Since you typically pick a BOV spring to just barely keep the BOV closed at idle vacuum, then it seems logical that the BOV would be open when vacuum is higher than idle — such as coasting in gear. Under these conditions, where vacuum is a lot higher than idle, wouldn’t a VTA BOV pull unfiltered air into the engine?

 

Yes, the BOV can open during decel while the throttle body is closed.

 

My car pulls about -8 psi vacuum at idle but significantly more at light throttle cruise. If I have a light BOV spring in something like a Tial Q, like an 8psi white spring, the valve is going to be cracked open during steady state cruise (when the throttle body isn't full closed) and ingest unfiltered air. Isn't this a problem for a VTA BOV on a street car? I'm thinking of getting the recirc model (QR) and putting a filter on it instead. Dumb idea?

 

I think you are over thinking it. Its not a logging truck spending its time in a dusty environment.

Personally, if its making less then 530whp ,A stock crushed bov works really well under all conditions.

 

The charge piping is either ambient pressure or above. I have two quick YouTube videos I displayed some of these situations. A greddy type s with single spring was my weapon of choice. Note the collapse of the bag is momentarily and pre tb reference ports are boost all the time no vacuum.

 

211ratsbud hit the nail on the head and backed it up with video. It makes more sense when you understand that the turbo is always spinning and blowing air into the charge pipe.

 

I agree. He convinced me. Tial Q ordered!

 

Take your time with the Tial. They can vent massive quantities of air but can be a little extra work to find the right spring to make driving perfect. I've always set my blow off valves to be open at idle and either open or light on the seat at cruise. This gives predictable part throttle behavior. ( very related to part throttle characteristics of your engine and turbo)

 

I got the flange changed in my upper pipe and installed the Tial Q. Works fantastic. Very happy with it. I used the 6 psi spring. It stays closed at idle even with the light spring.

 

nice install 👍.