having had one before, that I was in hopes that somebody on the FABO race forum might be able to explain to me whether I am on the right track in trying to understand how this thing operates.

Any information will be greatly appreciated!8)

Bill, in Conway, Arkansas

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