When it comes to forced induction, even the most peripheral turbocharger enthusiast is familiar with the component in the system known as a blow-off valve (often abbreviated “BOV”). While that may be due in large part to the fact that it is responsible for the trademark “woosh” noise associated with turbochargers, it serves a very critical purpose in a properly engineered turbocharger system.

However, as Ben Strader of EFI University has found in the course of his various performance classes, the blow-off valve’s true purpose is often misunderstood, even by the most knowledgeable enthusiasts. So even if you think you know what it’s for, read on, and we’re willing to bet you learn something new.

“Most people are familiar with the wastegate, and that its job is to control the amount of boost that is produced,” Strader says. “But a lot of people have misconceptions about what the blow-off valve actually does.” In the above video, he aims to clear up those misconceptions.

The Basic Problem

In order to understand what a blow-off valve is doing, you have to understand the inherent problem that exists in a turbocharged engine. “What will happen is when you are driving along at wide-open throttle in full boost, and you quickly close the throttle, all of the air that was coming out of the turbocharger gets stopped by the throttle plate. That air has to go somewhere,” explains Strader.

Without any kind of pressure relief, like some OEMs have done in the past, that air needs to go somewhere, and that usually means it comes back out of the turbocharger. “What happens is that air gets stacked up [in the charge pipe] and wants to reverse into the turbocharger. That causes a situation called ‘compressor surge’,” says Strader. “Surge is a condition where the blades of the turbocharger can no longer create pressure coming out of the turbo. That causes them to slip, or ‘surge’.”

To alleviate that problem, a pressure relief valve is placed in the system. There are several different types of valves, used for different reasons, but for the sake of this article, we’re going to stick with straight vent-to-atmosphere blow-off valves. “We can put this blow-off valve between the throttle and turbocharger. That way, when the throttle closes, the manifold sees vacuum, that in turn opens the valve, and lets the air that is otherwise trapped between the turbocharger and closed throttle plate ‘blow off’. Pretty simple,” says Strader.

What’s There To Understand, Then?

It seems simple, the blow-off valve relieves the pressure in the system, when there shouldn’t be any. Boom; end-of-article, right? Well, not really. This is EngineLabs and we like to dive a lot deeper into things than that, as does Strader, which is why we kinda like him.

“When I ask people: ‘Why do you want to prevent compressor surge?’ the most common answer I get is usually some form of, ‘So that the backup of air doesn’t slow the turbocharger down,’” says Strader of his experience in his classes. That answer, which is commonly held among the turbocharger community, is actually the opposite of correct.

“The real truth is, when the turbocharger gets backed up and experiences surge, the blades don’t actually slow down,” Strader relates. “In fact, the compressor wheel speeds up in surge. We see this all the time in our data, when we hear that surge whistle in the compressor.”

You might be wondering how a backup of excess air pressure can cause the blades to accelerate. One word: cavitation. “Think about it like this: when the compressor wheel is working properly, the blades in the air are like paddles in water. They are digging in and pushing the air out,” Strader elaborates. “When the air gets stacked up in the tract, the blades ‘lose traction.’ That causes the wheel to accelerate, just like a tire that loses traction under power.”

Why Speeding Up Is Bad

So the logical next question is, “If it doesn’t slow the turbo down, why is that bad?” Well, for the same reason that doing neutral drops is bad for your transmission.“In the turbochargers center rotating assembly, there is a thrust washer or thrust bearing. Because of the shape of the blades on the turbocharger, as they are creating pressure, they are also creating an axial load, pushing the compressor assembly towards the center of the turbo,” Strader says.

“When the compressor surges, the blades are losing their ‘traction’ in the air, which unloads that thrust bearing. Then once it regains traction, it is rapidly loading up the assembly again. Imagine what would happen to your car’s driveline, if you were driving down a hot, sticky asphalt road at full power, and then suddenly you hit a patch of ice. Your wheel speed would zing up. Then when you get past the ice and back to the sticky asphalt and get all your traction back, it would grab and overload the drivetrain, probably snapping an axle in the process”

This is the sound a turbocharger makes with no blow-off valve installed. That fluttering noise is compressor surge when the throttle blade closes under boost.

That loading and unloading of the compressor wheel while under power (remember, the exhaust is still driving the turbine wheel during all of this) will beat the snot out of that thrust washer in the rotating assembly. “There is only about .020 inch of clearance between your compressor blades and housing, and it’s the thrust bearing that sets that clearance,” explains Strader.

“You start jackhammering on the end of that because the compressor is surging, it’s going to wear that bearing out pretty quickly. Once that shaft has movement laterally through the turbo, the next thing that happens is that the blade from the wheel crashes into the housing and destroys your turbocharger.”

That’s a pretty serious cross to bear for a simple vacuum-operated poppet valve, but well-built blow-off valves handle the chore with almost boring reliability. And now you know the nitty-gritty of what the blow-off valve does, and that it’s job is to keep the compressor wheel from speeding up, not slowing down.