"Diverter valve" and "blow off valve" get used interchangeably in forum threads and product listings, but they describe two different ways of handling the same problem: what to do with pressurised air when the throttle slams shut. GFB makes both, and the right choice depends on how your engine measures air, not on which one sounds better. Here's what actually separates them.
Why a Boost Bypass Valve Exists at All
When you lift off the throttle in a turbocharged car, the throttle plate closes but the turbo is still spinning and still pushing air. That air has nowhere to go, so it stalls against the closed throttle and surges back toward the compressor wheel. The result is the fluttering noise some people chase, along with unnecessary load on the turbo's bearings and a slower spool-up when you get back on the throttle.
Every factory turbo car is fitted with a valve to release that pressure. Whether it's called a diverter valve, a bypass valve or a blow off valve, the mechanical job is identical — open under closed-throttle conditions and dump the excess pressure. The difference is purely where that air ends up.
Diverter Valves: Recirculated Air
A diverter valve (also called a bypass valve or recirculating valve) routes the released air back into the intake tract upstream of the turbo inlet. The air stays inside the system, gets measured once, and is simply used again. This is what almost every manufacturer fits from the factory, because it's quiet, emissions-friendly and keeps the air metering consistent.
Where GFB's DV+ Fits In
Many modern cars — particularly VW Group models like the Golf GTI, Audi S3 and Skoda Octavia — use an electronically actuated solenoid valve rather than a vacuum-operated one. Rather than replacing the whole unit, the GFB DV+ replaces the internal valve mechanism with a machined piston assembly that fits inside the factory housing, retaining the original solenoid and the factory wiring and control strategy. It stays fully recirculating, so there's no change to the intake noise or the way the ECU sees airflow.
You'll find recirculating options across the diverter valves range, including units for VW Group, Subaru and other platforms.
Blow Off Valves: Atmospheric Venting
A blow off valve vents the released air out to atmosphere instead of returning it to the intake. That's where the familiar "psssh" comes from. Mechanically it's the same closed-throttle release, just with an open outlet and, usually, a trumpet or nozzle to shape the sound.
The MAF Problem
This is the part that catches people out. If your car uses a mass airflow (MAF) sensor, that sensor has already counted the air before it reaches the turbo. Venting it overboard means the ECU is still fuelling for air that has left the engine, which can cause an over-rich condition, hesitation, stumbling or stalling on gear changes and can log fault codes. Cars running a speed-density or MAP-based setup are generally far more tolerant of atmospheric venting, because airflow is inferred after the fact rather than metered upstream.
Subaru WRX models are the classic example of a MAF-equipped platform where a fully atmospheric valve often causes drivability complaints, which is exactly why hybrid and dual port designs exist.
The Middle Ground: Dual Port and Hybrid Valves
GFB's dual port and hybrid designs split the difference. They have two outlets — one plumbed back into the intake and one vented to atmosphere — with an adjustable ratio between them. You can dial in enough atmospheric venting to hear the valve while still recirculating enough metered air to keep the fuelling sane. On some designs the split can be adjusted right down to fully recirculating if you decide the noise isn't worth the trade-off.
This is usually the sensible starting point on a MAF car if you want some sound, and it's a big part of why these valves are so common on the Subaru and Mitsubishi platforms. Browse the full blow off valves range to compare single port, dual port and hybrid layouts.
Electronically Controlled Options
GFB also produces electronically managed valves, such as the Deceptor Pro II, which use a control module to change the valve's behaviour depending on driving conditions rather than relying on a fixed mechanical split. Replacement parts for these — including looms and hardware — sit in the blow off valve accessories range.
How to Choose
Start With Your Air Metering
MAF-equipped car and you want stock-like manners: stay recirculating, or run a dual port with the atmospheric side wound most of the way closed. MAP-based car: you have far more freedom to vent fully to atmosphere.
Consider the Factory Valve's Limits
Some original valves are diaphragm-based and can split or leak over time, and many struggle to hold higher boost pressures once the car has been tuned. A piston-style aftermarket valve is generally chosen for sealing consistency at raised boost rather than for noise alone.
Check the Fitment Style
Valves are not universally interchangeable. Some bolt directly to the turbo or a factory housing, some require a flange, and some need an adaptor welded or clamped into the intake piping. If you're running an aftermarket intake or relocating the valve, check what mounting hardware is required and confirm clearance under the bonnet before ordering. Related intake components are grouped in the air intake range.
Think About Noise and Compliance
Atmospheric venting is loud by design, and noise and emissions rules vary between Australian states and territories. Check what applies where you drive before committing to a fully vented setup, particularly on a daily-driven or registered vehicle.
Installation
Valve replacement can involve removing intake piping, working with boost hoses and vacuum lines, and in some cases handling electrical connectors or adaptors. Incorrect sealing or a badly routed recirculation line can cause boost leaks and poor drivability. Installation by a qualified professional is recommended, and any adjustment of a dual port valve's venting ratio should be checked over afterwards for correct running.
You can view the full GFB blow off valves, diverter valves and boost control range in one place if you'd like to compare options side by side.
FAQ
Is a blow off valve the same as a diverter valve?
Functionally they do the same job — release pressure when the throttle closes. The difference is the destination of the air: a diverter valve recirculates it back into the intake, while a blow off valve vents it to atmosphere.
Will a blow off valve make my car faster?
Not on its own. A quality valve helps maintain boost pressure and reduce compressor surge, which can improve throttle response and protect the turbo, but it isn't a power-adding part by itself.
Can I fit an atmospheric BOV to a MAF car?
You can, but you may encounter over-rich running, hesitation or stalling on gear changes and possible fault codes, because the vented air has already been measured. A dual port or hybrid valve is generally the safer choice on a MAF-equipped car.
Why does my factory valve need replacing?
Diaphragm-style valves can tear or lose their seal over time, and some original valves struggle to hold higher boost once a car has been tuned, which shows up as boost leak or inconsistent pressure.
Does the DV+ change the sound of my car?
No. The DV+ retains the factory recirculating path and the original solenoid, so intake noise and airflow metering remain as they were from the factory.
Do I need extra hardware to fit a valve?
It depends on the valve and your intake setup. Direct-fit units generally don't, while universal valves often need an adaptor, flange or mounting piece. Check the listing for what's included before ordering.
Need help choosing? Contact our team at sales@chicaneaustralia.com.au.