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What limits a bypass valves hp capacity? Is it the overall flow from the valve? I don't think it could be anything else that would cause drivability issues.
They referring to the amount of air it can vent without causing the turbo to be dead headed. So, the Tial Q for example, its rated at 1800hp. It can vent 1800hp worth of airflow out of the charge system without barking the turbo.
And spring tension, along with boost pressure from the reference line, needs to be strong enough to stay closed for desired PSI.
As long as it stays closed and can flow enough to relieve pressure (as mentioned above), then it's all good.
And spring tension, along with boost pressure from the reference line, needs to be strong enough to stay closed for desired PSI.
As long as it stays closed and can flow enough to relieve pressure (as mentioned above), then it's all good.
spring tension in the BOV has nothing to do with how much boost you're running. It is only relevant to how much vacuum the engine creates with throttle closed so that the engine can open the valve effectively.
spring tension in the BOV has nothing to do with how much boost you're running. It is only relevant to how much vacuum the engine creates with throttle closed so that the engine can open the valve effectively.
On the OEM BOV, it has boost on both sides of the diaphragm through a small port on the charge side of the valve. Almost no aftermarket BOV has that port, and you can't change the spring in the OEM valve, so it's not exactly relevant. When it pertains to an aftermarket BOV like Tial, Synapse, TurboSmart, the spring does not dictate how much boost it will hold.
Crushing the stock metal DV compresses the spring inside. It takes more pressure to open the valve because of this extra compression. Here is a picture of the hole on the compressed intake side (to throttlebody). This hole allows the diaphram to see this pressure and uncrushed DV will open the DV around 26-27 psi without having a vacuum on other side. This then becones a boost leak.
On the OEM BOV, it has boost on both sides of the diaphragm through a small port on the charge side of the valve. Almost no aftermarket BOV has that port, and you can't change the spring in the OEM valve, so it's not exactly relevant. When it pertains to an aftermarket BOV like Tial, Synapse, TurboSmart, the spring does not dictate how much boost it will hold.
Thinking about this, and im not trying to argue just thinking out loud, your right that if you have boost pressure on one side of the diaphram and boost pressure on the other side of the valve they will effectively cancel themselves out. If you are recirculating though you will have vacuum on the other side of the valve going to the turbo inlet, and that will throw off that balance, and you will need a spring to counter act that. At least I think so lol
Crushing a stock DV allows it to hold more boost. I assumed that's because it adds preload to the spring. I also assumed OP was referring to an OEM DV, for some reason.
thinking about it some more, the surface area of the valve vs the size of the hose going to the diaphragm housing are pretty unequal. So the valve would see the initial pressure spike before the diaphragm could build enough pressure to push back against it causing the valve to blow open and vent the pressure, thus the spring is needed to help prevent that from happening.
Crushing the stock metal DV compresses the spring inside. It takes more pressure to open the valve because of this extra compression. Here is a picture of the hole on the compressed intake side (to throttlebody). This hole allows the diaphram to see this pressure and uncrushed DV will open the DV around 26-27 psi without having a vacuum on other side. This then becones a boost leak.
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I've been having an apparent boost leak and haven't been able to find the source. I wonder....
Thinking about this, and im not trying to argue just thinking out loud, your right that if you have boost pressure on one side of the diaphram and boost pressure on the other side of the valve they will effectively cancel themselves out. If you are recirculating though you will have vacuum on the other side of the valve going to the turbo inlet, and that will throw off that balance, and you will need a spring to counter act that. At least I think so lol
Don't forget that once the DV opens, the pressure is also vented on the bottom side of the diaphragm.
Last edited by 2006EvoIXer; Jun 20, 2017 at 03:17 PM.
Don't forget that once the DV opens, the pressure is also vented on the bottom side if the diaphragm.
not sure i follow. From what i understand the "bottom side" that is the side not getting pressure/vacuum from the intake is always exposed to the inlet vacuum of the turbo unless VTA. When the valve opens all that air bypassing it will just get sucked back up into the turbo inlet.
You actually raise a good point about the bottom side of the diaphragm though. It is also seeing vacuum just like the back side of the valve, which would equalize that out, pretty much cancelling out my first point, my second though about the diaphragm housing requiring more time to build pressure then the face of the valve i think still stands though.
Last edited by Biggiesacks; Jun 20, 2017 at 02:58 PM.
not sure i follow. From what i understand the "bottom side" that is the side not getting pressure/vacuum from the intake is always exposed to the inlet vacuum of the turbo unless VTA. When the valve opens all that air bypassing it will just get sucked back up into the turbo inlet.
You actually raise a good point about the bottom side of the diaphragm though. It is also seeing vacuum just like the back side of the valve, which would equalize that out, pretty much cancelling out my first point, my second though about the diaphragm housing requiring more time to build pressure then the face of the valve i think still stands though.
Let me try to explain what I understand and hopefully don't confuse you more.
The top side of diaphragm is connected to intake manifold between throttlebody and valves. When off throttle, it sees a vacuum from pistons sucking and throttlebody being closed.
The bottom side of diaphragm will only see boost because it is plumbed to the pressurized piping between throttlebody and intercooler (same as air we are recirculating with DV). When DV opens from vacuum at top of diaphragm or enough pressure from bottom of diaphragm over coming the spring and top diaphragm pressure, it opens and purges the pressure (recirculating to turbo intake). Onced purged, the bottom of diaphragm sees the same reduced pressure as what's in the pipe between throttlebody and intercooler. You are correct that the bottom side of diaphragm is slightly delayed because of the pathway from source pressure. I think (my opinion) this gives us the nice midrange boost spike before bleeding off the boost in the higher RPMs.
There should never be a vacuum between throttlebody and intercooler (including the "bottom side") because turbo intake can't suck it into a vacuum without the DV closing and cutting it off.
