Blowoff valve operation questions...

[Bill Dedman]

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

 

[danmc77]

The vacuum pulls the valve open to blow off excess pressure in the intake pipe. When the throttle plates close, manifold vacuum builds and pulls the valve open. It protects the charger from the pressure that will build when the carb/throttle body butterflies suddenly close.

Here's a Wiki explanation:
http://en.wikipedia.org/wiki/Blowoff_valve

To answer your one question - the intake manifold vacuum opens the valve because if the intake charge pressure opens it, it still would build initial pressure spike that can damage the supercharger or turbo. So the manifold vacuum does this and basically bleeds the intake tubing of the pressure rather than letting the pressure build and spike before being able to open the valve.

 

[Bill Dedman]

I appreciate the well-written explanation! I "get it," now!!!8)

Bill

 

[dgc333]

Also, in the case of a turbo the pressure plus will stop the turbo from spinning and when you get back on the gas there will be lag waiting for the turbo to spool back up, with the blow off valve the turbo will continue to spin.

 

[Bill Dedman]

Thanks, Dave; I never thought about that. I have a Vortech system, now, but I might build a turbo setup next... the thought is definitely appealing.
Bill