This video is by mightycarmods.com I highly suggest watching their videos. They’re awesome.
Note: This post is intended for turbocharged cars and for people with basic knowledge of engine parts, articles about all engine parts will be explained in later posts.
What does a blow off valve do?
When you press the gas pedal and accelerate, air goes into the intake system, goes through the turbo, passes the intercooler and piping, and finally to the engine through the throttle body. Now, when you let off the gas pedal, the throttle body closes, which creates a problem. What problem?
There is a huge compressed wave of air traveling towards the engine and the throttle body is closed. So now the air has nowhere to go, the compressor wheel slows rapidly and may even stall, and the driver will notice a fluttering air sound. The rapid slowing or stalling stresses the turbo and cause severe turbo lag if the driver accelerates immediately. Whats the solution?
A blow off valve, also know as a diverter valve. When the throttle body closes, a valve will open which will divert the excess air that is going towards the engine into the non-pressurized area (before the turbo), which will prevent compressor surge and keep the turbo spinning in the same direction.
What types of BOVs are there?
The excess air passing through the BOV will be recirculated back to the intake system. Most vehicles have recirculated BOVs from factory and should be remained that way. (explained below)
The air passing through the BOV will vented out of the system to the atmosphere. NOT RECOMMENDED
Why do people change their BOV?
Most people change their BOV to atmospheric BOVs as they create an OVERLY loud pshhhhhh sound which they THINK is cool. NOTTT, were in 2010 guyz, not 1992.
But I want to hear some pshhhh well I have some good news for you. When you change your intake system to an open filter intake, the stock BOV will be heard and make your beloved sound loud enough to hear it and not too loud to sound like a ricer.
Should I ever change my BOV?
BOVs are made to operate to a certain PSI level (boost pressure). as an example, the stock BOV of the Evo X works very well to 25 PSI of boost. If you tune your car to run over 25 PSI (mostly when changing to a bigger turbo), the BOV will start to leak and consequently lose power. Then, you will need to change your BOV to an aftermarket one, and don’t forget, it has to be recirculated.
One more advise?
For the love of god, DO NOT buy or install the HKS SSQV. I have seen way too many guys running those, even in Evo Xs. PLEASE if you have one, take it off. HKS has recently released a new version which is the HKS SSQV-R which is recirculated. So if you really need an HKS BOV, get that one.