Dual Piston BOV leaking/fluttering

[turbo bob]

Just installed a Stinger dual piston BOV in this 84 SVO I am working on.
For some reason it seems that starting around 70kPa it begins slightly fluttering and getting closer to atmospheric it gets worse.
The extra spring that comes with the BOV is that only a "replacement" spring and not a stiffer one? Is there a stiffer spring available for this valve?
BOV is installed currently between turbo and FMIC, with vacuum line being sourced from the vacuum tree on the firewall.

[Stinger]

The second spring is not the same as the one installed.

Is it fluttering at part throttle or at wide open throttle?

[turbo bob]

I havent been able to give it wide open yet to see, it has been doing it while holding any type of steady part throttle.
Is the second spring a bit stiffer because I can throw that in there and see what happens?

[Stinger]

It's been years since I messed with the springs so I can't honestly remember which is stiffer. I "think" the spare is but I'm not positive. Also inspect the o-rings while it's apart. Pay attention to which way the pistons are installed/oriented or you'll get confused when putting it back together.

[richr]

I just purchased the dual piston BOV and have questions about its operation, so I'm resurrecting this thread.
Is the second (large)spring stiffer than the one installed, or not?
My box included an aluminum shim, one that appears would also limit the lifting of the piston that releases boost pressure to atmosphere (thru the circumferential ring of holes).
What determines when to install either the second (large) spring, or the aluminum shim?
Is the smaller spring the one which controls venting at idle?

Is this dual piston BOV the same as a Forge FMDV004?

[Stinger]

It's not a shim, it's a plug to put in place of the BOV if you're pressure testing the system or if you're putting it in place of a bypass valve where you've now got an extra port you no longer use.

The spare spring is stiffer. All it changes is how easy the valve is to get to blow off (how much boost is required right as you decel before it releases). 99% of people prefer to use it as-is.

It NEVER vents/opens at idle, that's the point of the dual piston design. One of the valves will open (you can see it move when you give it throttle and let off) but the second valve is closed under these conditions, preventing any air from passing through it.

It is functionally and dimensionally the same. It doesn't say Forge on top, or come from Forge obviously.

[tutulate]

Good morning Shannon,
I am noticing my BOV is fluttering during part throttle decel after a boost pull.
It "whooshes" during closed throttle decel.

[Stinger]

Yeah, that's how BOV's work. If you reach a point where the combination of spring pressure is close to the boost pressure (and no boost is in the vac line to help push the valve closed) then it will open/shut/open/shut until the pressure changes on one side or the other. If you do a full throttle release (or close to it) then it "wooshes".

[tutulate]

Good to know! Thank!
I was concerned it might be causing "Hammer effect" stress to either the Compressor vanes or the Throttlebody butterfly!

[tutulate]

Shannon, the Mrs does not like the fluttering sound... Do the Bypass valves have the same side effect? I have one on the shelf, and can easily route it to the air filter under the fender.

[Stinger]

You don't have to route it to the air filter, it only has to be routed back into the intake on systems with a VAM or MAF.

[tutulate]

It can vent to the atmosphere? Awesome!
Will it flutter at part throttle decel?

[Stinger]

Won't know until you try it out.