The BOV vs BPV – What Is the Difference Between a Blow Off Valve and a Bypass Valve?

The BOV vs BPV – What Is the Difference Between a Blow Off Valve and a Bypass Valve?. Turbocharged engines are intricate systems. Many internal factors must be regulated for a turbocharged engine to function properly. This is particularly true for air temperature and pressure. For those who aren’t familiar with the abbreviations “BOV” and “BPV,” they stand for blow off valve and bypass valve, respectively. Both of these devices serve the same purpose: to regulate air pressure in a turbocharged system. Turbo damage can result if excess pressure is not controlled.

While both blow off valves and bypass valves serve the same function, they do so in different ways. Whereas blow off valves release pressured air into the atmosphere (while emitting a distinctive whistle), bypass valves recirculate air into the intake. Although bypass valves prevent compressor surge and protect your engine from internal damage, some enthusiasts can’t live without that satisfying blow off noise. Blow out valves are also a superior solution for high horsepower applications.

In this guide, we’ll explain how blow off and bypass valves work and why some people prefer one over the other.

The BOV vs BPV - What Is the Difference Between a Blow Off Valve and a Bypass Valve

Why Do Turbocharged Cars Require a Blow Off / Bypass Valve?

To understand why turbocharged engines require a blow off/bypass valve, you must first grasp how a turbo system works. A turbocharger, in its most basic form, takes in air, cools it, and then sends pressured air into the engine. On one side of the turbo, exhaust gas spins a turbine. The turbine then spins another fan, known as the compressor, on the opposite side of the system, drawing new air into the turbo system.

The air intake system of a turbocharged vehicle is critical to the operation of the turbo system. The throttle body is a vital component of the intake system. The throttle body controls how much air enters the engine. After passing through the throttle body and into the intake manifold, air enters the turbo and is cooled by the intercooler. The air is then drawn into the engine. If internal air pressure is not managed in some way, problems can emerge.

When you let up on the gas, the throttle body closes, preventing air from entering the combustion chamber. When you depress the throttle while under boost, a burst of compressed air goes through the turbo system and is abruptly stopped at the closed throttle plate. At that point, the pressured air has nowhere else to go but back. The air is driven back into the turbo, causing it to collide with the compressor wheel. This is known as ‘compressor surge,’ and it can cause damage to your turbo’s wheel shaft and bearings. If compressor surge is significant and frequent, it can eventually eliminate turbo responsiveness and overall drivability.

What Is the Fix for Compressor Surge?

As a result, when an engine’s throttle plate closes, there is nowhere for the additional boost to go. As it turns out, there are a few options for dealing with that problem. One approach is to put a valve in the intake piping of a turbocharged engine that vents to the atmosphere to completely eliminate the extra pressure. The alternative option is to install a different type of valve that returns the pressure to the turbo’s inlet.

BOV vs. BPV: What Is the Difference?

Now that we’ve covered how compressor surge occurs, we can look at how blow off valves and bypass valves work together to alleviate the problem. We’ll begin with the blow off valves. When the throttle plate is closed, blow off valves take a more direct approach to dealing with excess pressure in a turbo system. They just discharge the extra air into the atmosphere.

Blow off valves are commonly fitted between the turbocharger compressor and the throttle plate on an engine’s intake pipework. A spring, a diaphragm, and a valve are the three primary components found inside the main housing of a blow off valve.

When you press the accelerator, the pressure in the turbo pipework and the inlet manifold is equal. That is, the pressure on each side of the valve is equal and hence cancels itself out. The spring closes the valve with equal pressure on both sides. Internal pressure changes cause the internal diaphragm to contract. When you depress the accelerator, there is high pressure in the turbo piping and a vacuum in the inlet manifold. Because the pressure in the turbo piping can no longer enter the engine, the pressure at the bottom of the valve and the vacuum at the top combine to lift the valve open and release the pressure in the turbo piping.

What are the Drawbacks of Using a Blow Off Valve?

While BOVs eliminate compressor surge, they can have negative consequences if all excess air is vented into the atmosphere. Venting to the atmosphere can interfere with ECU air/fuel ratio readings on turbocharged engines equipped with mass airflow sensors. As a result, your car may run rich because the ECU fails to appropriately fuel the engine. Damage might occur if the ECU fails to properly feed the engine over a long period of time.

Furthermore, when excess boost is vented to the atmosphere, none of it is retained in the turbo system. As a result, the turbo has to take in new air again, which takes some time.

BOV vs. BPV: What Is the Difference?

Finally, a bypass valve is intended to tackle the same compressor surge problem as BOVs do. Bypass valves, on the other hand, solve the problem in an entirely different approach. Bypass valves loop surplus pressure back to the turbo inlet where BOVs vent excess pressure into the atmosphere.

When compared to a blow off valve, bypass valves generally keep internal pressures at a much more stable level. As a result, the throttle response is improved and the turbo performance is more predictable. Another advantage of bypass valves is that they often restore air into the system after the MAF, which eliminates the need for the engine’s ECU to compensate for the lost air.

Bypass valves reduce turbo lag by keeping surplus pressure inside the turbo system. That means air is ready to be introduced into the turbo inlet right away. This is not the case with BOVs, which do not have the advantage of instantly reintroducing air into the turbo. As a result, a bypass valve has fewer drawbacks than a vent-to-atmosphere blow off valve.

The majority of vehicle manufacturers who offer high-performance turbocharged automobiles employ bypass valves rather than BOVs. Bypass valves are typically the quieter alternative because they do not generate the loud vent noise that BOVs do. They are also beneficial in terms of performance, as they reduce lag and improve throttle responsiveness.

What precisely is a Hybrid Blow Off Valve?

Aftermarket manufacturers have experimented with the BOV formula throughout time and developed a hybrid between a BOV and a BPV. They are commonly referred to as hybrid blow off valves. A hybrid valve is adjustable, as opposed to a typical BOV, which vents all excess boost pressure into the atmosphere. That is, you can control how much air is vented and how much is recirculated back into the system manually.

Finally, this is the best design for a high-horsepower engine since you have the benefits of a blow off valve and recirculating valve without the drawbacks. If the huge turbo on your build is producing an absurd amount of extra pressure, a significant portion of it can be vented and a lesser portion recirculated back into the turbo to reduce lag and enhance throttle responsiveness. As an added bonus, you’ll still hear some noise from the atmospheric dump. It goes without saying that a hybrid valve avoids the possibility of compressor surge.

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Which is better, BOV or BPV?

There are several elements to consider when deciding between a blow off valve and a bypass valve. The BOV vs BPV debate can go either way. Both systems are extremely effective at removing surplus pressure from a turbo system and reducing compressor surge. Having said that, there are reasons to prefer one over the other.

As previously noted, automobiles with MAF-style intake systems benefit from a bypass valve. Again, this is because blow off valves can interfere with electronically metered systems, as there is no way for a vehicle’s ECU to effectively adjust for lost air once it is vented to the atmosphere. Bypass valves produce a closed system inside the confines of a turbo system. As a result, all of the air in the system has been accounted for.

That isn’t to say that BOVs aren’t a good choice for moderate-horsepower, MAF-style, turbocharged engines. On high-horsepower turbocharged vehicles, blow off valves are actually advantageous. The problem with using a bypass valve on a high-boost engine is airflow. In general, it is difficult to find a recirculating, or bypass, valve with adequate airflow to recirculate the volume of pressured air that a large turbo pushes. Most bypass valves’ physical structure cannot tolerate that much of air volume.

A blow off valve is a superior alternative for high horsepower applications. This is due to the fact that BOVs are far more efficient at removing surplus pressure from a turbo system. Unlike a recirculating valve, which has physical limitations in terms of airflow, a blow off valve can eliminate any amount of pressure almost instantly.