Five “Facts” about Blow-Off Valves that are wrong


Turbosmart was built on the blow-off valve. It was the first product we ever made, and since then we have been endlessly working to improve and perfect it. As a result, we’ve learnt a thing or two over the years, and we want to put some of the most common misconceptions we’ve heard to bed.

1. “Cars don’t have blow-off valves from the factory, so I don’t need one”
Back in the early days of turbo cars, this was true – most factory turbo cars did not come with a factory blow-off valve. However, these early turbo cars were also running relatively low boost levels.

Most modern turbocharged cars do have a blow-off valve from the factory, but it recirculates the vented air, so it doesn’t give the characteristic sound of a vent-to-atmosphere blow-off valve. Recirculating BOVs are sometimes called bypass valves (BPVs). Nevertheless, standard BOVs/BPVs are often very basic, sometimes made of plastic and suffer from poor valve response and do not cope well with increased boost levels.

However, it is important to note that the reason these cars run blow-off valves is to prevent compressor surge, which leads to the next ‘fact’…

2. “Turbo flutter is harmless”
That fluttering noise that some turbocharged cars make when the throttle is suddenly closed, such as during gear changes, is the result of compressor surge. It’s a fairly complex phenomenon which we explain in far more detail here. Basically, that flutter is the sound of a turbocharger trying to push air but failing due to a closed throttle, and it dramatically increases load on the bearings of the turbo. If it occurs at higher engine loads and/or boost levels it can cause premature wear to your turbo. A BOV prevents turbo flutter by venting the air pressure that causes surge.

3. “Blow-off valves only vent to atmosphere”
This misconception is likely due to the fact that the characteristic sound of a blow-off valve, that loud, sharp “pssh”, is created by a vent-to-atmosphere valve. Recirculating or plumb back valves do exist, and are very popular – you just don’t hear them. As mentioned above, most modern factory turbo cars have a recirculating blow-off valve as standard. Turbosmart manufactures high performance aftermarket recirculating blow-off valves as well as the more noticeable vent-to-atmosphere versions.

A Turbosmart Type 5 Plumb Back BOV – it recirculates 100% of vented air back to the car’s intake.

Our Dual Port valves feature a split between recirculating and atmospheric operation, giving quiet, computer-friendly operation on low boost, and that classic vent-to-atmosphere sound at higher boost. Our Type 5 Dual Port even lets you switch between fully vent-to-atmosphere, fully recirculating, or a mix of both as you wish – as explained here.

4. “A BOV is the cause of a boost leak if it leaks in a smoke test”
A smoke test attempts to diagnose boost leaks by pressurising the turbo system with smoke, and observing where the smoke is escaping. However, this test does not replicate real driving conditions.

Typically in a smoke test, the plumb back port of a BOV is pressurised, which never happens when driving. This means air leaks out of the BOV through areas which do not have to be sealed under operating conditions.

Another form of test which is not an accurate means of testing a leaking BOV is the soap test. A soap test is similar to a smoke test except soapy water is sprayed on the BOV (and around the rest of the turbo system) and the system is pressurised with air. Bubbles will form in the soapy mixture where air is escaping.

There is no piston style valve that does not show some leakage in these tests. They all do, and for a very good reason. Just like a piston within an engine, there needs to be a small amount of clearance to allow the BOV’s piston to move. There is a very small amount of clearance between the bore of the BOV and the piston so it can move freely.

This does NOT cause any boost leaks. The amount of air getting through is negligible, and would not cause any boost drop in the real world.

5. “A blow-off valve needs an O-ring to seal”

Some aftermarket blow-off valves require a rubber O-ring to seal the piston with the valve body and prevent leakage, which will perish or become displaced over time. That means the valve will have to be disassembled and that little O-ring replaced to ensure the valve continues to operate as it should. Turbosmart’s blow-off valves are precision machined to mere fractions of a millimeter to ensure they seal without the need for a delicate rubber O-ring, making them much more durable.

Is there anything else you want the experts to answer? Let us know in the comments!


23 thoughts on “Five “Facts” about Blow-Off Valves that are wrong

  1. Is your turbosmart blow off valve good for off-roading with an F 150 EcoBoost, if the blow off valve is submerged underwater could it suck water in?

    • There is potential for water to enter a submerged BOV, yes – but due to the nature of the operation the BOV would be closed while the engine is sucking in air, and when the BOV is open it will be venting air out, so water should not get in.

      Nevertheless, there is a chance it could happen so we’d advise against deep water crossings if you’re using an atmospheric BOV like the Smart Port for the 2013+ F150.

      A plumb back BOV (available for the 2011-2013 F150 EcoBoost) would in theory be no different to the standard valve – and being mounted on the compressor cover on the turbo, you’d probably be in a lot more trouble than just having water entering the BOV if you got it that deep.

  2. I have a mk4 jetta and am running full atmospheric. I’ve realized it is messing with my idle, causing it to Bob up and down, under 1000 rpm. I’ve also recently got a check engine light almost immediately after installing the BOV.

    • Hi Cody,
      Have you checked the adjustment of the spring tension of the BOV? If the engine is idling abnormally after installing a BOV it’s usually due to the BOV being adjusted too soft, meaning the valve remains open at idle. You need to turn the cap towards the harder setting until the valve stays closed at idle. This should solve the issue.

  3. Hello,

    I have just installed a dual port BOV on my Nissan Juke, and after installing it, it feels like the car is weaker, and the boost is opening later than usual. With the stock BOV, the throttle response was better, where it needed half a pedal to accelerate and open boost.

    Please do let me know if you have faced this issue before or what might be causing that?

    Thank you,

    • Hi Joseph,

      Can you double check that the vacuum line running to the top of the BOV is installed correctly, and has no splits or kinks in it.

  4. I have a 1990 Mustang GT, 70mm turbo. Having issues and I don’t know if its turbo related or not.

    so here is the issue. If I take it up to 5500 – 5800 rpms and shift it like a granny everything is fine. If I take it up to 5500 – 5800 rpms and shift it hard the car will loose all power and want to shut off once I bring it to a stop. I can give it a little gas but anything more than 10% throttle it will just bog down. I can here the blow off valve letting air out with any throttle i give it. I can put the car in neutral I can rev it up with not problem, but when i put it back in gear it gives me the same issues untill I shut my car off turn it back on and everything is ok. After I restart its like it never happened. this mainly will happen between 1st and 2nd, in between the shift. This makes not sense to me is why I am reaching out for help. Its either Fuel or Ignition. don’t think its fuel but hell who knows. PLEASE HELP.

    • Hi David,

      Are you running one of our blow off valves on your Mustang? What ECU are you running? have you checked that the BOV isn’t getting stuck open? Also is there a setting in the ECU sending the car into limp mode?

  5. I just put my turbo smart bov to fully atmospheric I should be running about 19 to 20 psi in my Subaru. I now have lost a ton of power and only hitting about 12 psi in sport # and can’t hear the bov at all it was working fine

    • Hi Nick,

      If all you have done is to install the blanking plate in place of the plumb back fitting, and use a hose plug on the recirculation pipe, then there is no way that the BOV could be leaking. Can you give me a quick run-down of what you did to make it full atmospheric please, or feel free to email me at tech@turbosmart.com.au

  6. Since installing my BOV when I accelerate it kind of sputters at the BOV, is this because I don’t have the BOV in the right position and it’s opening to early?

    • The BOV should close immediately when you begin accelerating or the engine returns to idle. Double check all the vacuum hose connections and make sure the BOV is getting a reliable manifold pressure reference. If it’s not closing quickly enough at idle, you will need to adjust the spring tension harder.

  7. Hi, I want the sound to be the famous sutututu valve that I recommend? Recirculated or vented? I have a bmw 325 thank you very much

  8. Hi,
    I run a supersonic on my 4.5 petrol landcruiser and it works perfect no problem
    Here, just concerns when off-road Robles it have the potential to suck and dust/mud
    Into the engine? I wasn’t sure so fitted a 38mm pof filter but not sure how
    Effective that is?

  9. Hello, I have a turbo smart dual port bov 25mm installed on my supercharged 07 civic si. It originally came with a recirculating valve but I wanted to replace it with the turbo smart. At idle it’s fine, but when I’m driving, and let off the gas, the bov seems to be open for a long period of time until I get back on he gas. Is this normal for the dual port?

  10. Hello, so I still can’t decide wether I should get an intake or a bov for my 2016 wrx? I know that neither will give me power but the sound is what I am after. Therefore, my question is Will it harm my engine in any way? I drive 80-100 miles a day and it would really suck if anything went wrong. I do have an acessport now and running stage one, no mods at all. Please help? Thsnjsn

  11. I just installed your BOV on my ecoboost mustang and now my check engine light is on..I checked the error code and it’s showing P0035 error code any solutions?

  12. I just installed a dual port bov on my 2016 mustang ecoboost and as soon as I turned it on I had a check engine light on I connected a ready to it and it said something about the turbo bypass connection. Any help?

  13. hello my name is kevin and im wondering if its bad to have a bov? I have a 2005 Subaru sti with perrin cold air intake, cobb AP, and catback. stage 1. I just recently bought the turbosmart dual port bov and im wondering if its safe to install?

  14. i installed my BOV into my 2017 ecoboost mustang, i get a little turbo flutter in low RPMs. if i go softer on the BOV setting the car wants to stall, is flutter okay in the low RPMs? thank you

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