Turbocharged cars are becoming ever more popular. Not only is a smaller turbocharged engine more efficient, but it also opens more opportunities for tuning. We see folks on forums — all the time — confused about what a bypass valve (BPV) is, and how it’s different from a blow-off valve (BOV). In this short guide, we’re going to set the record straight and explain the difference. To do so, we cornered an expert on the topic — Marty Staggs from Turbosmart USA — to share his wealth of knowledge with us.
What is the Purpose of a BOV/BPV?
When you introduce forced induction to an engine, you need a way to release the pressurized air when the throttle body is closed. If there isn’t a way for the air to escape when the throttle body closes, pressure can skyrocket and force air into the turbocharger the wrong direction. This issue can destroy your engine, turbocharger, and other components. We’ve seen bent throttle shafts, bent throttle blades, crashed turbochargers, and more as a result of inoperative or missing pressure relief components.
A BOV/BPV solves this issue by allowing that pressurized air to escape from the inlet tubing, and works by measuring the manifold pressure. When the throttle body closes, relative manifold pressure drops below atmospheric pressure. This drop in pressure is what opens the valve, which allows the excess pressure to escape. By plumbing manifold pressure to the valve, it allows the valve to sense when the pressure changes and become operative.
Traditionally blow-off valves vent 100-percent to the atmosphere, which is what makes that cool “pssssh” noise. This means if you’re at 20 psi of boost and let off the throttle, all 20 psi is vented out to the atmosphere immediately. If the vehicle uses a manual transmission this can be extremely obnoxious in between gears, especially on a track or canyon road where throttle modulation may be necessary.
If a BOV vents all of that remaining pressure to the atmosphere, boost pressure must start from zero. This can be really frustrating when you’re shifting and experiencing turbocharger lag in every single gear.
Adjustable or “hybrid” blow-off valves help solve this issue by venting some of the pressure to atmosphere and the rest back into the intake system to be recirculated. This helps decrease lag substantially when going from gear to gear.
“A blow-off valve vents all of the pressure to atmosphere. Our dual port BOV vents the first 50-percent of the air back into the intake and the last 50-percent to the atmosphere,” says Staggs.
“Racing applications will vent 100-percent to the atmosphere because they typically don’t have anywhere to recirculate the air to. They also don’t need to recirculate the air because they’re all speed-density tuned so there’s absolutely no need to meter that air back into the intake. Using this system is clean and simple.”
Building on what he’s talking about, another problem with a traditional BOV is that the air being vented has already been measured by the mass airflow sensor. Since the air is being vented after being measured, this can cause your car to run extremely rich during shifts. This is why you’ll occasionally see cars with forced induction puff out black smoke in between shifts. Luckily this issue can typically be fixed with some simple tuning.
A bypass valve is different from a blow-off valve because it doesn’t vent any air to atmosphere. Instead, it recirculates the air back into the intake before the turbo or supercharger inlet, but after the airflow sensor. This design keeps pressure much more consistent. In addition, by re-introducing the air after the sensor (since it has already been metered once) it prevents any tuning issues.
“A bypass valve is typically a recirculation valve. This means the boost pressure that you are ventilating is recirculated either back into the airbox or somewhere else in the intake system. OEM applications like Ford Ecoboost engines use bypass valves. There are a few companies who have C.A.R.B.-certified systems — such as Turbonetics — who use our (Turbosmart) 38mm bypass valves as part of their systems,” Staggs explains.
All is not rosy with the bypass valve, however. The major concern with this design is its ability to hold up to high horsepower applications.
“Anything over 800 horsepower you’re going to have a hard time getting a recirculating bypass valve to flow enough air to do the job, so a vent-to-atmosphere blow-off valve is really the only way to go,” says Staggs.
Should You Upgrade from a BPV to a BOV?
You may be wondering if you should upgrade from a BPV to a BOV or if it’s beneficial.
“An aftermarket bypass valve will be better than the factory plastic one because it’ll flow more air more consistently. Until you upgrade from the factory turbocharger and start to upgrade past the capabilities of a bypass valve, there is no need to switch to a 100-percent vent-to-atmosphere blow-off valve. Most people switch just because they want the ‘pssh’ sound,” Staggs says.
To make a long answer short: A blow-off valve vents the extra boost pressure to atmosphere when the throttle body closes. A bypass valve vents the extra boost pressure back into the inlet when the throttle body closes.
For a street car, it comes down to horsepower capability and your individual combination as to which one makes the most sense for you–or whether you want to wake up the whole neighborhood on your way to Cars and Coffee. On a race car, the answer is clear–a blow-off valve should be your only consideration.