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Blow-off Valves Explained - Tech

The Boost Is Loose!

Evan Griffey
Nov 6, 2012
Photographer: Sam Du
Sstp 1211 01+blow off valve explained+cover Photo 1/8   |   Model: Christine Huang

First off, if you don’t own a turbocharged car, enjoy these photos of Christine, then turn the page. But if you’re honestly driving around an STI, Evo or SR20-swapped 240, or perhaps you’ve thought about turbocharging your Civic or Z, keep reading because you’re going to want to invest in a good blow-off valve.

The underlying objective of a blow-off valve (BOV) is to protect the turbo against damage while ensuring smooth and reliable drivability. It just so happens it makes a loud sound that gets everyone’s attention! But the noise it makes shouldn’t be what you’re most concerned about; you have to make sure you have the right valve for your ride that’ll keep your turbo happy and healthy.

Compressor Surge
Every component of a turbo system needs to work together. But when there’s compressor surge, it can create headaches, mess with performance and even cause damage to your precious car. Compressor surge occurs when you suddenly lift off the gas pedal and the throttle plate closes. A rush of boost heads into the engine and when it hits the closed throttle plate, it has nowhere to go but back into the turbo.

The effect of this boost backtracking into the compressor outlet and interfering with the compressor wheel is called compressor surge. Now why is it bad? As the boost returns to the compressor wheel, it can slow or stall the wheel putting stress on the wheel shaft and bearings. Not only damaging, this surge can also mess with turbo response and kill drivability. The wheel loses its momentum and also creates lag. A good blow-off valve can come to the rescue and keep the surge out of the turbo by venting it to the atmosphere. Pssshhhh!

Blow-off Basics
Here’s how it works… A blow-off valve is connected to the intake tract. Inside the valve’s main housing is a vacuum chamber with a spring, a diaphragm and valve. The diaphragm reacts to pressure changes and at a predetermined vacuum it’s pulled toward the vacuum source compressing the spring inside the housing. The spring is connected to a valve that pulls away from its seat and releases the unwanted boost pressure.

Sstp 1211 02+blow off valve explained+diagram Photo 2/8   |   Blow-off Valves Explained - Tech

On some BOVs, an adjustment screw lets you control at what pressure in the intake system the valve is activated. You can also swap the spring to change the activation point.

Blow-off valves are often referred to by diameter, 40mm being a common size. Picking the best valve comes down to the how much boost you’ll be running and the physical space available for installation.

When to Bypass
Not all turbocharged engines are made for blow-off valves, they might need a bypass (aka diverter or recirculating) valve instead. They both accomplish the same task but there’s a big difference in how they do it.

Sstp 1211 03+blow off valve explained+broken down Photo 3/8   |   Blow-off Valves Explained - Tech

The type of engine management your car runs will come into play when choosing a valve. If you have a MAF-type system, it meters the airflow after it enters the intake system. In this case, when a blow-off discharges to the atmosphere, the ECU isn’t able to properly fuel the engine resulting in rich AFRs, hesitation, bad idle and even stalling. In these systems the excess pressure must be plumbed back into the turbo system before the compressor inlet, which is what a bypass valve does.

Remember that the key to picking a valve is to know your car and what kind of plumbing it needs. But if you know what you’re doing, here are our top picks for BOVs you should check out. They all come from reputable companies and they also offer a wide range of applications. Hit them up if you have questions but just don’t be “that guy” and ask about what noise it makes…

M7 Japan Hyper Sound
BOV Type GT-7

M7’s BOV utilizes a dual drive system that helps relieve compressor surge without reducing compressor speed. Its design creates a tight and sharp blow-off noise. The valve will fit most applications but there are also specific kits for the R35 GT-R, Impreza and Evo.
Price: From $292

Forge Motorsport 50mm BOV
This is Forge’s newest valve. It’s a universal BOV that features the largest bore and flow rate they’ve ever manufactured. The diameter of the inlet port is 50mm. Its top port will accommodate any position for the vacuum hose and the valve can be adjusted with any of its valve springs.
Price: $225

Sstp 1211 04+blow off valve explained+forge bov Photo 4/8   |   Blow-off Valves Explained - Tech

Forge Motorsport nissan GT-R Recirculation Valves
Got a R35 GT-R? Forge has you covered with either its BOV or recirculating valves. These recirc valves pictured here offer a higher flow rate capability compared to stock and feature a 50mm bore capable of handling up to 34psi of boost.
Price: $520

Synapse Synchronic BOV
This valve is completely modular and can be set to either blow-off or diverter mode. It uses an interchangeable mounting flange system and should fit most turbo cars. By design, it is a pull-type valve only and features no diaphrams. It’s also designed to remain open under vacuum conditions. While universal, Synapse offers vehicle-specific kits for the Mazdaspeed 3 and 6, Genesis, Evo and more.
Price: $243.95

One of the most popular BOVs around, HKS is on its fourth design of its Super Sequential valve. It’s a dual stage pull-type relief valve that won’t leak under boost and will work on most stock and heavily-tuned vehicles.
Price: $249.99

Sstp 1211 08+blow off valve explained+hks bov Photo 8/8   |   Blow-off Valves Explained - Tech

Win Me!
Thanks to Forge Motorsport, we’ll be giving away one of their 50mm BOVs to one lucky reader. Just e-mail us at and tell us why your turbo project needs it.


Los Angeles, CA 90061
Synapse Engineering
San Diego, CA 92121
Forge Motorsports
By Evan Griffey
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