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BOV fitted but still have compressor Surge?

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Hey all, ok so on my skyline r33, stock intake/exhaust manifold and turbo running 0.8-1bar boost.

Had a HKS SSQ BOV fitted (stock location) when I bought the car (China probly) the turbo would still flutter as well as the BOV woosh even with the adjuster screw wound right out...

I thought maybe the BOV was faulty so bought another, admittedly a china greddy RS and still pretty much the same... Under high rim/boost and sudden release of the throttle it's about 80% whoosh and 20% flutter. The lower the rpm or slower you release the throttle the more flutter there is.

So just wondering if it's just a symptom of the small factory turbo on medium boost? Or is it cheap china bovs that are too slow to react? Or maybe another cause?

A properly sized, quality BOV should eliminate audible surge at the sort of boost levels you're talking about. That being said though, a minor amount of surge won't do any damage to the turbocharger. With a MAF sensor you can get problems with the surge affecting the MAF sensor reading and this can cause stalling coming to a stop. Of course an atmospheric venting BOV with a MAF sensor can cause the same issues...

Hey Andre, Yeh the car uses a AFM and the surge and reversion is causing it to backfire and overfuel sometimes just after throttle lift leaving a nice black stain on my bumper :(

While on the topic of Bov's and recirculating... Currently the BOV is atmo vented (if it was working properly) Id imagine this would be better then no bov at all as far as issues with the AFM are concerned?

As I understand it without a Bov when you close the throttle the air is reversing and going back past the afm which it reads as more air causing more fuel to be injected..

With a Atmo vent Bov this should stop the air passing back past the afm and reduce the overfueling? What are the other issues you speak of?

It's actually not quite that simple. When an atmo venting BOV is open, air has already passed through the MAF and been metered, hence the ECU is providing the fuel to match this mass of air. Since the air doesn't make it into the engine though, the engine ends up running excessively rich when the BOV is venting. I can't say if the end result is better or worse than an engine running no BOV and suffering from surge when the throttle is closed. A lot I imagine will depend on the individual configuration, MAF sensor, and length of the intake pipe between the turbo and MAF element.

Really though A MAF sensor and a BOV that vents to atmosphere is not a match made in heaven unless the BOV is fitted pre MAF.

But in that situation the MAF has measured the Air but the throttle has closed, so even if it was a plumb back system since the throttle is closed then the air is not getting into the engine anyway? so wouldnt it still be a rich condition and just a symptom of having the MAF so far upstream of the engine?

With a plumb back BOV, the air is reintroduced to the intake between the compressor inlet and the MAF sensor so the air kind of just loops around in the intake while the BOV is open. This air is initially metered but while the BOV is open, the mass of air circulating through the BOV isn't lost so it isn't replaced.

Cheap chinese BOVs have very low quality of machining, it could open, the next time it won`t. Avoid them. Try Tial 50 mm BOV, it has a high flow capability, but don`t choose the spring rate with attached manual, because on a road car the spring that has been chosen using that method 100% will cause turbocharger fall below surge line. I`d suggest using black spring in any case.