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?
Recirculated:
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)
Atmospheric:
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.
Wa7d mn el rb3 ALLAH YASTER 3LEH ;p
shra blow off kit for his R35 o sa3at yf9el o yg3d ydos o y5fef fj2a 3shan y6l3 9ota el blow off LOL . msaken el pistons mlota ;p
lool, his doing that to an R35?? :S speechless
Ya, but how are you going to enjoy a car if you dont abuse it abit ?:p
You enjoy the car by driving it hard to its limits, not by putting junk in it
so simple and so meaningful at the same time, great work anoos….
but i hve a question ,, the pshh sound can be in any super-car !! or depending on the car’s BOV ???
A BOV is only available in turbo cars, so only turbo cars can make the sound.
Also, you cant hear it from my Evo or any stock turbo cars because I have the stock BOV and stock intake system (stock intake is inside a box which reduces the sound). If I change my intake to an open filter intake (like K&N intake) or I change the BOV, the sound will be loud enough to be heard.
Why recirculated?
Thanks
People running atmospheric on newer cars, e.g. Evo Xs are experiencing drivability issues, like hesitations. If you want to be technical, here is a good read from wikipedia:
In the case where a mass airflow sensor is used and must be located prior to the blowoff valve, the engine control unit (ECU) will meter out excess fuel because the atmospherically vented air is not subtracted from the intake charge measurements. The engine then briefly operates with a fuel-rich mixture after each valve actuation.
The rich mixing can lead to hesitation or even stalling of the engine when the throttle is closed, a situation that worsens with higher boost pressures.
I knew a girl, she had a great blow off valve
. Great post Anoos. Keep it up.
Looool. Lets keep it clean in here :/ haha
“The BOV is like a badge that says Hey I’m really into my car”
Well said just weeell said =D
I don’t get it, You kinda convinced me that the better option would be to install a recirculated bov, but when i watched the video there is not difference in power with atmospheric or recirculated. I get the part about the valve been closed when u off the throttle and the and the turbo wheel not spinning but in the video the dyno results show that it doesnt make a difference.
Please reply because i was kinda excited that i learnt something new about Bov’s but now i just feel like a fool
Having a recirculated BOV is not better because of performance advantages. Both will probably net you the same performance figures. Its because the ECU (Engine Control Unit) on newer cars do not respond well with atmospheric BOVs resulting in drivability issues. People experience hesitations and other negative effects. All vendors will strongly advise you to have a recirculated BOV. (also check my reply to Omars’ question above)
Plus, atmospheric BOVs sound ricey
If you need any more help, please ask me.
So what u are saying is that the atmospheric bov wont affect figures on the dyno but will affect my times on the 1/4 mile coz of lag???
Sean, I did not mention anything about 1/4 mile or lag. I’m saying, atmospheric BOVs will cause issues to the drivability of your car, meaning it will not drive smooth (hesitations), and cause other undesirable effects. Got it?
ok i get it now, I have a 2008 VW GTI and wanted to change to a atmospheric bov for the dumping sound, is it severe hesitations?
To get the BOV sound, change the air intake to an open filter intake. like the AFE Stage 2 intake (http://tinyurl.com/369yjj8)
When you change your intake your BOV will sound loud and sexy. I highly highly suggest not changing your BOV unless you have a boost leak, which only occurs if you highly modify your car (like changing turbo and stuff).
If I was you, Ill get the AFE intake, nice exhaust and Ill have one sexy sounding GTI, and the BOV will be heard.
Totally agree with you. I’ve seen people installing BOV on German cars as well (335i, GTI) which sound really wrong. I hear the sound and I look for a 1994 nissan 240SX or something only to be surprised that it’s coming from a GTI :S
My brother installed neuspeed CAI on his GTI and the sound is just amazing. no need for BOV at all.
Thanx man, perfect reply, u made my day
I hope Sean reads this..
Just to clarify on some points that Anas has already discussed, BOV’s are not necessarily a bad thing depending on where it is installed on the intake piping.
For example on some cars (such as the Skyline R34, Supra, etc.) the blow off valve is installed between the MAF sensor and the throttle body (mainly due to stock piping design). Unfortunately the pressurized air that vents from the BOV has already been metered by the MAF. Therefore the ECU will compensate for this amount of air by fuel enrichment. In this case, the engine momentarily runs rich since the ECU is assuming the air is going into the engine rather than being vented to the atmosphere.
This issue however is not present in cars such as the BMW 335i or turbocharged G35/G37/370Z where the BOV is located before the MAF sensor. In this case, any pressurized air that is vented from the BOV is not metered and therefore the air/fuel ratio is unaffected. In such cars, installing BOV’s has little negative effect on fuel compensation. The only other issue that may arise is explained below.
The other advantage for recirculated valves is turbo lag during off-throttle conditions. In such cases, and unlike BOV’s, recirculated valves will vent the pressurized air into the intake piping right before the turbocharger. This vented air helps keep the turbocharger spooling and thus increases engine response which greatly helps in racing situations when upshifting and coming out of a corner. This is particularly true in cars which have large turbochargers.
Othman.
Wow, thanx for the awesome write up
That clears things up Othman, thansk guys!