Updated July 2020: It's never a bad idea to review the basics, and this concise piece about blowoff valves originally from eight years ago still sums up nicely how they work in a car's turbo system and what to consider when choosing one. While modern factory forced-induction systems don't generally benefit from a BOV, once you start swapping in a bigger turbo or even give a car boost that didn't originally have it—that's when this info will become handy.
If you're driving around in an STI, Evo or SR20-swapped 240, or perhaps you've thought about turbocharging your Civic or Z, you're going to want to invest in a good blowoff valve. The underlying objective of a blowoff 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.
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 blowoff valve can come to the rescue and keep the surge out of the turbo by venting it to the atmosphere. Pssshhhh!
Here's how it works: A blowoff 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.
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.
Blowoff 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 blowoff 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.
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, the key to picking a blowoff valve is to know your car and what kind of turbo plumbing it needs.