A boost bypass is meant to bypass excess boost from hammering the TB when transitioning from WOT to idle and is placed pre-throttlebody. A vacuum bypass, as displayed on positive displacement blowers, is placed post-throttlebody and open while the engine is pulling vacuum at or near idle due to the TB blade being closed. The blower cannot make any boost, with or without a vacuum bypass valve in the mix, because it is still pulling from a vacuum on the inlet side due to the closed throttle blade. It is simple physics dealing with pressure ratios man. When you are WOT with a PD blower that has a vacuum bypass valve the blade is held closed by a spring to allow the blower to generate boost. Boost does sort of assist in holding it closed too but really all the boost does is make the valve snap closed more quickly is the line is plumbed to the lower manifold to see boost in addition to vac. Another reason the valve is closed is because there is no vacuum to pull against the diaphragm to rotate the bypass valve blade open at that time. When you let off the gas pedal the throttle body blade shuts right away, and in less than an instant the engine transitions to vacuum even while RPM's are still high. There is no excess boost to bypass since the engine is already in vacuum. Besides, the boost has nothing closed to prevent it from continuing to travel into the intake ports and opening valves of the cylinder heads so excess boost cannot be generated and thus need to be bypassed. Again, it is physics. If there is no post-blower obstruction present then there is no way to have a pressure spike when getting off of the gas. Thus if there is no chance that a pressure/boost spike can occur then that valve isn't exactly being used to bypass boost now is it? So one would surmise that perhaps it is being used for something else. That something else is what I already outlined in my previous post regarding allowing vacuum to equalize between the post-TB/pre-blower upper intake manifold and the post-blower lower intake manifold. Allowing this vacuum to bypass the blower completely is the goal. Again, when the valve is open and thus bypassing; the airflow is traveling from upper intake manifold, through the bypass valve, and to lower intake manifold hence the reason it is a vacuum bypass valve. If the blower were boosting then this airflow direction would be reversed and the valve could sort of be called a boost bypass valve. But it is not.
There is allot of misunderstanding regarding these valves and their operation. What some may think is interchangeable really is not. But after so many people get ideas stuck in their heads sometimes it is just easier to use their lingo when speaking to them than correcting them in that they are actually talking about a vacuum bypass and not boost bypass. This is why you see some companies referring to them as a boost bypass valve. Something similar happens when people who have turbo systems refer to their boost bypass valve as a BOV (blow off valve). While both do vent excess boost so turbo and TB damage don't occur, one recirculates it back into the pre-turbo intake tract while the other blows it off to atmosphere. While both valves are similar or even the exact same in construction, the way they are plumbed and used is not interchangeable especially if you are running a draw-through MAFS setup. If you want to call the valve that is on the bottom of your Eaton M90 blower a boost bypass then be my guest, but that isn't the operation it really is performing when operating. If by now you don't understand how that valve works, why it works that way, and why it is not a boost bypass valve then I don't know what to tell you at this point man. I have beaten the dead horse on this so much and taken this thread so far off topic in order for you to understand it that it just isn't worth it if you cannot get past yourself and pay attention to someone who has light years more knowledge on this and the experience to back it up. There is a reason I have been in a field of engineering for the better part of 20 years and why I am also transitioning over from my previous specialty to mechanical engineering with a focus on automotive and robotics. If you don't want to trust what I say then that is cool, but you cannot refute physics. Like I said, look at how pressure ratios work and think about how the vacuum would work taking that into account with an Eaton blown vehicle at idle.
I am burnt out on explaining this stuff. Sorry I took your thread off topic Don.
