Blow off valve operation?

[Rich Z]

Well, since I pulled off the blow off valve on my C5Z to check for fluid at the bottom of the intercooler, and also found a broken O-ring, I got to thinking about how the BOV actually works, and drew a blank.

Here's one description of the operation:

A blowoff valve is connected by a vacuum hose to the intake manifold after the throttle plate. When the throttle is closed, the relative manifold pressure drops to below zero bar and the resulting pressure differential operates the blowoff valve's piston. The excess pressure from the turbocharger is then vented into the atmosphere or recirculated into the intake upstream of the compressor inlet.

Hmm, what exactly does THAT mean? If the BOV opens when manifold pressure is below zero bar, then what keeps it from opening during normal driving when the intake manifold has a vacuum in it?

The reason I am thinking about this is because since the wastegate springs were increased from the stock STS 6.5 lbs to 10 lbs, does the BOV need to be adjusted or in any way changed to reflect the increased psi that will be in the intake manifold?

Obviously I don't understand exactly how this device works, but I don't want to just assume that what is on my car now is actually CORRECT.

[Rich Z]

Well, been thinking about how this must work, and I think I may have a handle on it.

If I understand this process correctly, the key word in that description quote I have in the post above is "differential". As best I can figure out, during normal driving, the pressure should be pretty much the same in the intake manifold as it is anywhere in the air intake on the air feed side of the turbos. So this means that in the BOV itself, there will be vacuum on both sides of the valve. Meaning NO differential, so the valve is CLOSED. During boost, pressure builds up, but it is STILL the same throughout the air intake from the intake manifold to the turbos, which means, again, that both sides of the valve are now under the same pressure. Meaning again, with NO differential, the valve is CLOSED.

However, when you take your foot off of the throttle and the throttle blade snaps shut, boosted air pressure is suddenly blocked from going into the intake manifold, which means the manifold suddenly develops vacuum because the piston action is STILL trying to draw air/fuel into the cylinders. So, at this point, the feed line from the intake manifold is providing vacuum on one side of the BOV and the other side of the valve is still under boost pressures, which causes the valve to open in order to vent the excess pressure. Meaning, there is now a DIFFERENTIAL, so the valve is OPEN. At least until the differential ends.

I can see where a BOV might actually be adjustable in order to control both the opening of the valve as well as the closing, in order to try to preserve some boost instead of dropping to zero, as it would likely be beneficial to have some residual boost remaining, so when you shift gears you don't start into the next gear with zip for boost from the turbos to the engine. So there would be a noticeable lag while the turbos again spooled up to produce boost.

In other words, if a BOV opens at any differential and drops the boost down to ZERO psi, then the turbos have to start spooling up again when you apply the throttle in order to reach boost again. But if a BOV still left a few psi of pressure remaining when it closes again, then there is that much boost left available that the turbos don't have to build up when you want it available. Obviously there must also be a point whereby the BOV determines that enough of a differential exists in order to open the valve. So I can see where a BOV might have both the opening and the closing differential points adjustable in order to fine tune how you would want it to work according to your preferences.

I have no idea if the Tial BOV on my car has those parameters fixed, or if it is in any way adjustable. Didn't look adjustable to me, but who knows? I'm just trying to figure out this stuff by the seat of my pants.