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I have a Ford 302 with a homemade hotside 70mm turbo, 44mm wastegate and 50mm BOV. Blow through Carb.
All On3 parts, I finally got it up and running and found the BOV stays open at idle. The engine is pulling about 17" of vac. at idle. I have done a bunch of reading on this to find there are many different schools of thought on the way a BOV is supposed to work.
On3 says you want the BOV fully open at idle to keep all back pressure off the turbo.
Tail says you want it closed at idle, and that will cause boost to come on quicker.
I really just don't like the idea of pulling unfiltered air into the engine.
I think I will get a stronger spring to just keep it barley closed at idle.
I would love to hear other peoples thoughts on this.
Thanks

 

Shim, or change spring so its just closed at idle. The spike in pressure, and vacuum should open it. Most will tell you that a bov isn't necessary at 10 psi or less, especially with an auto. I would take Tial's recommendation over On3's any day.

 

This is how to properly use a bypass (I call it a bypass)
1. The bypass should be as close to the turbo as possible. Some manufacturers (borg warner for example) are now placing them directly on the compressor housing. The purpose of the bypass is to protect the turbo so it needs to be as close as possible.

2. If the bypass is recirculated (like a factory unit, back to the intake pre-turbo tract) you want the bypass to hang open at idle and while off throttle. The Reason: The turbo imparts kinetic energy to the airflow as the air molecules gain velocity and leave the compressor. This is how turbos work. What most people overlook however is the conservation of that energy. By recirculating the air back to the intake tract, you conserve some of the work that was done on those molecules, thus reducing the necessary speed of the shaft to create a differential. In other words, you improve the response of the turbo by feeding it high energy air molecules, and conserve the fuel/energy that was initially imparted to them the first time they went through the compressor.
All factory turbo cars use this technique because it not only improves response, it increases economy. From the engineering point of view, using fuel (energy) to speed up the air molecules and then dumping them (open bypass to the atmosphere) is a huge waste of energy. No auto manufacturer would do that.

3. Another reason you may want to keep the bypass open while off throttle is surge conditional near idle speeds. If the bypass is closed, as you put it earlier, "to keep the backpressure off the turbo" is the basic idea. What that 'back pressure' could cause is an idle surge, meaning the compressor wheel gradually speeds up until the 'pressure' (its very low pressure) becomes too much, then the airflow has no where to go (bypass is CLOSED) so it simply flows backwards through the compressor, stalling the compressor, and reducing the shaft speed dramatically. Often this turns into an oscillation event where the compressor speeds up, hits the 'wall', surges, slows down, then starts speeding up all over again. While not particularly damaging to anything (low speeds air and mild surge, most turbos will tolerate for a long time) this situation definitely degrades performance, your compressor wheel will always be moving slower than it should be, the turbo will lag more.

4. Is there any situation where a fully closed bypass is "ok"?
Yes, I think so. If the turbo is so huge that it can never gain enough shaft speed to surge near or at idle, then you can leave the bypass closed full time because it never needed to open to release the building shaft speed/pressure. It would still benefit some from conservation of kinetic energy as above in #1 however the larger the turbo is the more difficult it becomes to recover something in a useful manner so I put less emphasis on it.

 

Just a update, I swapped in a spring designed for the vacuum my engine pulls and it works great now! Here is a video of it working properly!