cheap bosch bov replacement
11-20-2007, 05:58 PM
#84
The fact remains that pressure is what will open a BOV, not flow. When the pressure acting on the piston (pressure*area = force) override the force of the spring in the BOV, it will start to open. How FAST it opens will be determined by the flow rate of the air, but the fact it opens is due to the force applied by the pressure.
Think of it as a hydraulic cylinder. If you put a constant 200 lb load on the top of a cylinder with a 1" area piston, 200 psi will create an equilibrium. Anything above that pressure will cause the piston to move the load upward. If you increase flow the cylinder will move faster, decrease it and it will move slower. If you cut the flow then the piston will continue to move upward until the volume behind the piston expands and the pressure in the cylinder reduces to 200 lbs.
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11-28-2007, 10:15 AM
#85
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actually, in an oem situation at least, its the vacuum that opens the bov/bpv.
reason being the diaphragm is significantly larger than the valve itself.
i did a big write up recently on it to explain it to some monkeys on another mazda board, i can repost it here if there is interest. what it came down to though is this :
the purpose of these valves is two fold :
safety stop the turbo being damaged
performance stop the thing from "despooling" due to the compressor pushing against pressure when the turbine has none driving it (they are just an equilibrium at the end of the day)
in both cases, a diaphragm/poppet valve type outperforms a piston type significantly for some very basic reasons.
the trouble comes once the bov is open, and there is still significant air flowing through it despite the lack of vacuum from the manifold. if your oem style one is blowing open on a shift and staying there, its too small for the volume of compressed air in the intercooler and plumbing and/or the flow of the turbo thats still spinning. the wind speed keeps it open. if you upsize or double up on the stockers (bosch or whatever) you can have good flow and less wind and it should get to close, once closed, pressure builds, and the diaphragm takes control of the situation and forces it closed.
the piston ones on the other hand rely on the spring pressure to seal properly and close again, so it must be heavier, and by definition, the piston is the same area at both ends, so the vacuum has a hard time pulling against the spring by itself > enter back pressure, the pressure from the incoming air is then relied upon to force open the bov and by doing that, you are defeating the purpose somewhat as for both safety and performance things are less than optimal.
the piston type ones came about to allow a vented bov to exist with an afm. thus its biased strongly enough with the spring to remain closed at idle. bypass diaphragm ones are wide open at idle and couldnt be used like that with an afm.
in summary, stay plumbed back but just bigger unless you have a map sensor driven ecu. if you have map, chuck a filter at a diaphragm unit and it will work better if it is sized accordingly.
for reference, the silvia uses a 25mm in/out bpv for 250 crank hp stock. thats probably a fairly good sizing guide. i imagine most of you are pushing figures like that, and thus need that sort of size or bigger.
i went silly with mine :-) twin 38mm oem nissan skyline gtr bpvs open to air with high class sock filters on their outlets. i run ms with map, so they are just vented.
silvia vs gtr :
http://i50.photobucket.com/albums/f3...trbovsbpvs.jpg
gtr installed :
http://s50.photobucket.com/albums/f3...4/100_4469.jpg
http://s50.photobucket.com/albums/f3...4/100_4470.jpg
the write up just has the math to prove the point.
let me know if you are interested.
fred.
reason being the diaphragm is significantly larger than the valve itself.
i did a big write up recently on it to explain it to some monkeys on another mazda board, i can repost it here if there is interest. what it came down to though is this :
the purpose of these valves is two fold :
safety stop the turbo being damaged
performance stop the thing from "despooling" due to the compressor pushing against pressure when the turbine has none driving it (they are just an equilibrium at the end of the day)
in both cases, a diaphragm/poppet valve type outperforms a piston type significantly for some very basic reasons.
the trouble comes once the bov is open, and there is still significant air flowing through it despite the lack of vacuum from the manifold. if your oem style one is blowing open on a shift and staying there, its too small for the volume of compressed air in the intercooler and plumbing and/or the flow of the turbo thats still spinning. the wind speed keeps it open. if you upsize or double up on the stockers (bosch or whatever) you can have good flow and less wind and it should get to close, once closed, pressure builds, and the diaphragm takes control of the situation and forces it closed.
the piston ones on the other hand rely on the spring pressure to seal properly and close again, so it must be heavier, and by definition, the piston is the same area at both ends, so the vacuum has a hard time pulling against the spring by itself > enter back pressure, the pressure from the incoming air is then relied upon to force open the bov and by doing that, you are defeating the purpose somewhat as for both safety and performance things are less than optimal.
the piston type ones came about to allow a vented bov to exist with an afm. thus its biased strongly enough with the spring to remain closed at idle. bypass diaphragm ones are wide open at idle and couldnt be used like that with an afm.
in summary, stay plumbed back but just bigger unless you have a map sensor driven ecu. if you have map, chuck a filter at a diaphragm unit and it will work better if it is sized accordingly.
for reference, the silvia uses a 25mm in/out bpv for 250 crank hp stock. thats probably a fairly good sizing guide. i imagine most of you are pushing figures like that, and thus need that sort of size or bigger.
i went silly with mine :-) twin 38mm oem nissan skyline gtr bpvs open to air with high class sock filters on their outlets. i run ms with map, so they are just vented.
silvia vs gtr :
http://i50.photobucket.com/albums/f3...trbovsbpvs.jpg
gtr installed :
http://s50.photobucket.com/albums/f3...4/100_4469.jpg
http://s50.photobucket.com/albums/f3...4/100_4470.jpg
the write up just has the math to prove the point.
let me know if you are interested.
fred.
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11-28-2007, 01:34 PM
#88
Sorry if my brief reply came across mean spirited, just bushed from lack of sleep. I honestly don't care about post counts. Lets see now:
actually, in an oem situation at least, its the vacuum that opens the bov/bpv. This is the case with pretty much all mechanical dv/bovs that I am aware of. The only exception coming to mind right now are the kits that we make for VW diesels.
reason being the diaphragm is significantly larger than the valve itself. Actually the diaphragm, being contained within the valve, cannot be larger than the valve. Correctly stated, the surface area available to the manifold is significantly larger than that available to charge air. This is one of the areas where valve design is important and this is also the case with piston valves.
i did a big write up recently on it to explain it to some monkeys on another mazda board, i can repost it here if there is interest. what it came down to though is this :
the purpose of these valves is two fold :
safety stop the turbo being damaged
performance stop the thing from "despooling" due to the compressor pushing against pressure when the turbine has none driving it (they are just an equilibrium at the end of the day)
in both cases, a diaphragm/poppet valve type outperforms a piston type significantly for some very basic reasons. In bench racing and on internet forums, the reality of it is that piston valves usually have the advantage in drivability.
the trouble comes once the bov is open, and there is still significant air flowing through it despite the lack of vacuum from the manifold. if your oem style one is blowing open on a shift and staying there, Staying there after the manifold has been pressurized? If that happens you need a stiffer spring or the difference between the top and bottom surface areas is not great enough. If you mean while the manifold is in vacuum then the valve is doing what it should. Lightly spring valves will usually stay open longer, hurting spool and power during boost onset. its too small for the volume of compressed air in the intercooler and plumbing and/or the flow of the turbo thats still spinning. the wind speed keeps it open. if you upsize or double up on the stockers (bosch or whatever) you can have good flow and less wind and it should get to close, once closed, pressure builds, and the diaphragm takes control of the situation and forces it closed. In this scenario you’ve just fixed a valve that takes too long to close by adding a second valve to do the same. It MAY work sometimes but only when it is a flow limitation causing the valve to stay open, but usually the problem comes down to spring pressure.
the piston ones on the other hand rely on the spring pressure to seal properly and close again, Actually they rely on manifold pressure to close, spring tension is just there to make up for a pressure differential on boost onset and for pressure drop due to intercooler heatsoak, latency etc. This is the case with pretty much all valves, diaphragm or piston. so it must be heavier, and by definition, the piston is the same area at both ends, You will find this to be the case for most diaphragm valves as well, until closed at which point surface area available to the charge air is determined by the size of the pressure port on the valve, which is the same in piston valves. so the vacuum has a hard time pulling against the spring by itself > enter back pressure, the pressure from the incoming air is then relied upon to force open the bov Working in conjunction with vacuum, again, this is the case with nearly all valves. and by doing that, you are defeating the purpose somewhat as for both safety and performance things are less than optimal.
the piston type ones came about to allow a vented bov to exist with an afm. This would be why twin piston units were developed, piston units exist to address reliability issues with diaphragm units. thus its biased strongly enough with the spring to remain closed at idle. Relying on spring tension to keep a valve closed at idle is usually one of two things: first is when people are using a valve not meant to be closed at idle on an AFM system so they crank up the spring tension to try and make it work, second is with a single piston valve that is designed for an AFM system. These units will usually have a leak engineered into them so that they close at idle, this allows them to use a light spring so that opening under low boost conditions isn’t compromised. bypass diaphragm ones are wide open at idle and couldnt be used like that with an afm.
in summary, stay plumbed back but just bigger unless you have a map sensor driven ecu. if you have map, chuck a filter at a diaphragm unit and it will work better if it is sized accordingly.
for reference, the silvia uses a 25mm in/out bpv for 250 crank hp stock. thats probably a fairly good sizing guide. i imagine most of you are pushing figures like that, and thus need that sort of size or bigger. A single 25mm valve should be good for close to twice that power, we have 34mm valves running on cars making 700whp.
i went silly with mine :-) twin 38mm oem nissan skyline gtr bpvs open to air with high class sock filters on their outlets. i run ms with map, so they are just vented.
actually, in an oem situation at least, its the vacuum that opens the bov/bpv. This is the case with pretty much all mechanical dv/bovs that I am aware of. The only exception coming to mind right now are the kits that we make for VW diesels.
reason being the diaphragm is significantly larger than the valve itself. Actually the diaphragm, being contained within the valve, cannot be larger than the valve. Correctly stated, the surface area available to the manifold is significantly larger than that available to charge air. This is one of the areas where valve design is important and this is also the case with piston valves.
i did a big write up recently on it to explain it to some monkeys on another mazda board, i can repost it here if there is interest. what it came down to though is this :
the purpose of these valves is two fold :
safety stop the turbo being damaged
performance stop the thing from "despooling" due to the compressor pushing against pressure when the turbine has none driving it (they are just an equilibrium at the end of the day)
in both cases, a diaphragm/poppet valve type outperforms a piston type significantly for some very basic reasons. In bench racing and on internet forums, the reality of it is that piston valves usually have the advantage in drivability.
the trouble comes once the bov is open, and there is still significant air flowing through it despite the lack of vacuum from the manifold. if your oem style one is blowing open on a shift and staying there, Staying there after the manifold has been pressurized? If that happens you need a stiffer spring or the difference between the top and bottom surface areas is not great enough. If you mean while the manifold is in vacuum then the valve is doing what it should. Lightly spring valves will usually stay open longer, hurting spool and power during boost onset. its too small for the volume of compressed air in the intercooler and plumbing and/or the flow of the turbo thats still spinning. the wind speed keeps it open. if you upsize or double up on the stockers (bosch or whatever) you can have good flow and less wind and it should get to close, once closed, pressure builds, and the diaphragm takes control of the situation and forces it closed. In this scenario you’ve just fixed a valve that takes too long to close by adding a second valve to do the same. It MAY work sometimes but only when it is a flow limitation causing the valve to stay open, but usually the problem comes down to spring pressure.
the piston ones on the other hand rely on the spring pressure to seal properly and close again, Actually they rely on manifold pressure to close, spring tension is just there to make up for a pressure differential on boost onset and for pressure drop due to intercooler heatsoak, latency etc. This is the case with pretty much all valves, diaphragm or piston. so it must be heavier, and by definition, the piston is the same area at both ends, You will find this to be the case for most diaphragm valves as well, until closed at which point surface area available to the charge air is determined by the size of the pressure port on the valve, which is the same in piston valves. so the vacuum has a hard time pulling against the spring by itself > enter back pressure, the pressure from the incoming air is then relied upon to force open the bov Working in conjunction with vacuum, again, this is the case with nearly all valves. and by doing that, you are defeating the purpose somewhat as for both safety and performance things are less than optimal.
the piston type ones came about to allow a vented bov to exist with an afm. This would be why twin piston units were developed, piston units exist to address reliability issues with diaphragm units. thus its biased strongly enough with the spring to remain closed at idle. Relying on spring tension to keep a valve closed at idle is usually one of two things: first is when people are using a valve not meant to be closed at idle on an AFM system so they crank up the spring tension to try and make it work, second is with a single piston valve that is designed for an AFM system. These units will usually have a leak engineered into them so that they close at idle, this allows them to use a light spring so that opening under low boost conditions isn’t compromised. bypass diaphragm ones are wide open at idle and couldnt be used like that with an afm.
in summary, stay plumbed back but just bigger unless you have a map sensor driven ecu. if you have map, chuck a filter at a diaphragm unit and it will work better if it is sized accordingly.
for reference, the silvia uses a 25mm in/out bpv for 250 crank hp stock. thats probably a fairly good sizing guide. i imagine most of you are pushing figures like that, and thus need that sort of size or bigger. A single 25mm valve should be good for close to twice that power, we have 34mm valves running on cars making 700whp.
i went silly with mine :-) twin 38mm oem nissan skyline gtr bpvs open to air with high class sock filters on their outlets. i run ms with map, so they are just vented.
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11-28-2007, 03:30 PM
#89
Junior Member
Join Date: Feb 2007
Location: 11368 miles from where i would like to be
Posts: 269
Total Cats: 92
actually, in an oem situation at least, its the vacuum that opens the bov/bpv. This is the case with pretty much all mechanical dv/bovs that I am aware of. The only exception coming to mind right now are the kits that we make for VW diesels.
oem : vacuum does the vast majority of the work
single piston : vacuum shares the job with pressure against the valve
the latter is far less good for flow and residual pressure for the compressor to have to fight with no driving force. ie, the latter causes the turbo to slow down more and faster.
reason being the diaphragm is significantly larger than the valve itself. Actually the diaphragm, being contained within the valve, cannot be larger than the valve. Correctly stated, the surface area available to the manifold is significantly larger than that available to charge air. This is one of the areas where valve design is important and this is also the case with piston valves.
valve = poppet valve, and just like the wastegate valve is MUCH smaller than the diaphragm above it, so too is the rubber lined poppet valve in a typical oem bov/bpv application MUCH smaller (in area, thanks for clarifying) than the diaphragm above it.
the single piston design on the other hand has the same area exactly at each end and thus relies solely on spring pressure to return closed and seal when under boost.
usually, the valve in an oem bov/bpv will not be significantly larger than the orifice feeding it with charge air to release.
In bench racing and on internet forums, the reality of it is that piston valves usually have the advantage in drivability.
the trouble comes once the bov is open, and there is still significant air flowing through it despite the lack of vacuum from the manifold. if your oem style one is blowing open on a shift and staying there, Staying there after the manifold has been pressurized? If that happens you need a stiffer spring or the difference between the top and bottom surface areas is not great enough. If you mean while the manifold is in vacuum then the valve is doing what it should. Lightly spring valves will usually stay open longer, hurting spool and power during boost onset.
at low rpm and on a long term trailing throttle with no turbine speed, all valves close as good as instantly when you stab the throttle. at that point, either they can hold pressure or they cant, the oem style one will be holding it better at that point. dual piston IF the pistons were different sizes could do just the same thing as oem.
only when flow exceeds that the valve was designed to operate at (3/4inch valve on 15psi bpt) will it hang open forced there by the sheer speed of the air flowing past it.
its too small for the volume of compressed air in the intercooler and plumbing and/or the flow of the turbo thats still spinning. the wind speed keeps it open. if you upsize or double up on the stockers (bosch or whatever) you can have good flow and less wind and it should get to close, once closed, pressure builds, and the diaphragm takes control of the situation and forces it closed. In this scenario you’ve just fixed a valve that takes too long to close by adding a second valve to do the same. It MAY work sometimes but only when it is a flow limitation causing the valve to stay open, but usually the problem comes down to spring pressure.
the piston ones on the other hand rely on the spring pressure to seal properly and close again, Actually they rely on manifold pressure to close, spring tension is just there to make up for a pressure differential on boost onset and for pressure drop due to intercooler heatsoak, latency etc. This is the case with pretty much all valves, diaphragm or piston.
in reality BOTH designs rely upon a spring bias to ensure that they close at all, however, the spring pressure required with a diaphragm style one is by design and definition lighter than a single piston unit, leading to the lower restriction that i initially stated here :
so it must be heavier, and by definition, the piston is the same area at both ends, You will find this to be the case for most diaphragm valves as well, until closed at which point surface area available to the charge air is determined by the size of the pressure port on the valve, which is the same in piston valves.
all single pistons by definition have equal area at each end.
so the vacuum has a hard time pulling against the spring by itself > enter back pressure, the pressure from the incoming air is then relied upon to force open the bov Working in conjunction with vacuum, again, this is the case with nearly all valves. and by doing that, you are defeating the purpose somewhat as for both safety and performance things are less than optimal.
the piston type ones came about to allow a vented bov to exist with an afm. This would be why twin piston units were developed, piston units exist to address reliability issues with diaphragm units.
the piston type ones came about to allow a vented bov to exist with an afm. This would be why twin piston units were developed, piston units exist to address reliability issues with diaphragm units.
thus its biased strongly enough with the spring to remain closed at idle. Relying on spring tension to keep a valve closed at idle is usually one of two things: first is when people are using a valve not meant to be closed at idle on an AFM system so they crank up the spring tension to try and make it work,
second is with a single piston valve that is designed for an AFM system. These units will usually have a leak engineered into them so that they close at idle, this allows them to use a light spring so that opening under low boost conditions isn’t compromised.
TBC, post too long...
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11-28-2007, 03:31 PM
#90
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Join Date: Feb 2007
Location: 11368 miles from where i would like to be
Posts: 269
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.....
I have two responses to this :
firstly, your 34mm valve has an area of flow of 908mm^2 vs the silvia one at 491mm^2 if you assume that you can crank the boost on the silvia in stock form and max out the t28 at 300hp or so, then the ratio of area gives us the 34mm unit flowing 550hp adequately, but not better than adequately :
although 25mm works for 300hp or so, the ideal bov for any given engine will flow exactly the same as the ideal throttle body for the same engine. for a 2.0, thats about 50mm dia. this is not really practical though, and oems like to save $$$$ as much as the aftermarket like to make it, so they do what will "just work". just look at the pathetic 6" diff the 1.6 miata comes with to know this is the truth. on a less low end budget car like the godzilla, the expenses arent spared and you end up with the twin 38mm setup that the GTR comes with. according to silvia ratios, they will flow about 1200hp worth, or 1500hp worth in slightly stretched configuration. thats because they wanted zero restriction to flow on the gtr as their flagship vehicle. so they went bigger.
so, no, a 25mm valve is far from ideal even on its stock application, but if you up the stock power a bit, you can consider this a maximum hp for that size valve.
once again, the difference between working at all, and working VERY well.
we seem to agree on most things, just your wording differs from mine. i hope i have been suitably explicit with this flakey language that we all use such that my points are crystal clear :-)
spot you later,
fred.
for reference, the silvia uses a 25mm in/out bpv for 250 crank hp stock. thats probably a fairly good sizing guide. i imagine most of you are pushing figures like that, and thus need that sort of size or bigger. A single 25mm valve should be good for close to twice that power, we have 34mm valves running on cars making 700whp.
firstly, your 34mm valve has an area of flow of 908mm^2 vs the silvia one at 491mm^2 if you assume that you can crank the boost on the silvia in stock form and max out the t28 at 300hp or so, then the ratio of area gives us the 34mm unit flowing 550hp adequately, but not better than adequately :
although 25mm works for 300hp or so, the ideal bov for any given engine will flow exactly the same as the ideal throttle body for the same engine. for a 2.0, thats about 50mm dia. this is not really practical though, and oems like to save $$$$ as much as the aftermarket like to make it, so they do what will "just work". just look at the pathetic 6" diff the 1.6 miata comes with to know this is the truth. on a less low end budget car like the godzilla, the expenses arent spared and you end up with the twin 38mm setup that the GTR comes with. according to silvia ratios, they will flow about 1200hp worth, or 1500hp worth in slightly stretched configuration. thats because they wanted zero restriction to flow on the gtr as their flagship vehicle. so they went bigger.
so, no, a 25mm valve is far from ideal even on its stock application, but if you up the stock power a bit, you can consider this a maximum hp for that size valve.
once again, the difference between working at all, and working VERY well.
we seem to agree on most things, just your wording differs from mine. i hope i have been suitably explicit with this flakey language that we all use such that my points are crystal clear :-)
spot you later,
fred.
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11-29-2007, 11:30 AM
#91
oem : vacuum does the vast majority of the work I’d wager that the overwhelming majority of OEM valves are simple single diaphragm units, not poppet style (cost being the reason, take a look in a junkyard most valves are Bosch diaphragm valves)
single piston : vacuum shares the job with pressure against the valve
Vacuum alone will open even our stiffest spring. There is no difference in principle between the two. And I imagine that the majority of OEM valves are simple single diaphragm units which are no different than piston valves. The only OEM valves that I’m aware of that have a design that allows surface areas to differ are poppet style, and many of those incorporate some means of slowing response, to offset how easily they open.
the latter is far less good for flow and residual pressure for the compressor to have to fight with no driving force. ie, the latter causes the turbo to slow down more and faster.
Actually when logging pressure in the intake tract all of the different designs operate almost identically (assuming an adequate design) once the throttle body is closed: a small 1-4 psi spike than boost tapering off until the TB is opened and the manifold is pressurized again… Unless the valve is very under-sized or the spring is overly stiff. Then you get a larger spike, maybe fluttering, maybe surge… but that is beyond what we are discussing.
valve = poppet valve, and just like the wastegate valve is MUCH smaller than the diaphragm above it, so too is the rubber lined poppet valve in a typical oem bov/bpv application MUCH smaller (in area, thanks for clarifying) than the diaphragm above it.
I am familiar with them. Your wording made this seem to be an advantage of diaphragm valves. It is not. Diaphragm valves are almost identical functionally to piston valves. Poppets are the exception. But poppet valves can be either piston or diaphragm and can’t be lumped with either category to support your (nor my) claims.
the single piston design on the other hand has the same area exactly at each end and thus relies solely on spring pressure to return closed and seal when under boost. As do diaphragm valves.
usually, the valve in an oem bov/bpv will not be significantly larger than the orifice feeding it with charge air to release.
Quote:
In bench racing and on internet forums, the reality of it is that piston valves usually have the advantage in drivability.
i honestly can not conceive of a single reason why either would be better than the other in this respect. provided that they function correctly, preventing surge and releasing excess charge air, what can possibly be different other than the speed of onset of boost? enlighten me :-) Response time. Valves that respond too quickly cause problems with driveability, especially in transient situations. As I mentioned before, many OEM valves incorporate some way to delay response…
no, its not possible for the intake tract to pressurise whilst the valve is still allowing air past. Not true. You can build boost with a vacuum leak. And valves are typically open under vacuum… but some how boost is made. Look around at my posts, you will see a dyno I posted of one of our valves compared against a stock WRX unit… it picked up power over stock in the low rpm ranges because the valve would close sooner. Point being, the car was making boost while the OEM valve was open, it was just less than with ours. meaning that the back side of the diaphragm sees 100kpa, and the front side of the valve sees 100kpa air thats moving rapidly towards it, and the spring pressure sees that energy carried though inertia in the moving air. 100kpa is 100kpa. What you are saying makes no sense. What you are describing would be pressure higher than 100kpa… I think. at the that time, the spring is doing the majority of the work because the front side of the diaphragm is also seeing 100kpa, and the back side of the valve is seeing less than 100kpa due to the velocity of the air past the valve. This is the same for piston and diaphragm valves. Poppet is the exception.
at low rpm and on a long term trailing throttle with no turbine speed, all valves close as good as instantly when you stab the throttle. at that point, either they can hold pressure or they cant, the oem style one will be holding it better at that point. dual piston IF the pistons were different sizes could do just the same thing as oem. Holding it better? Closed is closed… but not really. Look at that dyno I posted… single piston versus OEM. Single piston wins. You are working with theory. Theory has to be applied correctly to survive reality.
it would be fair to say that you could "fix" the problem by either increasing flow by upsizing or duplicating, OR by increasing spring pressure, however the larger valve with the lower resistance to opening (pulled strongly there by the vacuum on the larger diaphragm:valve ratio rather than pushed open by intake pressure) will offer less resistance to flow and increased performance as far as bovs go (which isnt much, they really just have to work or not) No. The dyno again. Valves opening easily is not always good. So, yes, flow has been increased, but you’ve added a second leak to the system… not good.
Quote:
the piston ones on the other hand rely on the spring pressure to seal properly and close again, Actually they rely on manifold pressure to close, spring tension is just there to make up for a pressure differential on boost onset and for pressure drop due to intercooler heatsoak, latency etc. This is the case with pretty much all valves, diaphragm or piston.
a single piston BOV CAN NOT rely on manifold pressure to close fully, if it had virtually no spring pressure, at the point when it was nearly closed, there would be equal and opposite force EXCEPT the velocity of the air rushing out holding it open, and it would never close. if it started closed, and there was so much as a slight restriction of air from the front of it to the back, it would blow open all the time. on the contrary, if you took the spring all but out of a diaphragm one, it would still close and stay there securely because even if there was slight restriction to the flow to the back side of it, the slight restriction through the release path would cause intake pressure to rise just a little bit, and once it did, the force closing it would outweigh that opening it even at very low levels of intake tract pressure. No difference. Except in poppet. Which can be either piston or diaphragm.
in reality BOTH designs rely upon a spring bias to ensure that they close at all, however, the spring pressure required with a diaphragm style one is by design and definition lighter than a single piston unit, leading to the lower restriction that i initially stated here : Sometimes this is true. Take two equivalent piston and diaphragm based valves, same surface areas for both, same port sizes, same size pressure chamber, same travel etc. And it will come down to which assembly has more mass to determine which needs the stiffer spring (also friction for the piston… which is why it is usually the piston style with the higher spring rate. But in many diaphragm setups there are components added to reinforce the diapgragm to prevent failure that makes them much heavier than a piston).
Quote:
so it must be heavier, and by definition, the piston is the same area at both ends, You will find this to be the case for most diaphragm valves as well, until closed at which point surface area available to the charge air is determined by the size of the pressure port on the valve, which is the same in piston valves.
i beg to differ, the silvia one is the obvious example, the diameter of the diaphragm is about twice that of the poppet valve that lets the air out. twice. the gtr ones, the diameter is roughly 30% bigger. for the silvia thats a 4:1 ratio of closed to open pressure, and vice versa for vacuum. Poppet again is the exception. Do you have a cut-away? Once sealed ALL valves that I know of have a smaller pressure surface area… Once openeded does charge air act on the bottom of the Silva valve or is the diaphragm in a separate chamber with a stem connecting it to the poppet? If so (and that is the only way that it would retain that ratio) than it wouldn’t matter if they had used a piston in that top chamber… that is why I say poppet vavles can’t be used to substantiate the virtues of either.
all single pistons by definition have equal area at each end. Until sealed….
Quote:
so the vacuum has a hard time pulling against the spring by itself > enter back pressure, the pressure from the incoming air is then relied upon to force open the bov Working in conjunction with vacuum, again, this is the case with nearly all valves. and by doing that, you are defeating the purpose somewhat as for both safety and performance things are less than optimal.
the piston type ones came about to allow a vented bov to exist with an afm. This would be why twin piston units were developed, piston units exist to address reliability issues with diaphragm units.
twin pistons units with a larger piston exposed to the plenum kpa work exactly the same as the oem diaphragm ones. i have no argument against these whatsoever. Except that most OEM diaphragm valves are the same as single piston vavles… which as you said have equal area at each end.
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thus its biased strongly enough with the spring to remain closed at idle. Relying on spring tension to keep a valve closed at idle is usually one of two things: first is when people are using a valve not meant to be closed at idle on an AFM system so they crank up the spring tension to try and make it work,
lets keep the idiots out of the discussion and stick to discussing valves used as designed. I was trying to help you
single piston : vacuum shares the job with pressure against the valve
Vacuum alone will open even our stiffest spring. There is no difference in principle between the two. And I imagine that the majority of OEM valves are simple single diaphragm units which are no different than piston valves. The only OEM valves that I’m aware of that have a design that allows surface areas to differ are poppet style, and many of those incorporate some means of slowing response, to offset how easily they open.
the latter is far less good for flow and residual pressure for the compressor to have to fight with no driving force. ie, the latter causes the turbo to slow down more and faster.
Actually when logging pressure in the intake tract all of the different designs operate almost identically (assuming an adequate design) once the throttle body is closed: a small 1-4 psi spike than boost tapering off until the TB is opened and the manifold is pressurized again… Unless the valve is very under-sized or the spring is overly stiff. Then you get a larger spike, maybe fluttering, maybe surge… but that is beyond what we are discussing.
valve = poppet valve, and just like the wastegate valve is MUCH smaller than the diaphragm above it, so too is the rubber lined poppet valve in a typical oem bov/bpv application MUCH smaller (in area, thanks for clarifying) than the diaphragm above it.
I am familiar with them. Your wording made this seem to be an advantage of diaphragm valves. It is not. Diaphragm valves are almost identical functionally to piston valves. Poppets are the exception. But poppet valves can be either piston or diaphragm and can’t be lumped with either category to support your (nor my) claims.
the single piston design on the other hand has the same area exactly at each end and thus relies solely on spring pressure to return closed and seal when under boost. As do diaphragm valves.
usually, the valve in an oem bov/bpv will not be significantly larger than the orifice feeding it with charge air to release.
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In bench racing and on internet forums, the reality of it is that piston valves usually have the advantage in drivability.
i honestly can not conceive of a single reason why either would be better than the other in this respect. provided that they function correctly, preventing surge and releasing excess charge air, what can possibly be different other than the speed of onset of boost? enlighten me :-) Response time. Valves that respond too quickly cause problems with driveability, especially in transient situations. As I mentioned before, many OEM valves incorporate some way to delay response…
no, its not possible for the intake tract to pressurise whilst the valve is still allowing air past. Not true. You can build boost with a vacuum leak. And valves are typically open under vacuum… but some how boost is made. Look around at my posts, you will see a dyno I posted of one of our valves compared against a stock WRX unit… it picked up power over stock in the low rpm ranges because the valve would close sooner. Point being, the car was making boost while the OEM valve was open, it was just less than with ours. meaning that the back side of the diaphragm sees 100kpa, and the front side of the valve sees 100kpa air thats moving rapidly towards it, and the spring pressure sees that energy carried though inertia in the moving air. 100kpa is 100kpa. What you are saying makes no sense. What you are describing would be pressure higher than 100kpa… I think. at the that time, the spring is doing the majority of the work because the front side of the diaphragm is also seeing 100kpa, and the back side of the valve is seeing less than 100kpa due to the velocity of the air past the valve. This is the same for piston and diaphragm valves. Poppet is the exception.
at low rpm and on a long term trailing throttle with no turbine speed, all valves close as good as instantly when you stab the throttle. at that point, either they can hold pressure or they cant, the oem style one will be holding it better at that point. dual piston IF the pistons were different sizes could do just the same thing as oem. Holding it better? Closed is closed… but not really. Look at that dyno I posted… single piston versus OEM. Single piston wins. You are working with theory. Theory has to be applied correctly to survive reality.
it would be fair to say that you could "fix" the problem by either increasing flow by upsizing or duplicating, OR by increasing spring pressure, however the larger valve with the lower resistance to opening (pulled strongly there by the vacuum on the larger diaphragm:valve ratio rather than pushed open by intake pressure) will offer less resistance to flow and increased performance as far as bovs go (which isnt much, they really just have to work or not) No. The dyno again. Valves opening easily is not always good. So, yes, flow has been increased, but you’ve added a second leak to the system… not good.
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the piston ones on the other hand rely on the spring pressure to seal properly and close again, Actually they rely on manifold pressure to close, spring tension is just there to make up for a pressure differential on boost onset and for pressure drop due to intercooler heatsoak, latency etc. This is the case with pretty much all valves, diaphragm or piston.
a single piston BOV CAN NOT rely on manifold pressure to close fully, if it had virtually no spring pressure, at the point when it was nearly closed, there would be equal and opposite force EXCEPT the velocity of the air rushing out holding it open, and it would never close. if it started closed, and there was so much as a slight restriction of air from the front of it to the back, it would blow open all the time. on the contrary, if you took the spring all but out of a diaphragm one, it would still close and stay there securely because even if there was slight restriction to the flow to the back side of it, the slight restriction through the release path would cause intake pressure to rise just a little bit, and once it did, the force closing it would outweigh that opening it even at very low levels of intake tract pressure. No difference. Except in poppet. Which can be either piston or diaphragm.
in reality BOTH designs rely upon a spring bias to ensure that they close at all, however, the spring pressure required with a diaphragm style one is by design and definition lighter than a single piston unit, leading to the lower restriction that i initially stated here : Sometimes this is true. Take two equivalent piston and diaphragm based valves, same surface areas for both, same port sizes, same size pressure chamber, same travel etc. And it will come down to which assembly has more mass to determine which needs the stiffer spring (also friction for the piston… which is why it is usually the piston style with the higher spring rate. But in many diaphragm setups there are components added to reinforce the diapgragm to prevent failure that makes them much heavier than a piston).
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so it must be heavier, and by definition, the piston is the same area at both ends, You will find this to be the case for most diaphragm valves as well, until closed at which point surface area available to the charge air is determined by the size of the pressure port on the valve, which is the same in piston valves.
i beg to differ, the silvia one is the obvious example, the diameter of the diaphragm is about twice that of the poppet valve that lets the air out. twice. the gtr ones, the diameter is roughly 30% bigger. for the silvia thats a 4:1 ratio of closed to open pressure, and vice versa for vacuum. Poppet again is the exception. Do you have a cut-away? Once sealed ALL valves that I know of have a smaller pressure surface area… Once openeded does charge air act on the bottom of the Silva valve or is the diaphragm in a separate chamber with a stem connecting it to the poppet? If so (and that is the only way that it would retain that ratio) than it wouldn’t matter if they had used a piston in that top chamber… that is why I say poppet vavles can’t be used to substantiate the virtues of either.
all single pistons by definition have equal area at each end. Until sealed….
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so the vacuum has a hard time pulling against the spring by itself > enter back pressure, the pressure from the incoming air is then relied upon to force open the bov Working in conjunction with vacuum, again, this is the case with nearly all valves. and by doing that, you are defeating the purpose somewhat as for both safety and performance things are less than optimal.
the piston type ones came about to allow a vented bov to exist with an afm. This would be why twin piston units were developed, piston units exist to address reliability issues with diaphragm units.
twin pistons units with a larger piston exposed to the plenum kpa work exactly the same as the oem diaphragm ones. i have no argument against these whatsoever. Except that most OEM diaphragm valves are the same as single piston vavles… which as you said have equal area at each end.
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thus its biased strongly enough with the spring to remain closed at idle. Relying on spring tension to keep a valve closed at idle is usually one of two things: first is when people are using a valve not meant to be closed at idle on an AFM system so they crank up the spring tension to try and make it work,
lets keep the idiots out of the discussion and stick to discussing valves used as designed. I was trying to help you
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11-29-2007, 11:50 AM
#92
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go back to the beginning of the post, read it all again.
this time, when you read "diaphragm" realise that i mean the one above the poppet valve below it, not anything else.
poppet with actuating diaphragm
VS.
single piston
is the discussion as far as i intended the initial comments, and as i have written everything thereafter
bump the thread when you have edited your post to reflect the difference in comprehension based on the above assumption.
i failed apparently at making it clear enough.
take two...
fred.
this time, when you read "diaphragm" realise that i mean the one above the poppet valve below it, not anything else.
poppet with actuating diaphragm
VS.
single piston
is the discussion as far as i intended the initial comments, and as i have written everything thereafter
bump the thread when you have edited your post to reflect the difference in comprehension based on the above assumption.
i failed apparently at making it clear enough.
take two...
fred.
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11-29-2007, 12:13 PM
#94
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Ok, fair enough. I did go back and look through your posts btw. hard luck with the crossed up ms wiring :-( good that you got it going in the end though! :-)
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11-29-2007, 12:57 PM
#95
Did you find the dyno?
Poppet valves are a different animal... so reading your post thinking you meant a common diaphragm valve made you seem very wrong... but in the end it still comes down to theory and execution.
We have a single piston valve called the RS that can be configured with a GReddy style flange. People are starting to use these to replace their GReddy RS valves (especially with the VR6 turbo kits), which is a poppet design, and they absolutely love it. In theory the GReddy design is superior but our execution of a more simplistic design makes for a better valve.
Poppet valves are a different animal... so reading your post thinking you meant a common diaphragm valve made you seem very wrong... but in the end it still comes down to theory and execution.
We have a single piston valve called the RS that can be configured with a GReddy style flange. People are starting to use these to replace their GReddy RS valves (especially with the VR6 turbo kits), which is a poppet design, and they absolutely love it. In theory the GReddy design is superior but our execution of a more simplistic design makes for a better valve.
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11-29-2007, 02:08 PM
#96
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No, I didnt find the dyno, but i did find other information.
basically in many ways i WAS totally wrong, but in others i wasnt.
i'm writing a post to admit it like a man and explain now.
damn it, why didnt you stop me sooner??? lol
woops.
if you care to link the graph i'm interested to see it anyway.
fred.
basically in many ways i WAS totally wrong, but in others i wasnt.
i'm writing a post to admit it like a man and explain now.
damn it, why didnt you stop me sooner??? lol
woops.
if you care to link the graph i'm interested to see it anyway.
fred.
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11-29-2007, 02:24 PM
#97
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apparently you guys dont have a blushing smiley, so i stole that from another board...
This doesnt happen tooooooo often, but I have indeed made a fool of myself in grand style.
they say the trouble with people isnt what they dont know, but rather what they think they know that isnt true.
because i am a man and not a wimp, i'll admit it in equally grand style :-p
i was assuming (incorrectly) that the back of the diaphagm (above the poppet :-p) was open to atmos.
I am unsure about the gtr valves, but, it would seem that all the other poppet style oem valves have a port open to boost pressure that feeds the back side of the diaphragm above. this causes them to open earlier, but requires a stronger spring than the piston style to stop them opening under boost because effectively the top diaphragm does a grand total of zero once boost is applied to both sides and the back side of the poppet only sees 100kpa. there are modifications floating around to block that port and vent it to atmos which makes the valve operate exactly how i described, but apparently the oem springs are too stiff for part throttle driving and cause surge in that situation when used like that.
there is a kit that switches the back side pressure between boost and atmo depending on manifold vacuum, but mixed results are reported.
the really annoying thing about being this wrong is that i actually knew about that port, but had forgotten.
it would appear that at higher boost levels a piston one would be better though, quite apparently i am too tired to think clearly about it right now. i'll be thinking about this some more and will post if i come up with anything further.
i'll stand by what i said about the sizing though. bigger definitely = better, but with negligible real world benefit i guess.
fred.
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11-29-2007, 03:42 PM
#98
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https://www.miataturbo.net/showthrea...195#post170195
the dyno.
interesting. also interesting is that it doesnt aid spool at all. i would have expected that it would. i guess the compressor is spinning at much the same speed and the engine just isnt seeing all of it.
the difference from where the boost curves become one and up must be increased back pressure in the exhaust needed to provide boost and leak.
fred.
the dyno.
interesting. also interesting is that it doesnt aid spool at all. i would have expected that it would. i guess the compressor is spinning at much the same speed and the engine just isnt seeing all of it.
the difference from where the boost curves become one and up must be increased back pressure in the exhaust needed to provide boost and leak.
fred.
Last edited by rb26dett; 11-29-2007 at 04:34 PM. Reason: add word "one" in after become.
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11-29-2007, 04:23 PM
#99
It helps spool. At every RPM point up until ~3800rpm the car is making more boost (note the two green lines...). That is where the power is coming from (maybe some is coming from increased compressor efficiency cuz a leak has been eliminated and the turbo is spinning slower... but that is just conjecture).
Back pressure has nothing to do with it.
Back pressure has nothing to do with it.
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11-29-2007, 04:40 PM
#100
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oh boy, you and me just dont communicate well do we.
two green lines were noted and commented on. i missed out one word "one"
back pressure behind the turbine = poor compressor efficiency = spinning faster to produce the same pressure because its flowing more because some is leaking out through the valve.
its conjecture to say that it helps spool, especially when the graph shows that full boost is reached at EXACTLY the same rpm, that is what spool is. ie, rpm target boost is reached. it could easily just be that with full exhaust flow and the wastegate closed pre full boost that the compressor speed generates a certain flow, and if some of that flow is lost, then the pressure is lower given that the appetite for air from the engine is exactly the same.
i have to ask, exactly which back pressure were you talking about??
fred.
two green lines were noted and commented on. i missed out one word "one"
the difference from where the boost curves become one and up must be increased back pressure in the exhaust needed to provide boost and leak.
its conjecture to say that it helps spool, especially when the graph shows that full boost is reached at EXACTLY the same rpm, that is what spool is. ie, rpm target boost is reached. it could easily just be that with full exhaust flow and the wastegate closed pre full boost that the compressor speed generates a certain flow, and if some of that flow is lost, then the pressure is lower given that the appetite for air from the engine is exactly the same.
i have to ask, exactly which back pressure were you talking about??
fred.
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