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Blow off Valves 101

So you’ve heard the infamous blow off valve sound. It’s the woosh or the squeak in between shifts in a turbo car. It may even be the reason why you purchased a turbo car! This article will help you to understand what they really are, what they do, and the positive and negative side of things

What is a BOV?
For the purposes of this write up, I will refer to a blow off valve as a BOV and a bypass valve (recirculated BOV) will be revered to as a BPV.

A BOV/BPV is a valve on a turbo car that will stay shut when the car is under boost and it will open when the car is off of boost to let air pressure out of the system so that you do not back spin the turbo or rupture a pipe in your intake tract. The difference is that a BOV will vent the air to the atmosphere and will tend to be loud while a BPV will re-circulate the air around the back of the turbo to prevent it from being spun back wards.

A BOV or a BPV is essential on most modern cars for the simple fact that they prolong the life of your turbo. Most modern turbos can spin up to 100,000 RPMS and if you let off the gas pedal to shift, you have just shut the throttle plate almost all of the way and so the air that was pressurized in the system has no place to go but back through the turbo. This will force the turbo to back spin against it’s will and can severely damage it. Older cars like the Buick GNX or GM’s Typhoon and Syclone did not require a BOV or BPV because they ran big turbos that pushed very little boost and so they could be back spun with little damage, and even still, they did not always last long.

So now that it’s been established that you have a turbo car and you need a BOV or a BPV, which one do you get?

If you have purchased a turbo’d Subaru of any kind, it has a BPV from the factory. The reason why it has this is because the Subarus are also equipped with a Mass Airflow Sensor (MAS). A MAS sits right after your air box and records the amount of air and the temperature at which it has entered the system. As soon as the air comes into the filter it is calculated by the ECU and an appropriate amount of fuel is ready for it once it gets in past the intake manifold. Your factory BPV will open when the car comes out of boost and it will re-circulate air AROUND the turbo so that it does not back spin it, but it still keeps it in the system so that when the throttle plate opens again, the air still in the system has already been accounted for and the fuel is there for it.

The factory BPV on our cars is good for about 19-20psi, which means, if you are going to be running more than that, it will start to leak so that it makes it very hard for the turbo to build that much boost within the system. If you are going to be running much more than factory boost then you will need to upgrade your BPV. Most BOVs on the market are also able to be used as a BPV by simply hooking up the recirculation fitting or pipe. Some of the more popular BOVs that can do this are the Greddy Type S, TurboXS H34, HKS Super Sequential Valve (with the optional recirculation kit), and the Forge BOV.

It is a fact that a BPV will be quieter than a BOV so some people choose to not re-circulate their BPV. From a technical standpoint, this is a very bad thing to do. Your Subaru’s ECU knows how much air enters the system because of its MAS. If you dump air from the system when you shift, then there is going to be a temporary rich condition which is sometimes followed by a lean condition while the ECU tries to figure out how much fuel it should be putting into the motor. It sees air coming in but according to what the Oxygen sensors see, the calculations come out wrong and the ECU thinks it needs to adjust the amount of fuel entering the motor. If this is repeated a lot over time, then the ECU finds it hard to tune the motor to run the best based on your atmosphere and gasoline quality. Imagine if you were doing a complex math equation and every so often someone came over and ripped off half of your piece of paper. It would be impossible to find the correct “answer” in a decent amount of time as you would have to keep doubling back on your work.

For those of you who want the sound without any problems, then you need to do what is called a blow through setup. A blow through setup is basically one where the MAS is in between the BOV and the Intake manifold. This means that instead of the turbo sucking air through the MAS, it is now blowing it through. Air is measured as it actually enters the motor instead of when it enters the system. With a setup like this, you can actually vent your BOV with no ill affects.

Some people are reporting that they are able to run their cars fine with a draw through setup (the factory setup) and also have a BOV on the car. I tried this myself on (non-Subaru but still had a MAS) my old car and after watching the logs over the course of a week, the ECU had a very difficult time finding the proper fuel trims and they ended up being all over the spectrum. The ECU was holding back fuel where it needed it and was adding fuel when it didn’t need it and the car ended up burning up a set of copper spark plugs pretty quickly from repeated rich and lean conditions. Based on my experience, I will not ever be venting my BOV and I will always re-circulate it.

So what should you do?
If you do not have time to mess around with things and want your car to run at its best then stick with the BPV setup. If you crave this sound then by all means, go with the BOV setup and try it out. You should log your car over mixed driving conditions and see how the car runs. If it runs poorly and you still crave this sound then consider asking a local tuner shop if they are able to tune your car to run a BOV on your setup. If not, then look into running a blow through setup on your car as that is the only truly vent your BOV and also allow the car to “tune” itself and adjust its calculations properly.

Are all aftermarket BOVs bad for your car? No. I actually encourage you to get one, as long as you can re-circulate it. They are ESSENTIAL if you are going to moving to a larger turbo or running more boost out of the factory turbo (in some cases). Most aftermarket BOVs that are configured to be re-circulated will be louder than the factory BPV. All of the valves I mentioned earlier in the article are valves that I have run on my car or on friends’ cars with great results. In all cases, the valves were tested circulated and vented and we all came back to running them re-circulated.

I hope this has answered all of your questions regarding BOVs and BPVs and if you have any questions, feel free to shoot me a PM.
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