What is Blow off Valve and How it Works

A blow off valve or a BOV is responsible for releasing high-pressure air every time the car stops throttling. The device basically gives a car the whoosh noise which is also accountable for the avoidance of surge loading of the engine as well as a turbocharger. A blow off valve is also referred to as compressor bypass valve (CBV) or dump valve. This valve is widely known as a pressure release system found in most turbocharged engines.

Nowadays, blow off valve is used in many applications. It is mainly because this automotive device helps a lot in the improvement of a vehicle’s overall performance. The blow off valves also makes the turbocharger life span increase at the same time. If you have the best blow off valve, you can save more money in the future because you can save more money in repair works. Considering the best brand with best features, you will surely hold higher boost levels and improve throttle response compared to ordinary devices.

Blow Off Valve Functions

A BOV is just similar to a wastegate. The only difference is that it works on the intake side of the turbocharger. That once a certain level of pressure is reached, the inlet air opens a spring. This will allow for the intake air to go downstream of the turbocharger to either escape to the atmosphere or redirected back into the turbo. The valves will then give iconic “pssshh” sound. When that sound is heard, the driver wants to let off the throttle. However, instead of letting this back through the compressor with the high-pressure air, the blow off valves allow this to vent. 

A compressor bypass valve or the CBV is also known as a pressure relief valve or diverted valve. It is a manifold vacuum valve designed to release pressure in the intake system of a turbocharged car. The air pressure therein is recirculated back into the non-pressurized end of the intake. A blow off valve performs the same task as the CBV. The only difference is that it releases the air into the atmosphere instead of circulating it. 

This is the reason why its action produces a range of distinctive hissing sounds but the sound it makes greatly depend on the exit design. There are different kinds of blow off valves. Some of them are sold with trumpet-shaped exit which intentionally amplifies the sound. And some owners with turbocharged vehicle prefer to purchase a blow-off valve solely for the auditory effect even if the function is not anymore needed for normal engine operation. However, most motorsports are governed by the FIA and have made it against the law to vent unmuffled blow off valves to the atmosphere.

One other best thing of blow off valve is that it is used to prevent compressor surge. This is a phenomenon that readily happens when lifting off the throttle of a turbocharged engine that is not vented. Every time the throttle plate on a turbocharged engine closes, the high-pressure air in the intake is being trapped by the throttle and a pressure wave is forced back into the CBV. The compressor wheel then moves rapidly. This would be the time where a driver may notice a fluttering air sound. If the surge event happens but the driver accelerated immediately, the severe lag may occur due to the rapid slowing or stalling which stresses the turbo.

Advantages of a BOV

It is great to have turbocharged cars and driving them is awesome at the same time. However, most of the time, there is one thing sadly lacking with them and that is the performance-type sound when doing the shifting. Good thing that a blow off valves is invented and when installed in the vehicle, they can make a head-turning sound while boosting vehicle performance. 

Remember that back in the early days of turbo cars, most factory turbo cars did not come with a factory blow-off valve. These early turbo cars were running relatively low in performance. Most modern turbocharged cars today come with a blow-off valve from the factory but they do recirculate the vented air so it doesn’t give the characteristic sound and performance. 

Standard BOVs are very basic as well because most of them are just made of plastic and suffer from poor valve response.

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