BOV me

Today let's talk about the blowoff valve (BOV) used in the Saab. Ok, the stock one used in this car is actually a compressor bypass valve, or CBV, but I have heard both being grouped under the generic name of BOV. I know one can get off eBay a nice shiny metal one that is a direct replacement, or a custom one that can be mounted somewhere more convenient or dramatic, like this one: but for this chapter the factory one is the object of my attention. So, here is how it looks like:As you can see, there are two large ports and a smaller one (the nipple on its top). The large port that rests against my palm, port A, is connected to the pipe that goes to the compressor inlet of the turbo. The other large pipe, port B, is routed all the way to the throttle body. Finally, connected to the little nipple, port C or just the nipple, is a hose that goes to the plenum of the intake manifold. Let's see how it works. If we turn it so that port B is facing us, we can see a piston inside the BOV housing. By default, it presses against port A, closing it. So, air cannot flow between ports A and B.
No, how does it open? If we apply vacuum to the nipple, it will open the piston. That is fine, but why would we need this extra piece of hardware? To explain that, let's talk about what happens when you are under boost and shift gears in a car with throttle plate.

When you let off the gas pedal, the throttle plate shuts, separating the volume from the turbo compressor outlet to the intake to the volume inside the intake manifold. That also tells the engine to slow down, so it is not producing as much exhaust as before. That exhaust powers the turbo. The engine is still running so it will use some of the air that is inside the intake, which will create a lower pressure area inside the intake as not much air is moving around the throttle plate.

But, let's go back to the exhaust. It is, as we mentioned, not generating as much energy to power the turbo as before. So, the turbo cannot pump as much air as it was doing before you backed off the gas pedal. However, now we have air between the turbo and the throttle plate that was compressed when you were in full power. It has to go somewhere. Between a closed throttle plate and the turbo, which is not getting as much power as before, the path of least resistance is back through the compressor stage of the turbo.

So, we have air trying to go backwards through the compressor wheel. That can cause the wheel to slow down and even spin backwards.

Then, you finished shifting and once again stomp on the gas pedal. Throttle plate is open. Between the turbo and the low pressure area in the intake, the path of least resistance now is through the intake manifold. Also, exhaust is now trying to power the turbo. Still the turbo now has to fight with the air that was trying to escape through its compressor and persuade it to reverse flow one more time.

That is a lot of persuading... and time wasted. And, depending on your turbo, it can only take that so many times before snapping.

What if we could provide a path for this air that is trying to escape that is more attractive than going backwards through the compressor wheel? One possible solution is to let the air dump to atmosphere, which is what the BOO does. A lot of people like to do that because they enjoy the sound of the air being dumped to atmosphere.

Another option is to feed the extra air ahead of the turbo, which is what the CBV does. Problem with that is it does end up feeding hot air to the compressor side of the turbo, which then heats it up an additional amount because of thermodynamics. A possible solution is to suck this extra air after the intercooler, where it would be somewhat cooler. One of the main benefits of using a CBV is that no metered air is being thrown away so we do not end up having a rich mixture.