BOV: Blow off Valve Subaru FAQ


As to blanket “you’ll run rich” statements, a BOV will only run “rich” during hard acceleration and shifting as 99% of the time it stays closed.

Can my tuner or engine management tune out this rich period? Yes. There are some forms of engine management that can tune this out. Buying your engine management solely for this purpose is a poor method of choosing an engine management system though.

What about a 50/50 or BOVs that you can portion the VTA portion? This is a bad analogy, but if a BOV is a person in a wheelchair, a 50/50 BOV is a person in leg braces. It’s not as bad, but not good enough to say bolt it up. If you find a deal on one or happen to like the sound of a particular model, go for it, but don’t think you are doing your car better vs. a 100% VTA model.

Won’t I be labeled as a ricer? It’s your car, do with it what you like, but be aware that many people are prejudiced against BOVs just like neon underbody lights and other accessories. Be aware that a BOV can be seen as a dinner bell for street racers though. It attracts attention both good and bad.

So a BOV is not bad then right? Not so fast…most BOVs leak. Even ones that are religiously maintained, installed, and are from quality manufacturers. A leak in essence means less boost and less power or your car will be working harder to produce boost than it means, which can decrease reliability.

How do I set up my BOV to not leak/stand less chance of leaking? Read the BOV manual/instructions first off. Learn how to adjust it. If it doesn’t have a manual, just look at the valve. You can almost always figure out how to adjust it. Some have screws, some use washers under the spring, others use replacement springs. Now that you know how to adjust it, here is what you do:

First off, start the car, get out and open the hood. If you can see the piston of the valve (look in the hole it vents out of if its a VTA valve), check to see if it is moving at all at idle, or worse, partially open. You want that piston rock solid, not moving at idle.
Now give it a little gas. It shouldn’t move when you step on it, and when you lift, it should pull up a bit. Depending how hard you hit the gas, it will either JUST move, or fully open.
If its doing anything but what it should be, adjust it a little tighter.

Now go drive the car.