Since I'm nowhere near completion, this is one of those details that I've

to work out... all I know is this is the right way to do this job and how

will attempt to setup my system.  It seems to me that if the popoff valve

spring controlled, then it should be a simple thing to size and rate the
spring to an absolute pressure.  Let's go through this little bit of

and you let me know where the holes are...

Your recent post explained the relationship between MAP in inches of

and PSI of boost, that being 1psi = about 2" MAP.  That says to me that at

MAP the deck pressure available is 18psi.  Imagine that the popoff valve

has 2 square inches of surface area.  Opposing that 18psi would require 9
pounds of spring pressure against the 2sqin popoff valve flapper or poppet

whatever mechanical device is used to control the MAP inside the intake
manifold.  Once the MAP reaches 36" (18psi) the pressure on both sides of

valve flapper are the same... if the MAP goes higher it overpowers the

and forces the flapper door open, venting the excess pressure beyond

 If you remember that the "A" in MAP stands for absolute, we are simply
defining the amount of pressure available inside the manifold.  Since the
spring rate is an absolute value that is unaffected by ambient pressure

the area of the valve face is fixed, I can see no mechanical reason why

shouldn't work. (This is essentially how the alternate intake air doors

on most of the Lancair IV fleet... if the air filter clogs (ices up) and
causes a bit of suction inside the air box, the little spring is

the alt air door opens and allows the intake to breathe using the air
available inside the cowling.)   It seems a simple problem in mechanics

not one of plumbing, at least to me.  What am I missing?

        <Marv>


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