I'm posting this thread because we previously had a pretty lively discussion about DV flutter, and whether or not the Synchronic BOV would be a fix. To clarify, we aren't talking about the kind of flutter or surge when you let off the throttle. This is about the part-throttle rapid opening and closing of the DV/BOV.
I spent today in 4 sessions at California Speedway, in an EVO and will drop what we found and the configuration that worked for the EVO. The car was setup for street use initially. On the street, DV flutter was not a major issue. Once on the track, in the 1st & 2nd session, we would get DV flutter at part throttle and modulation. Back to the pits to take the BOV apart and see what was in it. It was a super soft main spring and no pre-load spring. Both ports on the back are connected to vacuum. This was a one-off setup that I did.
So, I took out the prototype spring and put in both the production main spring that comes in every standard Synchronic BOV, and the pre-load spring. I cranked down some pre-load and only connected Port B (see pic below). Using only Port B is still enough force for vacuum to open the BOV properly, but is less surface area to be opened up by a difference in pressure pre and post throttle.
For the rest of the day, there was NO MORE DV FLUTTER. Not tipping into the throttle, not at partial throttle, or anywhere else. And there was also a complete absence of compressor surge/flutter any time the throttle was let off.
The only time I heard DV flutter was when the car was severely out of shape, sideways, and tires spinning so that there was boost pre-throttle and less boost post-throttle, because the car was unnaturally lightly loaded.
The configuration above (one vac hose only) works really well for the race track. For the street, I would recommend running both hoses connected to eliminate any light load surge/flutter, when letting off the throttle.
