So this is a dumb question but it stumps me.. On a recalculating bypass valve/ blow off valve. With a 30 psi spring. How does the spring open under low boost condition like street driving?

 

BOV's and DV's are pressure differential based not just straight PSI based. A correctly built DV and BOV will open at some pressure differential not matter what the pressures are.

The spring in the valve provides the difference in pressure the valve must stay closed for. Since the valve is connected to the intake which is under 30psi of boost one side of the valve has 30psi on the top of it and the bottom. When the throttle plate closes suddenly the pressure in the intake drops but there is still 30psi of pressure in the boost tube, the pressure exceeds the springs capacity and the valve pops open.

However there are some valves which don't have the 1:1 ratio of pressure on both sides due to chamber designs or other design ideas they have employed. These type's of BOV's tend to have high pressure issues since they either relied on the spring to hold the valve closed or some other principle besides the simple difference in pressure engine.

 

Good info thanks!! makes more sense to me now!

 

Thanks roadspike