Should You Delete Your Subarus Blow Off Valve? Here's What Nobody Tells You

We've seen it come up in the local Subaru groups more than once lately. Someone's selling a BOV blanking plate for their WRX. Someone else wants to straight-up delete their factory blow off valve entirely. Both of them are WRX EJ owners. Neither of them could fully explain why.

So we did what we do — dug into the forums, the tech, the tuner knowledge, and the real-world experience — and we're going to break it all down for you. Whether you're a seasoned EJ tuner or you've just bought your first WRX and you're trying to figure out what all the turbo noise fuss is about, this one's for you.

First Things First: What Does a Blow Off Valve Actually Do?

Let's start from the beginning, because if you don't understand what the BOV is doing, you can't understand what happens when you remove it.

Your WRX's EJ engine is turbocharged. The turbocharger compresses air and forces it into the engine at high pressure — that's boost. When you're accelerating hard, the throttle is open and all that pressurised air is flowing into the engine smoothly. The turbo is happy. You're happy. Life is good.

But then you lift off the throttle to change gear. The throttle plate snaps shut. The engine no longer wants that pressurised air. But the turbo doesn't know that yet — it's still spinning, still pushing compressed air into the intake tract. Now you've got a wall of pressurised air with absolutely nowhere to go.

That air has to go somewhere.

Without any valve to relieve it, that pressurised air hits the closed throttle plate and bounces back — straight through the turbo compressor in reverse. This is called compressor surge, and it's the source of the distinctive flutter or chatter sound you've heard on modified Subarus.

The Blow Off Valve (BOV) or Bypass Valve (BPV) — more on the difference in a moment — exists to give that pressurised air somewhere to go so your turbo doesn't have to deal with it. It's a pressure relief valve, vacuum-actuated, that opens when the throttle closes and relieves the boost spike cleanly.

Simple. Elegant. Essential.

BOV vs BPV — They're Not the Same Thing

This is where a lot of WRX owners get confused, and it matters a lot on an EJ-powered car.

Blow Off Valve (BOV) / Atmospheric Dump Valve: Opens and vents pressurised air directly to the atmosphere — outside the intake system entirely. This is the valve that makes the loud PSSSHHHT whoosh sound you hear on highly modified turbo cars. It looks cool, sounds cool, and on certain setups it's completely fine. But on your EJ WRX in stock or mildly modified form, it's actually a problem.

Bypass Valve (BPV) / Recirculating Valve: This is what Subaru fits from the factory. Instead of venting to atmosphere, it recirculates the pressurised air back into the intake tract before the turbo inlet. The air goes: compressed → throttle closes → BPV opens → air loops back to the turbo inlet → turbo keeps spinning → boost builds again faster when you get back on the throttle.

The recirculating BPV is the smarter valve in almost every real-world WRX application. Here's why:

Your EJ engine uses a MAF sensor. The Mass Air Flow sensor sits in your intake tract and measures exactly how much air is entering the engine. The ECU reads that measurement and calculates exactly how much fuel to inject to match it. The system is always accounting for the air that the MAF has already measured.

When a recirculating BPV opens, the air loops back through the system. The MAF has already counted it. The ECU is happy. Fuelling stays correct. Your car shifts cleanly.

When an atmospheric BOV opens, that air exits the system entirely. But the ECU still thinks that air is in there — because the MAF already counted it. So it injects fuel to match air that no longer exists. The result is a momentary rich condition every single time you shift. On a MAF-based car like your EJ WRX, this can cause rough shifts, stalling at idle, fouled spark plugs, backfires through the exhaust, and over time, accelerated catalytic converter wear.

The stock EJ BPV is good for well over 20 PSI. Unless you're building a serious power car, you almost certainly don't need to change it at all.

So Why Are People Deleting Their BOV Entirely?

Here's where it gets interesting. We've identified five reasons people remove the valve and fit a blanking plate. We're going to be straight with you about which ones are valid and which ones aren't.

Reason 1: They Want The Flutter Sound (the honest answer, most of the time)

We'll just say it plainly: the majority of BOV deletes on EJ WRXs happen because the owner wants that turbo flutter sound. You've heard it — that rapid stutter, stu-stu-stu-stu, on gear changes. It's the signature sound of a serious Subaru, and people love it.

What most people don't know is that the flutter sound IS compressor surge. It's the sound of pressurised air reversing back through the compressor wheel because it has nowhere else to go. It's the turbo choking on its own boost. Yes, it sounds tough. Yes, it is genuinely stressing your turbo every single time you hear it.

The surge that happens on deceleration — when you're lifting off and there's no boost building — is less damaging than surge under acceleration. But it is still measurable wear on the compressor wheel and the thrust bearings inside the turbo. The guys running blanking plates often say "I've been running it for years with no issues." That may be true. It's also true that the turbo is quietly accumulating wear it wouldn't otherwise have.

If you want more turbo noise from your EJ, a short ram intake or a cone filter will allow you to hear the recirculating BPV operating — which has its own satisfying sound — without the turbo damage.

Reason 2: Speed Density Tune With MAF Delete (the only legitimate reason)

This is the one situation where deleting the BOV or fitting a blanking plate genuinely makes sense from an engineering standpoint.

When a heavily modified WRX owner maxes out their MAF sensor and makes the switch to a speed density tune, they remove the MAF sensor entirely. The ECU now calculates fuelling from manifold pressure (MAP sensor) and intake air temperature (IAT sensor) rather than from measuring mass airflow directly.

Without a MAF sensor in the system, an atmospheric BOV no longer causes fuelling problems — because the ECU is no longer relying on counting the air that just exited through it. Some tuners on big builds then choose to run a blanking plate instead of any valve at all, particularly because at very high boost levels they may want to maintain maximum pressure consistency between shifts.

This is where the IAT sensor port on the blanking plate becomes important. On a speed density setup, the IAT sensor often needs to relocate to after the intercooler for accurate charge air temperature readings. A blanking plate with a pre-drilled and tapped IAT sensor port solves this neatly in one part.

If you're seeing blanking plates with IAT ports being made and sold, they are specifically designed for speed density tuned cars. They are not for stock or mildly tuned EJ WRXs. If you're not on a speed density tune, this product is not for you — yet.

Reason 3: They Had a Leaky or Badly Tuned Atmospheric BOV and Gave Up

This is more common than it should be. Someone buys a cheap aftermarket atmospheric BOV. The spring tension isn't right. The valve leaks under boost. The car stumbles, stalls, runs rough. They try adjusting it, can't get it right, and eventually fit a blanking plate because at least that doesn't leak.

The correct fix here is always to go back to a quality recirculating BPV — either the OEM Subaru unit or a reputable aftermarket recirculating valve from a brand like Go Fast Bits, Turbosmart, or Cobb. Don't let a bad aftermarket part send you down the blanking plate road unnecessarily.

Reason 4: They Think It Improves Throttle Response (It Doesn't)

The forum myth: "Blanking off the BOV keeps boost in the system between shifts, so the turbo spools faster next gear."

The reality: The opposite is true. When the BOV is deleted and compressor surge occurs, the surge is actually decelerating the compressor wheel faster — not keeping it spooled. A functioning recirculating BPV keeps the compressed air in circulation and keeps the turbo spinning more efficiently between shifts. The BPV is your friend on an EJ WRX, not your enemy.

There is no measurable 0–60 or quarter-mile time improvement from deleting the BOV on an EJ WRX. Multiple forum users have tested this back-to-back with data. It doesn't exist.

Reason 5: They Think the Factory BOV is a Weak Point

Some owners running elevated boost are concerned about the stock BPV leaking under pressure, and rather than upgrade it, they blank it off. This is understandable but misguided. The OEM BPV on most EJ-era WRXs is rated well beyond factory boost levels. If you're running enough boost that it's genuinely a concern, the correct move is an upgraded aftermarket recirculating BPV — not a blanking plate.

What Actually Happens When You Run a Blanking Plate on a MAF-Based EJ WRX?

Let's be clear about the chain of events:

1. You fit the blanking plate. The BOV port on the intercooler is sealed.
2. You accelerate. Boost builds. Everything feels normal under load.
3. You lift off to change gear. The throttle closes.
4. Pressurised air in the intercooler and intake tract has nowhere to go.
5. It reverses through the turbo compressor — compressor surge occurs.
6. You hear the flutter sound. The turbo compressor wheel is momentarily stressed.
7. The ECU, still on a MAF-based tune, sees a momentary rich condition as fuelling doesn't adjust cleanly.
8. You repeat this every single gear change, every drive.

Over time: compressor wheel wear, thrust bearing wear, potentially a shorter turbo life. Whether it takes 20,000 km or 200,000 km to become a problem depends on your driving style, boost levels, and whether the car is used on track.

The Verdict: Should You Do It?

Here's our straightforward guide:

You're on a stock or mildly modified EJ WRX with a MAF-based tune: Don't delete the BOV. The factory BPV is doing exactly what it should. If you want more turbo noise, upgrade your intake. If you want better BPV performance above 20 PSI, upgrade to a quality aftermarket recirculating valve.

You've got a badly performing atmospheric aftermarket BOV: Remove it and go back to a recirculating BPV — OEM or quality aftermarket. Don't use the blanking plate as a shortcut.

You're on a serious power build, MAF deleted, running speed density: A blanking plate with an IAT sensor port is a legitimate solution and makes engineering sense in your specific application. Make sure your tuner signs off on it.

You just want the flutter sound: Understand what you're trading. It's compressor surge. It is accelerating wear on your turbo. How much risk you accept is up to you — but go into it with open eyes, not forum myths.

A Note on the EJ Turbo Specifically

Everything above applies to the EJ20 and EJ25 turbo family — the EJ205, EJ207, EJ255, EJ257 engines in the GC8, GD, GE/GH, GR/GV, and VA WRX generations. These are all MAF-based engines from the factory, and the rules above apply uniformly.

The EJ turbo is a tough, proven, and well-understood platform. It responds brilliantly to the right modifications done in the right order. The BPV is not the place to compromise — there are better places to spend your money and your effort on these engines.