• The Case for Blow-Off Valves: Why Turbocharged Engines Need Them and Supercharged Engines Don’t

    One of the most important additions you can make to a turbocharged engine is the installation of a turbo blow off valve. While many factory turbocharged vehicles will come with a BOV from the factory, they are not commonly included in most low-boost aftermarket turbo kits. While they are not absolutely required in these low boost situations, a BOV is a good idea for any turbocharged application.

    A Question of Performance and Durability

    Turbochargers introduce large amounts of pressurized air into your engine. When your throttle plate closes however, this airflow has nowhere else to go but backwards, which sends it back towards the compressor. This pressure wave tries to reverse the rotation of the compressor which at times can be spinning at many tens of thousands of rpms, and the resulting stress can cause failure of the bearings or the turbo itself. What a blow-off valve or recirculation valve does then, is give this pressure wave a place to go, reducing stress on the turbo and improving response. While this compressor surge can be more of an annoyance than a danger in low boost applications, it still can dull boost response. In high boost applications, this compressor surge as it is called can cause outright failure.

    Who Needs One

    Everyone with a turbocharger-equipped engine should have some form of relief valve installed for both longevity and improved performance, especially in the case of daily street driven vehicles. There should be some care taken when selecting one however, especially in vehicles with a mass airflow system. Installing a vent-to-atmosphere type of BOV or VTA BOV will cause incorrect air readings and overly rich idle, stumbling and stalling problems. MAF equipped vehicles should always have their BOV properly recirculated back into the intake tract to prevent these problems.

    While a BOV may only seem to help response in low boost applications, in reality, a properly installed BOV can vastly improve longevity in low and high boost applications alike. The simple answer to this question then is that all turbocharged vehicles should have a BOV installed for best results.

    Recirculating Blow-Off Valve

    A recirculating blow off valve is a necessary part of cars with a turbocharger. The blow-off valve or BOV relieves the pressure created in the air intake system when the driver lets off the gas and the throttle closes. The excess pressure builds up and is forced back into the turbo, causing the compressor wheel to slow and even stop the blades from turning. This results in a compression surge causing a turbo lag and thus a drop in performance. A recirculating blow off valve releases the pressure from the turbo, diverting the excess back into the engine, preventing the turbo lag caused by the excess pressure, maintaining the proper and efficient spool up of the turbo.

    There a few notable advantages of having a recirculating blow off valve.

    For some car enthusiasts wanting performance this may or may not be a big deal. Vent-to- atmosphere blow off valves release the pressure buildup by allowing the excess pressure to vent outward instead of back into the engine. Think of a teapot when the boiling point of the water is reached. The design of the teapot causes the buildup of the air to vent through the hole in the spout. The same concept applies to the blow off valve. Not all BOV’s are noisy but for those wanting a quiet turbo for either aesthetic or due to state regulations on noise levels, a recirculating BOV is the better choice.

    The air diverted back from the intake contains small particles of oil, which when recirculated back through the compartment, lubricates the engine.

    Finally, the recirculating blow off valve is a contained system and will not pose problems with debris and dust for vehicles used in rally or on roads where particles of mud or sand could cause major damage to the engine.

    Why Superchargers Don’t Need Blow Off Valves

    It isn’t necessary and doesn’t provide any benefit whatsoever. Blow off valves are used only in turbocharged-engines and its purpose is to relieve the pressure built up within the air intake system that is capable of stalling the compression wheel. A true supercharged engine works using forced induction which internally channels the air pressure back into the combustion system and compresses it using a pump. A turbocharger works on the same principle however it uses a turbine and the exhaust from the car’s engine to provide the necessary compression in the air system, not a crankshaft driven pump.

    Blow off valves are sometimes confused with bypass valves, which in a supercharger, diverts the air back into the engine directly, it doesn’t vent out into the atmosphere. If your supercharger is centrifugal, a blow off valve can be used however you should only use them if your supercharger has a blow-through mass-air meter (mass-air flow, or MAS) installed or your vehicle will suffer performance problems. If your supercharger isn’t a centrifugal one or utilizes a mass-air meter, a blow off valve is pointless. A supercharger typically is always on, and doesn’t need a blow off valve to sense the pressure build up and engage to vent the pressure. Some Roots type superchargers can have a blow off valve installed, but still you would need to have a bypass valve to properly run the supercharger as designed.