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.

The Holden Commodore VL Turbo is an example of an early factory turbo car with no BOV. VLs are known for the flutter sound on modified cars, but they ran low boost pressures from the factory.

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 misconception…

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 areas which do not have to be sealed under normal 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!

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6 thoughts on “Five “facts” about blow-off valves that are wrong

  1. My question is that my 88 toyota pickup had a ct20 i bought a new one it has an internal wastegate. Should i still run a blow off vavle if the new turbo builds more boost than the original one?

  2. Hi there

    Just wondering what options you have for a BOV for a built 2JZ-GTE engine. I’m particularly interested in a plumbed back item but I’m not sure if this is possible for boost in access of 35psi.

    Cheers, Ross

    • Hi Ross,
      There are indeed plumb back options for when you need serious flow! Our Big Bubba BOV would cope with a built engine pushing big power, but it is (as it’s name suggests) big, so you’d have to make sure there’s enough room under the bonnet where you plan to mount it. However, if you were thinking of going vent-to-atmosphere, the Race Port is pretty much unbeatable in its flow rate and compact size.

  3. I recently putchased a TS-0203-1222 BOV for my falcon but damn i can barely hear my turbo flutter. Great piece of hardware im happy with it but is there another bov for a ba xr6t that will make the reverse oscillation sound louder?

    • Hi Rodrigo,

      The volume of your intake (flutter, induction noise, etc.) is determined by the type of intake pipe you use on the turbo, a free flowing metal intake pipe will probably give you the loudest noise. The BOV only affects whether the turbo flutters or not, not how loud it is (the primary purpose of a BOV is to stop the turbo from making this noise).