Stock EVO X BPV Modification
Jan 17, 2010, 01:08 PM
#31
There's a port in the stock BOV that puts boost pressure on the bottom of the valve so it's more responsive at light throttle. I don't think he's making a hole, I think he's making the existing hole smaller so the bottom of the valve sees less boost pressure.
Jan 19, 2010, 10:33 AM
#33
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Hi Clipse3GT,
I'm interested to hear more about your concerns with the GFB Stealth FX, since in your original review you seemed very happy with it.
Can you tell me more about the cold-weather problems you had?
Best regards,
GFB Pete
I'm interested to hear more about your concerns with the GFB Stealth FX, since in your original review you seemed very happy with it.
Can you tell me more about the cold-weather problems you had?
Best regards,
GFB Pete
Jan 19, 2010, 11:33 AM
#34
I find the GFB does well in summer and when the spring is set to 0-3 turns from softest. However, when driving the EVO X. The throttle modulation becomes a issue at times especially driving in areas with hill or in the mountains. Throttle modulation becomes very hard. Either you have to be in boost or out of boost. Otherwise the valve with flutter or you will have compressor surge.
The stock valve remedies the issue with the throttle modulation by adding a reference port that allows the the underside of the diaphragm to see boost and have the valve respond faster to throttle changes. Problem is that it makes the valve hold less boost.
I think if GFB would look into creating a valve that has a reference port to help increase throttle modulation it would really have a large market in the EVO community.
But this is coming from one person. I am sure more people that want a new BOV, that holds boost, but retains the stock driveability of the car.
Jan 19, 2010, 07:09 PM
#37
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I used to do this on my Subaru stock BOV and it would hold 2+bar no problems.
Jan 10, 2011, 02:24 PM
#40
Jan 13, 2011, 07:31 PM
#41
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hi guys, I've just been going back over this thread and would like to clear a few things up regarding how the factory valve works.
The primary function of the transfer port that feeds the underside of the diaphragm is not valve response, even though that's what it may appear to do. It is in fact part of the design that makes these valves deliberately open at elevated boost pressure.
There are 4 forces acting on a stock valve. The spring closing the valve, manifold pressure/vacuum acting on the top of the diaphragm, intercooler pressure acting on the underside of the diaphragm via the port in question, and the intercooler pressure pushing up on the valve itself.
Now, when you are at WOT, the pressure in the intercooler and the manifold is the same, therefore the resulting forces of these two opposing pressures is zero. That leaves the spring holding the valve shut against intercooler pressure trying to push it open. Therefore the spring DIRECTLY relates to how much boost pressure the valve will hold.
When you lift off, the manifold pressure goes to vacuum and the resulting forces are unbalanced and the valve opens.
Ok, here’s the tricky part. In order for the valve to hold the required 21-22psi (that’s when the EVO valve begins to leak, even though the car can still make more boost than this with a stock valve, more on this soon), the spring needs to be quite stiff – stiff enough to hold back 22psi as it were. Because the spring is quite stiff, manifold vacuum and intercooler pressure on the underside of the valve do not open the valve sufficiently, so the extra push from the transfer port helps open the valve, so yes, in that sense it does speed up the opening of the valve. However, it needs to help the valve open faster because the spring is so hard in order to combat the opening force provided by the transfer port in the first place. It is because the transfer port design allows the valve to open under excessive boost that it is also needed to speed up the opening of the valve. It is in fact very simple to design a valve without this transfer port which will stay shut at WOT.
Clipse3GT, the problem I can see with the mod you made is that a restrictor in this transfer port does not reduce the boost pressure the underside of the diaphragm sees, it only slows how long it takes for the full pressure to get there. For it to limit the pressure, you would need a second hole to leak the air from the diaphragm chamber after passing through the restrictor – this is how the restrictor pill in a boost control system works – the restrictor can only lower pressure if there is constant flow through the chamber. Restricting flow into a sealed chamber (i.e. the diaphragm) will only slow the build-up of the pressure. Additionally, a 1/16” hole will likely slow the build-up of pressure in such a small volume by fractions of a second. Air at 20psi will pass through a 1/16” restrictor fast enough to fill the diaphragm very, very quickly, negating any possible benefit. With the restrictor in place, you can see this simply by pushing the valve open, then letting it go as fast as you can. You will likely find that the valve closes as fast as you can remove your thumb, meaning the diaphragm is pumping air through the restrictor pretty quickly.
On the topic of boost holding, measuring boost pressure does nothing to tell you whether the valve is opening or not. As previously mentioned, cars can run higher than 22psi on the stock valve. This doesn’t mean the stock valve isn’t leaking, it just means the turbo is able to make up for the loss at the expense of working harder. The ECU will very quickly compensate by adjusting the wastegate duty cycle to make up for lost boost.
I can absolutely guarantee that if anyone were to run an EVO X with a stock valve over 22psi on a dyno with the recirc valve disconnected, they would find a very, very large amount of air escaping.
Back to designing a valve that does not open under boost, well that’s actually very simple. Our valves are designed this way, which is why we do not have the transfer port that the factory one does.
There are only 3 forces acting in our valve – manifold pressure/vacuum pushing down on top of the piston, spring pre-load, and intercooler pressure pushing up on the piston. At WOT, again the manifold and intercooler are at the same pressure, therefore the two resulting forces are equal and opposite, cancelling each other out. You could have 500psi of boost pressure and there would still be no resulting force pushing the piston open. The spring then has a very easy time holding the piston closed on its seat regardless of boost pressure.
The reason fluttering may be experienced with a valve like the Stealth FX is because it is designed to allow air to be vented to atmosphere, meaning it must be closed at all times when it is not required to vent air. The spring pre-load required to hold the piston shut is equal to the maximum manifold vacuum. So therefore when the car is idling with no boost and maximum manifold vacuum trying to lift the piston, the spring must be just firm enough to hold it shut. This pre-load however can cause low RPM/light throttle flutter.
There is a very simple solution – use a softer spring so the piston opens more easily, and all the flutter will be gone. However, it also means you would then not be able to vent it to atmosphere, as the car would stall and generally drive horribly.
There’s plenty more on this topic, but I’ve already made this an essay of epic proportion. I’m very happy to elaborate or discuss any of these points further.
Cheers,
GFB Pete
The primary function of the transfer port that feeds the underside of the diaphragm is not valve response, even though that's what it may appear to do. It is in fact part of the design that makes these valves deliberately open at elevated boost pressure.
There are 4 forces acting on a stock valve. The spring closing the valve, manifold pressure/vacuum acting on the top of the diaphragm, intercooler pressure acting on the underside of the diaphragm via the port in question, and the intercooler pressure pushing up on the valve itself.
Now, when you are at WOT, the pressure in the intercooler and the manifold is the same, therefore the resulting forces of these two opposing pressures is zero. That leaves the spring holding the valve shut against intercooler pressure trying to push it open. Therefore the spring DIRECTLY relates to how much boost pressure the valve will hold.
When you lift off, the manifold pressure goes to vacuum and the resulting forces are unbalanced and the valve opens.
Ok, here’s the tricky part. In order for the valve to hold the required 21-22psi (that’s when the EVO valve begins to leak, even though the car can still make more boost than this with a stock valve, more on this soon), the spring needs to be quite stiff – stiff enough to hold back 22psi as it were. Because the spring is quite stiff, manifold vacuum and intercooler pressure on the underside of the valve do not open the valve sufficiently, so the extra push from the transfer port helps open the valve, so yes, in that sense it does speed up the opening of the valve. However, it needs to help the valve open faster because the spring is so hard in order to combat the opening force provided by the transfer port in the first place. It is because the transfer port design allows the valve to open under excessive boost that it is also needed to speed up the opening of the valve. It is in fact very simple to design a valve without this transfer port which will stay shut at WOT.
Clipse3GT, the problem I can see with the mod you made is that a restrictor in this transfer port does not reduce the boost pressure the underside of the diaphragm sees, it only slows how long it takes for the full pressure to get there. For it to limit the pressure, you would need a second hole to leak the air from the diaphragm chamber after passing through the restrictor – this is how the restrictor pill in a boost control system works – the restrictor can only lower pressure if there is constant flow through the chamber. Restricting flow into a sealed chamber (i.e. the diaphragm) will only slow the build-up of the pressure. Additionally, a 1/16” hole will likely slow the build-up of pressure in such a small volume by fractions of a second. Air at 20psi will pass through a 1/16” restrictor fast enough to fill the diaphragm very, very quickly, negating any possible benefit. With the restrictor in place, you can see this simply by pushing the valve open, then letting it go as fast as you can. You will likely find that the valve closes as fast as you can remove your thumb, meaning the diaphragm is pumping air through the restrictor pretty quickly.
On the topic of boost holding, measuring boost pressure does nothing to tell you whether the valve is opening or not. As previously mentioned, cars can run higher than 22psi on the stock valve. This doesn’t mean the stock valve isn’t leaking, it just means the turbo is able to make up for the loss at the expense of working harder. The ECU will very quickly compensate by adjusting the wastegate duty cycle to make up for lost boost.
I can absolutely guarantee that if anyone were to run an EVO X with a stock valve over 22psi on a dyno with the recirc valve disconnected, they would find a very, very large amount of air escaping.
Back to designing a valve that does not open under boost, well that’s actually very simple. Our valves are designed this way, which is why we do not have the transfer port that the factory one does.
There are only 3 forces acting in our valve – manifold pressure/vacuum pushing down on top of the piston, spring pre-load, and intercooler pressure pushing up on the piston. At WOT, again the manifold and intercooler are at the same pressure, therefore the two resulting forces are equal and opposite, cancelling each other out. You could have 500psi of boost pressure and there would still be no resulting force pushing the piston open. The spring then has a very easy time holding the piston closed on its seat regardless of boost pressure.
The reason fluttering may be experienced with a valve like the Stealth FX is because it is designed to allow air to be vented to atmosphere, meaning it must be closed at all times when it is not required to vent air. The spring pre-load required to hold the piston shut is equal to the maximum manifold vacuum. So therefore when the car is idling with no boost and maximum manifold vacuum trying to lift the piston, the spring must be just firm enough to hold it shut. This pre-load however can cause low RPM/light throttle flutter.
There is a very simple solution – use a softer spring so the piston opens more easily, and all the flutter will be gone. However, it also means you would then not be able to vent it to atmosphere, as the car would stall and generally drive horribly.
There’s plenty more on this topic, but I’ve already made this an essay of epic proportion. I’m very happy to elaborate or discuss any of these points further.
Cheers,
GFB Pete
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