The blow-off valve is a device placed between the turbo and the throttle plate. It is used to eliminate potential damage to the turbine blade when the throttle is lifted. The blow-off valve works by using an internal spring and pressure differential above and below the diaphragm. The pressure above the diaphragm is equal to the manifold pressure and can vary from your maximum engine vacuum to your maximum boost pressure. The pressure below the diaphragm is equal to the pressure between the throttle plate and turbo and can vary from zero to your maximum boost pressure. This causes a pressure differential. This is accentuated when you realize the areas above and below the diaphragm aren’t the same. The area below is approximately the area of the valve, which is about 3.04 in². The area above the diaphragm is approximately the inside diameter of the blow-off valve body or 5.85 in². This allows us to find an equation for the forces acting on the valve of the BOV. A force, F, greater then zero means the valve is closed and less then zero means the valve is open.

F = S_f + P_a * A_a – P_b * A_b

Where S_f is the spring force, P_a is the pressure above the diaphragm or manifold pressure, A_a is the area above or 5.85 in², P_b is the pressure between the turbo and throttle plate, and A_b is the area below.

F = S_f + P_a * 5.85 in² – P_b * 3.04 in²

At idle the pressure in front of the throttle plate is zero, but the manifold pressure is negative or vacuum. We want to ensure that the BOV won’t open at idle so our above equation is changed as. We will assume a positive force holds the valve shut.

0 < S_f - P_a * 5.85 - 0 * 3.04

Since P_a is now a vacuum pressure its sign changes from positive to negative. If we solve this equation for S_f we get:

S_f > P_a *5.85

This says that as long as we know the engine vacuum at idle, we will be able to select a spring that will deliver enough force to keep the valve closed at idle.

At full throttle we will be making full boost. At full throttle the pressure on both sides of the throttle plate is equal. This means that the pressure above and below the diaphragm are equal. So in our equation P_a = P_b in the full throttle case we will call these pressures P.

F = S_f + P * 5.85 – P * 3.04

or,

F = S_f + P * (5.85 – 3.04)

finally,

F = S_f + P * 2.81

S_f is the spring force and it holds the valve shut so it is always positive. Also P is the maximum boost pressure so it has a positive value. This tells us that F is always positive. This makes it impossible for the BOV to open at full throttle. Furthermore, since the area above the diaphragm is bigger then the area below it, we can set S_f equal to zero and the BOV still won’t open. What this means is that, at full throttle only, you could use a BOV with no internal spring and the BOV would still function properly.

Idle and full throttle are relatively easy cases to discuss, but dynamic driving is slightly harder. If you want to know when the BOV will open you need to examine the original equation again. Like before we will assume that a positive force holds the valve shut. In our case we want to know when the valve will open. This will happen when our equation is negative, or:

0 > S_f + P_a * A_a – P_b * A_b

In the idle and full throttle conditions we were able to make assumptions about P_a and P_b which made this equation easier to solve. Here P_a and P_b aren’t equal to each other or zero. To make satisfying the above equation easier we need to set P_a or P_b and solve for the other. So we will say for a given P_b what manifold pressure, P_a, will cause the BOV to open. Our equation will look like this:

P_a < ((P_b * A_b) – S_f) / A_a

This is easier to answer by looking at the chart below. The lines are the critical values or when:

P_a = ((P_b * A_b) – S_f) / A_a

The x-axis is our given value, P_b. If you go vertically upward from your selected P_b until you intersect the line corresponding your BOV’s rated pressure, and then go from that point horizontally till you intersect the y-axis you will find the manifold pressure, P_a, that will cause the BOV to open. As an example say P_b is 10 PSI, and we have a -11 PSI BOV. Moving vertically up to the -11 PSI line from 10 on the x-axis, and then horizontally to the y-axis we find that the valve will open at about -6 PSI. Solving the above equation to the right of the -11 PSI line gives -5.8 PSI. So you can solve for P_a by looking at the graph or solving the equation next to the line.

The BOV remains open until your manifold pressure becomes greater then critical value obtained from solving the equation next to the line corresponding to your BOV.