Buyer's Guide: Blow Off Valves
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Blow Off Valve Buyer's Guide
Blow off valves (also called dump valves, bypass valves, and recirc valves) are used in turbocharged engines to relieve excess boost pressure in your intake system when the throttle closes. Blow off valves protect against compressor surge, which causes the characteristic turbo flutter sound ("stututu"). As cool as this sounds, compressor surge slows the turbocharger and causes excess stress, reducing the turbo's lifespan. A blow off valve is an essential component in any good turbocharged system. This guide walks you through everything you need to know about blow off valves, how they work, and how to choose the right one for your vehicle.
Compressor Surge Explained
When you're accelerating under boost, the turbo spins up and pressurizes the intake system. When you lift off the throttle (say, to shift gears or brake), the throttle plate snaps shut. Now the turbo is trying to push air through a closed door. Without a blow off valve, the compressed air has nowhere else to go except backwards through the compressor wheel.
This reversal, called compressor surge, produces the characteristic "stututu" sound. This process damages the turbocharger bearings, reducing its lifespan. It can even damage your intercooler and intercooler piping from the excess pressure.
A blow off valve prevents compressor surge by releasing the built-up pressure when the throttle closes.
How Blow Off Valves Work
A blow off valve (BOV) is installed on the intercooler piping between the intercooler outlet and the throttle body. A vacuum hose connects the top of the BOV to the intake manifold, providing the pressure signal that tells the valve when to open and close.
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Open throttle (under boost): When you're accelerating, boost pressure pushes on both sides of the BOV piston, and the spring holds the valve closed. This results in the charged air flowing like normal to the engine.
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Throttle closed: When you lift off the throttle, the intake manifold pressure drops to vacuum. This vacuum signal travels through the hose to the top of the BOV, creating a pressure differential across the piston. This pressure difference overcomes the spring force, opening the valve and venting excess pressure before it can surge back through the compressor.
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Idle: The intake manifold is under vacuum at idle, but the BOV spring is calibrated to remain closed under normal idle vacuum levels. Many BOVs allow you to adjust the spring preload based on the amount of vacuum your engine pulls.
Vent-to-Atmosphere BOV vs. Recirculating BOV
Blow off valves can either vent to the atmosphere, or recirculate the air back into the intake system.
Vent-to-atmosphere BOVs release excess charge air directly into the open air, producing the characteristic "psssht" sound. On vehicles equipped with a mass air flow (MAF) sensor, this causes a temporary rich condition because the ECU is still expecting that air to enter the engine.
Recirculating BOVs (also called bypass valves) route excess air back into the intake system upstream of the turbocharger. This maintains proper air metering and is fully compatible with MAF-based systems, which is why recirculating valves are commonly used on factory turbocharged vehicles.
General guidance:
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MAF-based systems (most modern vehicles) → recirculating BOV recommended
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MAP-based systems → either option works
BOV Sizing
The primary performance characteristic of a blow off valve is its airflow capacity, which is determined by valve diameter. Aftermarket BOVs typically range from 38mm to 60mm. Larger valves can flow more air, allowing them to more effectively relieve pressure in high horsepower or high boost applications.
As a general reference, a 50mm BOV can handle approximately 1,000 hp. Manufacturers often publish maximum horsepower ratings for their BOVs to help with proper sizing. It's practically impossible to oversize a BOV, although you may be spending unnecessary money.
Mounting and Installation
BOVs mount directly to the intercooler piping, and the connection must be a precise match. Common mounting types include:
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V-band
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Hose clamp
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2-bolt flange
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4-bolt flange
Confirm your intercooler piping's connection type and diameter before purchasing. A mismatched mount is the most common installation issue with aftermarket BOVs and is easy to avoid with a quick check beforehand.
Conclusion
Blow off valves release excess pressure from your intake system when your throttle closes to protect your turbocharger. When choosing between BOVs, first decide between a vent-to-atmosphere or a recirculating BOV based on your engine's air metering system. MAF-based systems should use a recirculating BOV. Size the BOV appropriately for your horsepower and boost level — when in doubt, larger is better. Most importantly, confirm your mounting connection type. A properly selected and installed BOV improves system reliability and enhances the overall driving experience of a turbocharged vehicle. You can find all the parts you need and plan your build on RideBuilder.
