Top 10 Blow-Off Valve Myths (Explained Properly)
Blow-off valves (BOVs) are one of the most misunderstood components in turbocharged engines. They’re often chosen for sound, blamed for drivability issues, or incorrectly adjusted in the pursuit of boost.
At Walton Motorsport, we see the same myths repeated across street builds, track cars, and high-power motorsport applications. So let’s clear the air — what a blow-off valve actually does, what it doesn’t do, and how to choose and set one up correctly.
This guide is structured in bite-sized sections:
- A simple explanation first
- Followed by deeper technical detail if you want it
No hype. No bias. Just engineering.
Myth 1: A Blow-Off Valve Reduces Turbo Lag
The Simple Explanation
A blow-off valve doesn’t make boost faster — it protects the turbo when the throttle closes. It can make sure you're keeping the boost you're making, but it's not magic.
The Technical Reality
When you lift off the throttle under boost, pressurised air has nowhere to go. Without a BOV, that pressure reverses direction and loads the compressor wheel, causing compressor surge. Special Note - Pressure, is not flow. The Air doesn't go backwards, nor does it make your compressor wheel go backwards... But the dynamic changes do rattle the compressor wheel and that's no good for anyone (except Turbo Salesmen) ;-)
A properly functioning BOV:
- Relieves pressure instantly
- Keeps the turbo spinning freely
- Seals Again Quickly, Improving response and consistency between gear changes
It doesn’t create boost — it preserves turbo speed and longevity.
Myth 2: Blow-Off Valves Add Horsepower
The Simple Explanation
A BOV doesn’t make power — it prevents damage. As above, a well suited valve will ensure the horespower you're making isn't lost, but it's not magical.
The Technical Reality
Power is determined by airflow, fuel, ignition, and efficiency.
A blow-off valve is a pressure control device, not a power-adding component.
However, a poorly specced, sized or incorrectly set valve can reduce performance by:
- Leaking under boost
- Opening too early
- Disrupting air-fuel ratios
This is why quality design and correct valve and spring selection matter.
Myth 3: Tighter Spring = More Boost
The Simple Explanation
- Cranking the spring tighter doesn’t let you run more boost — it usually causes problems. The sharpt sound may give you tingless, but it's potentially reducing he abilty of the valve to respond quickly. Equally, too soft and you're leaving boost response on the table.
The Technical Reality
Blow-off valves operate on pressure differential, not boost pressure alone.
If the spring is too stiff:
- The valve won’t open fast enough
- Compressor surge increases
- Turbo bearing load rises
Correct setup means:
- The valve stays shut under boost
- Opens cleanly on throttle lift
- Matches your vacuum and boost levels
This is where modular spring systems, like those used in the Turbosmart range, make accurate setup possible.
Myth 4: All Blow-Off Valves Sound the Same
The Simple Explanation
The sound depends on how the valve vents, not how well it works. Vent to Atmosphere Valves specifically should be sized appropriately to the setup... Smaller valves will be larger for any given flow and equally, sound quiter with less. Chooe Wisely!
The Technical Reality
There are three common configurations:
- Plumb-back (recirculating) – quiet, OEM-style
- Vent-to-atmosphere – the classic “whoosh”
- Dual-port – a mix of both
Performance doesn’t come from sound — it comes from response, sealing, and control.
Many modern builds run plumb-back valves for:
- Better drivability
- Stable fuel trims
- OEM-level refinement
Walton Motorsport Suggestion? Do whatever makes you happy. Noise or no noise, we're not biased - but do take it seriously!
Myth 5: Vent-to-Atmosphere Valves Are Bad for Engines
The Simple Explanation
They’re not bad — they just need to match the engine management. MAF (Mass Air Flow) metered engines fitted with VTA (Vent to atmosphere) valves will dump air it's already measured and this will generally mean a momuntary rich moment, often seen by a sharp pop (of fuel burning in the exhaust system).
The Technical Reality
On MAF-based systems, venting metered air can cause rich conditions.
On MAP-based systems, this is not an issue.
This is why many performance builds:
- Switch to MAP-based tuning
- Or use recirculating BOVs
The valve isn’t the problem — system compatibility is.
Myth 6: Compressor Surge Is Harmless (And Sounds Cool)
The Simple Explanation
That flutter noise isn’t harmless — it’s stress. Don't beleive us? Check any Turbo Pro Mod Racer for a Valve and ask them why they ue them? Can you get away with it? On mild tunes and boost, yes. But who wants that?
The Technical Reality
Repeated compressor surge:
- Loads the thrust bearing
- Increases shaft stress
- Shortens turbo life
Occasional light surge isn’t catastrophic, but consistent surge means the system isn’t controlled properly — often due to:
- Incorrect BOV setup
- Poor valve placement
- Undersized valve
Special Mention... Did you know you a turbo can surge under acceleration? It's true and this type of surge is even worse than the trailing throttle kind. Make sure you've sized your Turbo correctly or have a strategy to mitigate this, pronot!
Myth 7: A “Leaky” BOV Is Always a Problem
The Simple Explanation
Not all test methods reflect real-world operation. In most cases, these vacuum or leak tests are a false positive.
The Technical Reality
Pressure testing can show leakage that never occurs under dynamic conditions. Both sides of a piston (for example) should be exposed to the testing pressure (OR vacuum). One sides tests aren't fair or relevant. Put your Mity-Vac down and back away from the car.
What matters:
- The valve seals under boost
- Responds instantly to throttle changes
- Remains stable at high RPM
High-quality valves are engineered to balance sealing and response, responding to a pressure differential between the pre and post throttle body inputs — not just static pressure tests.
Myth 8: Bigger Blow-Off Valves Are Always Better
The Simple Explanation
Too big can be just as bad as too small.
The Technical Reality
Oversized valves can:
- Open too early
- Cause drivability issues
- Reduce throttle response
- Simply take up too much space!
Correct sizing depends on:
- Turbo size
- Boost pressure
- Throttle response requirements
This is why valve flow characteristics matter more than just diameter.
Myth 9: Factory Valves Are Fine for High Boost
The Simple Explanation
Factory valves are designed for factory boost levels.
The Technical Reality
Many OEM valves are:
- Plastic
- Diaphragm-based
- Designed for low boost and longevity, not performance
As boost increases, they often:
- Leak
- Fail
- Become inconsistent
Upgraded valves use:
- CNC-machined housings
- Piston-based designs
- Heat-resistant materials
Myth 10: All Blow-Off Valves Are Basically the Same
The Simple Explanation
They absolutely aren’t.
The Technical Reality
Key differences include:
- Piston vs diaphragm
- Spring calibration
- Seal design
- Flow efficiency
- Thermal stability
This is where brands like Turbosmart stand out — not because of marketing, but because of engineering depth, testing, and motorsport validation.
Blow-Off Valves Are Part of a System
A blow-off valve doesn’t exist in isolation, Its performance depends on:
- Turbocharger selection
- Manifold design
- Wastegate control
- Boost strategy
- EC calibration
When chosen and set up correctly, a BOV improves:
- Turbo longevity
- Response consistency
- Drivability
- System reliability
At Walton Motorsport, we view blow-off valves as critical supporting components, not accessories — and we spec them accordingly. Get in touch if you're not sure what you need!
Need More Details? Not Convinced? Check out the FAQ.
We've heard it all before!
Blow-Off Valve 'Opinions...'
My Mate (Including but not limited to the below) said...
- Facebark Expert
- Influencer
- Youtoob Personality
- Old boy down the pub...
Blow Off Valves Aren't needed otherwise high performance cars would have them fitted from the factory!
Sorry Mate, but they do. A Diverter Valve, Blow Off Valve or Dump Valve... call them what you will, original turbocharger petrol engines almost ALWAYS come with a divierter vavle from factory. Do you know of anything else on that car the engineers thought they would spend extra on for no good reason?
Rally Cars Never Used Blow Off Valves, proving they must be pointless!
Tell me you've seen some footage of Group B Rallying without telling me you've seen some footgae of Group B Rallying! First of all, there are 2 types of people here. Those who developed this opinion observationally and those who blindly listened and beleived the first kind. We're sorry, but you've overlooked a couple things...
Top Tier Motorsport Teams heave big budgets and want to win. So much so they will generally know the service life of a component and be willing to replace it just before it lets them down in competition. We've all heard it, so many of these teams also (but less conveniently to the nariative) replaced turbo's every event, occasionally every day and on the odd occasion, each stage. Is this the sort of service life you're hoping for with your build?
Assuming Race Cars did it, so it must be good is on one hand, smart. On the other, scary. Many Motorsport decisions are guided by managed risk, component lifecycle management and more often than not; technical regulations. Yes, Motorsport is where innovation is often born and we would be smart to keep an eye on opportunities, but often these innovative solutions are a workaround for technical regulations. So becaeful what you're 'inspired by'.
Turbo chargers never go backwards. It's all Bollocks.
True - They generally don't... When the throttle shuts and the momentum of the turbo contiues to build pressure within the charge pipe, its unlikely to turn the turbo backwards. What is happening is a matter of harmonics. The chopping sound is an airflow phenomenon more like airflow disconnecting from a surface in an aerofynamic sense. While the turbo doesn't stop and go backwards - You have introduced a violent airflow situation that your turbo is simply not bulit for. And yes, at low load you might even get away with this for years, but we assuse you it isn't good and the vehicle manufacturers put a diverter valve on there to avoid it for a reason. Wind the wick up where the stakes are higher and see how long it all lasts.
All Blow Off Valves Leak, Look let me show you with my hand pump...
Put the mity-vac down and walk away from the car... This (literally) one side test method is certainly going to prove atleast one, if not two things.
- A False Positive Test Result
- You don't understand how these work.
Yes, the factory valve is sealed and if you put a smoke machine on the inlet, or some soap near the valve and a little positive pressure in the inlet the original gear doesn't show this failure. However...
Most aftermarket valves don't work on the same principal (or have the same shortcomings...). They're based on a piston with an O Ring seal and operate on a pressure differential. That is, they operate based on the pressure on either side of them either being the same (valve held closed) or slightly different (valve opens - Until balance is restored). Sort of life when the the throttle closes until the manifold pressure normalises...!
Testing this design with pressure or smoke only on one side of the piston, or with low (relative to an engine running atleast at idle) flow is simply not how you do it... You're right to be looking for boost leaks, but this is not your smoking gun. If the valve is closed, or nearly closed at idle and stays, moves or snaps closed under throttle - You're valve is more than likely just fine.
You can mount them anywhere and they'll work fine!
Not always, lets discuss why...
Blow Off Valve Priority - Yes, like Wastegate Priority, same but different. If you have the option of putting it on the outside of a radius rather than the inside, do it. Airflow always prefers the path of least resistance and your valve will perform better on the outside of the radius.... If you need the air to subconciously stop, turn a hard corner and go backwards a bit in order for your valve to perform, you've seriously underestimated the energy in wind. Make it easy for the valve to do it's thing and put it in the airflow.
Pre, Or Post Intercooler. You can make it work either way, however in a performance application you really want it as close to the throttle-body as possible. Yes, some manufactures put it right on the turbo, but this doesn't mean we're wrong. It just means they weighed up and balanced the compromises and chose to go that way. Why would it matter? Remember the pressure differential we talked about? The one usually triggered by the pressure behind a closed throttle being a little lower than the pressure in front of the closed throttle? Well image you're now running 40psi and you have a steadily increasing pressure drop (not uncommon) across you're intercooler core... Now, without your throttle closing you've got the sort of pressure differential required to open you're valve. That valve opening at big boost isn't the valves fault... It could simply be the accumulative pressude drop and poorly chosen valve location working against you.