How is boost input controlled? For example, on a centrifugal supercharger after 2500 rpm or so I'm technically able to make boost...but I don't automatically have boost until the throttle is opened and I see loss of vacuum? What controls allowing the boost to enter the engine?

 

On applications that use a blow-off valve of some sort, intake manifold vacuum is what defines whether boost "can be built". When in vacuum, those valves are open, when not in vacuum (or sometimes at low vacuum) those valves close. When the valve is open air is still being moved through the blower, but it just goes out of the valve instead of being forced into the motor.

 

The throttle.

 

So, if I understand this right, at what ever the RPM the manufuacturer sets the turbine to make boost - at that RPM and above, boost is alway present at the throttle body?

 

Not quite. Sorry, my answer was oversimplified, and intended as a supplement to diynoob's post.

As diynoob metioned, a blow off valve keeps pressure from being excessive between the throttle body and blower. With the throttle closed, all that air would have no place to go, so the BOV bleeds boost when manifold vacuum is present.
But for manifold vacuum to be present, one must close the throttle.

Scenarios go like this:
You're at full throttle and want to slow down. You close the throttle enough to produce manifold vacuum instead of boost. Blow off valve opens and releases excess air that the compressor is making.
On a centrifugal, the BOV may stay open as long as the manifold shows vacuum. On a turbo, it may just stay open until the turbines spool down in response to less exhaust output.

If you're cruising at part throttle and nail it: Throttle opens, manifold vacuum goes close to zero, BOV closes that big leak that was there.

On a centrifugal, the BOV also helps economy when not under boost, since the blower isn't working against as much pressure and creating as much parasitic loss.

Edit: I shouldn't have said "On a turbo, it may just stay open until the turbines spool down in response to less exhaust output."
It may stay open any time there's manifold vacuum, but it may not be bleeding any pressure since the compressor rpm can be independent of engine rpm.

 

Good explaination - thanks. Just wanted to try and understand how my Procharger actually works.

I would like to see "boost" come on sooner than 2500 RPM....but guess that is an engineered aspected of the blower unit itself not the pulley?

 

On a centrifugal supercharger you can increase boost virtually across the RPM curve by lowering your pulley diameter. This assumes your valve is working properly and closing when it should. You will also increase top end boost, not just low end boost -- give ECS or A&A a call, they're experts in this stuff. I think they even sell restrictor plates that will allow you to pulley down without increasing top end boost and give you a little more low end grunt.