Everything you ever wanted to know about DV/BOV, etc.
07-24-2007, 01:38 PM
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Everything you ever wanted to know about DV/BOV, etc.
From Forge rep:
Mike@Forge
I'll try to be as clear as possible in my explanation of things and I have simplified a few areas for ease of understanding, but I will gladly elaborate if necessary. The points I touch on will be those that are most commonly brought up on these and other forums, but other relevant information is included for everyone to consider.
The basics.
What a Diverter/Blow-Off/Bypass Valve is and what it Does:
There are numerous names given to this part, and it should be understood that they are all interchangeable terms used in different ways by different people all to describe the same basic thing.
Diverter Valve
Blow-Off Valve
Dump Valve
(Compressor) Bypass Valve
Pop-Off Valve
Discharge Valve
Boost Valve
Hooter Valve (Yes, I have even seen people call them "hooter" valves
)
Etc.
Whether the valve is venting or recirculating, it is still a “bypass valve”, and since this term gives no implication to recirculating or venting function, it is the best term to use when talking about all of these valves in general terminology.
Basically, a bypass valve exists to relieve a residual amount of boost pressure in a pressurized application when the throttle on the application is abruptly closed preventing the air from “backing up” into the compressor wheel of the turbo, slowing it down, thus creating “lag” when the throttle is reapplied.
Whether the air is vented or recirculated makes NO difference to the amount of “lag” created.
"Lag" would only be created if there was no bypass valve in place at all, and the residual charge air inside the intercooler piping "backed up" into the compressor wheel at throttle lift. Whether an atmospheric or a recirculating valve is used, the valve is still able to bypass the residual charge pressure at throttle lift allowing the compressor wheel to maintain it's rate of speed, thus reducing “lag”.
The OEM Valves on the Evo's:
While the OEM valves on the Evo's, both the black plastic and the metal JDM/MR/9 units, are fine and dandy for certain applications and performance needs, they are not necessarily the best solution for every application.
I appreciate the views of those who like the OEM valves, their performance, and operation, and that they recommend the JDM/MR/9 valve as an upgrade for those whose cars still have the black plastic valve, but even then, there are some faults to be found.
Both of the OEM valves are a multi-chambered design, and while the design works well under certain scenarios, the valves typically will not hold boost levels as high as other aftermarket alternatives. When looking at the OEM valves, take note of the channel/vein running up the back side of the valve. This channel is used to help equalize the pressure differential on either side of the valve's diaphragm under partial throttle/partial boost conditions. (more on this later) This equalization of pressure helps give the valve very smooth response and operation.
The limiting factor of the OEM valves, however, is boost holding capacity. As mentioned above, a valve's ability to hold boost is not solely a function of spring pressure. The small vacuum line sourced from the intake manifold to the valve provides a positive pressure reference to the valve under positive pressure/boost conditions to aid in the valve's ability to maintain a seal and retain boost in the system until throttle lift. While other factors such as surface areas also apply, when the boost pressure in the intercooler piping surpasses the holding capacity of the spring and pressure reference combined, the valve will begin to leak/bleed off boost pressure causing a loss of an indeterminable amount of power dependant upon the size of the leak.
The OEM black plastic valve has been tested to hold roughly 18-19 PSI reliably. This figure may vary from valve to valve and car to car, but this is a good estimate to go by.
The metal JDM/MR/9 valve has been tested to hold roughly 21-22 PSI reliably, but really no more. Again, these figures may vary slightly from your car, but these are best estimates for the sake of argument.
Again, this, in no way means that these valves are insufficient at holding boost under those circumstances, but as the vast majority of Evo owners undertake modifications to boost their cars beyond these levels, the valves are then lacking compared to other manufacturer's valves in their ability to hold boost.
Aftermarket Valves:
Options abound. That's all really.
There are so many choices available on the market, it's perfectly understandable how people get confused, so let's be sure to take it easy on the "newbies" to the turbo world who need some guidance. We were all there once too.
With that said, valve selection IS a very important thing and should not be taken lightly. Valve selection should be made considering a number of different factors each of which will be specific to the application it will be used on and the use the valve will see.
Design
Boost Holding Capacity
Flow Volume
Adjustment Range
Quality
Reliability
After-Sale Support
Etc.
One VERY important factor is atmospheric vs. recirculating, and while I will elaborate on this more in depth below, if anyone is not comfortable making the decision themselves, please consult with a reputable and trusted valve manufacturer or your vehicle's tuner before making your choice.
ATMOSPHERIC vs. RECIRCULATING vs. 50/50:
The BIG question.
The simple answer:
What kind of valve is your engine management system designed for and/or capable of allowing for the use of?
Yes, I answered a question with another question, but the real answer is, there is no simple answer. Each person will need to consider what their plans for their car are, what type of engine management system they will ultimately be using, and what valve can or can't be used with that type of system. “Blow-off valve sound” aside.
Firstly, we need to know what type of engine management can use what type of valve.
Mass AirFlow systems are designed as "closed-loop" systems requiring the use of a recirculating valve.
Speed Density systems are typically setup as closed-loop systems from the factory, but they can typically use either a recirculating OR atmospheric valve without any major detriment to the system.
MAP based systems (manifold absolute pressure) are typically capable of allowing for the use of either valve as well, but most MAP-based systems are fully stand-alone and require significant tuning to overcome any changes.
Now, with that said, the OEM ECU used on the Evo is a Mass Airflow system. It requires the use of a recirculating valve. If anyone, through the course of modifying their car, plans to continue to use the OEM ECU as the base for their engine management (even if some aftermarket tuner has uploaded new programming or a piggyback system is used), it is HIGHLY recommended to continue to use a recirculating valve.
The use of an atmospheric valve will cause a rich fuel mixture due to the loss of already metered air which the ECU is expecting to remain in the system. When the air is vented, the ECU dumps fuel into the system expecting the air to be there, and it's not. This rich fuel condition can sometimes be severe enough that the ECU cannot compensate for the condition and the car will run rough, not idle properly, experience a loss of power, and also experience poor fuel economy.
I have personally seen, on my own vehicle, a loss of as much as roughly 40 miles per full tank of fuel from using an atmospheric valve on a tuned, but otherwise stock engine management system.
If anyone, through the course of modifying their car, plans to switch their engine management from the OEM ECU to some form of standalone engine management (NOT a piggyback system, as even a piggyback still uses the OEM ECU), they can then, and only then, consider the use of an atmospheric valve, as such a system can compensate for, or be tuned to allow for the use of such a valve without any of the problems mentioned above.
50/50 valves, while seemingly great, are not an ideal solution by any measure, in my personal opinion.
(Yes, everything below is a personal opinion and subject to argument, but I’d like to think I know what I’m talking about.)
While they are designed to accomodate those with the desire for an increase in the "blow-off valve sound" from their car, they do so in a way that is still venting metered air, still causing a somewhat richer fuel condition, and still potentially leading to the above mentioned problems. While they may appear to work on any given vehicle, they are only doing so within a window that is not yet necessarily a largely detrimental problem to the OEM ECU at that time and it's ability to alow for the venting of metered air.
The difficulty lies in that there is no precise way to measure, at least cost effectively for aftermarket companies anyway, the PRECISE amount of air that can "safely" be vented out of the system and not cause a problem for the ECU, in whatever it's current state of tune may be, and it's ability to correctly add fuel to the system in the appropriate ratio. There are innumerable factors to consider that will never be perfectly "tuned" in a single 50/50 valve design to suit all applications.
50/50 valves are essentially trying to "trick" the ECU into believing that enough air is still being recirculated to maintain a proper air/fuel ratio under any given load condition, when, in fact, there is no way to effectively know what ratio should be used, if any at all.
The safe bet is to just use a recirulating valve where recommended and enjoy the security that you know that your ECU is not struggling to maintain a proper air/fuel ratio.
If the added sound is REALLY the most important thing to you, however, you must be willing to accept a level of risk that some problems MAY occur.
While it may be nice to have that blow-off valve (whooshing) sound, I personally feel that it's significantly more important to have a valve that will perform and operate properly for the given application regadless of the amount of noise it makes. I do not feel that valves should be designed to make a particular sound, nor to trick their engine management system, whether factory or aftermarket, into thinking the car is operating properly. Valves are meant to perform a specific function that should be done in a manner best suiting the specific application.
Valve Tuning:
Once you have made your valve choice, regardless of which manufacturer’s valve you select, the same basic tuning principles will apply.
If you are finding that your valve is dumping more air than required, venting too soon, or it is leaking boost before the redline of your application is reached, it is normally an indication that the valve is adjusted or tuned too softly and that a stronger spring or more spring tension is required. Conversely if the valve is failing to dump boost pressure, or you are experiencing valve fluttering at full boost throttle lift, it may be necessary to install a weaker spring or tune the valve to a softer setting.
If you have a valve that uses different springs for tuning and, during the tuning of your valve, you are faced, for example, with a given spring being to strong and another spring being too weak, you can add spacers/shims/washer (which are typically included) to the softer spring to increase the tension by small increments, thus achieving a setting between the two springs.
If your valve uses some sort of adjustment ****, screw, or bolt at the top used to adjust the amount of pre-load on the spring to increase or decrease its tension and, subsequently, its boost holding capacity, unfortunately, there are other things, you must consider first, so read carefully.
First, you must consider what type of spring is used in the valve.
Does it use a cylindrical spring in the shape a can of food, or a conical spring in the shape of a funnel?
A cylindrical spring is the most commonly used type of spring, and while great for most applications, has limited tenability. Cylindrical springs can only be compressed to a certain point to which all of it’s coils are stacked; resting on top of one another. At this point, the spring has reached its full range of travel and can compress no further. In tuning a valve with a cylindrical spring, adding pre-load through an adjustment **** or adding spacer/shims/washers must be done carefully so as not to add to much pre-load that the valve’s piston then has limited travel. Limited travel can result in limited airflow which can inhibit the valve’s ability to perform optimally for the application.
A conical spring offers a much larger range of adjustability than a cylindrical spring within a given valve. A conical spring of the same height as a given cylindrical spring can be compressed much further as each smaller spring coil will rest inside the inner diameter of the next largest coil, preventing a “stacking” effect of the coils. This typically means that more pre-load can be added without limiting piston travel allowing for unobstructed airflow at all pre-load levels.
Also, consider how the spring used in the valve of your choice is manufactured, regardless of which type it is. Cold-winding of springs is a practice used to ensure the strength of the spring and to prevent any relaxing or sagging of any individual coils as the spring is heat cycled through daily usage. This will ensure that the spring retains its rate and boost holding capacity for an extended period of time. Not all manufacturers use cold-winding techniques, so keep that in mind.
With these things in mind, tuning of the valve can commence with caution given to how much pre-load is added, and an understanding that while there may appear to be more adjustment available from the ****, screw, or bolt, the amount of pre-load may already be the maximum allowable amount to not inhibit piston travel, and subsequently airflow volume.
Valve Fluttering:
Valve fluttering is commonly thought to be an automatic indicator of compressor surging, and I would like to put that rumor to rest right now. This could not be further from the truth.
Compressor surging (caused by a bypass valve) implies that the bypass valve is not opening fully to allow the optimal amount of airflow required to keep the compressor wheel of the turbo spinning at an optimal speed.
Bypass valve fluttering will occur under various circumstances, so please consider under what situations you are experiencing fluttering before you presume that compressor surging is taking place, or more importantly, before it is assumed that a problem even exists.
Valve fluttering under wide open throttle or full boost throttle lift, again, typically means that a valve is tuned or adjusted to stiffly, and while this can lead to compressor surging and potential damage over an extended period of time, if the user fixes the issue quickly, no significant damage will occur. It would only be after prolonged use of a valve in an improperly tuned configuration that compressor surging MAY lead to damage or excessive wear on the turbocharger.
Valve fluttering under partial throttle or partial boost throttle lift, on the other hand, is a completely normal occurrence and IS NOT an indication of compressor surging by any measure.
Partial throttle or partial boost valve fluttering is solely an indication that the valve is directly responding to the inconsistent pressure differential on either side of the throttle plate (throttle body).
An internal combustion engine naturally creates a vacuum effect during the intake stroke of a given cylinder. When boost pressure is built from the turbocharger, it will reach a certain level inside the intercooler piping, but as it enters the intake manifold, it is almost instantly reduced by a given amount of vacuum created by the intake stroke of the engine, thus resulting in a marginally lesser amount of boost pressure inside the intake manifold compared to inside the intercooler piping.
Since the bypass valve sees references from both of these pressure sources, the sealing surface of the valve, be it a diaphragm or a piston, will respond to these differences in pressure, as minor or severe as they may be. This sealing surface response is what is creating the fluttering effect at partial throttle or partial boost throttle lift.
It may be more pronounced on some applications than others, and as mentioned above, the OEM Evo valves incorporates a feature to combat this issue, and while it will always be present to a small extent, it is not a problem for the vehicle in any way whatsoever.
Mike@Forge
Jul 3, 2006, 11:33 PM
Bypass Valves and Boost Controllers:
The bypass valve of any forced induction application must always have a intake manifold pressure reference. You should NEVER reference the bypass valve from the turbo outlet nipple nor from the intercooler piping at any point before the throttle body.
With that said, if the boost controller and bypass valve share the same reference from the intake manifold, you can figure that they are essentially sharing the same volume of air (pressure/vacuum).
When the boost controller (lock-ball-and-spring type) actuates, meaning when the spring allows the ball to come off of it's seat and send the pressure signal to the actuator or external wastegate, some of the "shared" pressure helping to hold the bypass/blow-off valve closed under boost is now momentarily diverted to the actuator or wastegate potentially causing a "flutter" in the seal surface of the bypass valve and a slight loss of boost pressure altogether within the system.
When the boost controller and bypass valve each have their own individual reference from the intake manifold, the possibility for this flutter is greatly decreased, however, it may still exist to some extent because both references are still sharing a given volume of intake manifold pressure.
It is always highly recommend to ensure that any boost controller is referenced from the turbo outlet nipple where available, or from the intercooler piping before the throttle body if no turbo outlet nipple exists.
This is also important in that the boost controller only ever needs to see a positive pressure reference, and never vacuum.
Boost Controller
Positive Pressure Only
(Turbo Outlet)
Bypass/Blow-Off Valve
Positive Pressure AND Vacuum
(Intake Manifold)
Mike@Forge
I'll try to be as clear as possible in my explanation of things and I have simplified a few areas for ease of understanding, but I will gladly elaborate if necessary. The points I touch on will be those that are most commonly brought up on these and other forums, but other relevant information is included for everyone to consider.
The basics.
What a Diverter/Blow-Off/Bypass Valve is and what it Does:
There are numerous names given to this part, and it should be understood that they are all interchangeable terms used in different ways by different people all to describe the same basic thing.
Diverter Valve
Blow-Off Valve
Dump Valve
(Compressor) Bypass Valve
Pop-Off Valve
Discharge Valve
Boost Valve
Hooter Valve (Yes, I have even seen people call them "hooter" valves
)Etc.
Whether the valve is venting or recirculating, it is still a “bypass valve”, and since this term gives no implication to recirculating or venting function, it is the best term to use when talking about all of these valves in general terminology.
Basically, a bypass valve exists to relieve a residual amount of boost pressure in a pressurized application when the throttle on the application is abruptly closed preventing the air from “backing up” into the compressor wheel of the turbo, slowing it down, thus creating “lag” when the throttle is reapplied.
Whether the air is vented or recirculated makes NO difference to the amount of “lag” created.
"Lag" would only be created if there was no bypass valve in place at all, and the residual charge air inside the intercooler piping "backed up" into the compressor wheel at throttle lift. Whether an atmospheric or a recirculating valve is used, the valve is still able to bypass the residual charge pressure at throttle lift allowing the compressor wheel to maintain it's rate of speed, thus reducing “lag”.
The OEM Valves on the Evo's:
While the OEM valves on the Evo's, both the black plastic and the metal JDM/MR/9 units, are fine and dandy for certain applications and performance needs, they are not necessarily the best solution for every application.
I appreciate the views of those who like the OEM valves, their performance, and operation, and that they recommend the JDM/MR/9 valve as an upgrade for those whose cars still have the black plastic valve, but even then, there are some faults to be found.
Both of the OEM valves are a multi-chambered design, and while the design works well under certain scenarios, the valves typically will not hold boost levels as high as other aftermarket alternatives. When looking at the OEM valves, take note of the channel/vein running up the back side of the valve. This channel is used to help equalize the pressure differential on either side of the valve's diaphragm under partial throttle/partial boost conditions. (more on this later) This equalization of pressure helps give the valve very smooth response and operation.
The limiting factor of the OEM valves, however, is boost holding capacity. As mentioned above, a valve's ability to hold boost is not solely a function of spring pressure. The small vacuum line sourced from the intake manifold to the valve provides a positive pressure reference to the valve under positive pressure/boost conditions to aid in the valve's ability to maintain a seal and retain boost in the system until throttle lift. While other factors such as surface areas also apply, when the boost pressure in the intercooler piping surpasses the holding capacity of the spring and pressure reference combined, the valve will begin to leak/bleed off boost pressure causing a loss of an indeterminable amount of power dependant upon the size of the leak.
The OEM black plastic valve has been tested to hold roughly 18-19 PSI reliably. This figure may vary from valve to valve and car to car, but this is a good estimate to go by.
The metal JDM/MR/9 valve has been tested to hold roughly 21-22 PSI reliably, but really no more. Again, these figures may vary slightly from your car, but these are best estimates for the sake of argument.
Again, this, in no way means that these valves are insufficient at holding boost under those circumstances, but as the vast majority of Evo owners undertake modifications to boost their cars beyond these levels, the valves are then lacking compared to other manufacturer's valves in their ability to hold boost.
Aftermarket Valves:
Options abound. That's all really.
There are so many choices available on the market, it's perfectly understandable how people get confused, so let's be sure to take it easy on the "newbies" to the turbo world who need some guidance. We were all there once too.
With that said, valve selection IS a very important thing and should not be taken lightly. Valve selection should be made considering a number of different factors each of which will be specific to the application it will be used on and the use the valve will see.
Design
Boost Holding Capacity
Flow Volume
Adjustment Range
Quality
Reliability
After-Sale Support
Etc.
One VERY important factor is atmospheric vs. recirculating, and while I will elaborate on this more in depth below, if anyone is not comfortable making the decision themselves, please consult with a reputable and trusted valve manufacturer or your vehicle's tuner before making your choice.
ATMOSPHERIC vs. RECIRCULATING vs. 50/50:
The BIG question.
The simple answer:
What kind of valve is your engine management system designed for and/or capable of allowing for the use of?
Yes, I answered a question with another question, but the real answer is, there is no simple answer. Each person will need to consider what their plans for their car are, what type of engine management system they will ultimately be using, and what valve can or can't be used with that type of system. “Blow-off valve sound” aside.
Firstly, we need to know what type of engine management can use what type of valve.
Mass AirFlow systems are designed as "closed-loop" systems requiring the use of a recirculating valve.
Speed Density systems are typically setup as closed-loop systems from the factory, but they can typically use either a recirculating OR atmospheric valve without any major detriment to the system.
MAP based systems (manifold absolute pressure) are typically capable of allowing for the use of either valve as well, but most MAP-based systems are fully stand-alone and require significant tuning to overcome any changes.
Now, with that said, the OEM ECU used on the Evo is a Mass Airflow system. It requires the use of a recirculating valve. If anyone, through the course of modifying their car, plans to continue to use the OEM ECU as the base for their engine management (even if some aftermarket tuner has uploaded new programming or a piggyback system is used), it is HIGHLY recommended to continue to use a recirculating valve.
The use of an atmospheric valve will cause a rich fuel mixture due to the loss of already metered air which the ECU is expecting to remain in the system. When the air is vented, the ECU dumps fuel into the system expecting the air to be there, and it's not. This rich fuel condition can sometimes be severe enough that the ECU cannot compensate for the condition and the car will run rough, not idle properly, experience a loss of power, and also experience poor fuel economy.
I have personally seen, on my own vehicle, a loss of as much as roughly 40 miles per full tank of fuel from using an atmospheric valve on a tuned, but otherwise stock engine management system.
If anyone, through the course of modifying their car, plans to switch their engine management from the OEM ECU to some form of standalone engine management (NOT a piggyback system, as even a piggyback still uses the OEM ECU), they can then, and only then, consider the use of an atmospheric valve, as such a system can compensate for, or be tuned to allow for the use of such a valve without any of the problems mentioned above.
50/50 valves, while seemingly great, are not an ideal solution by any measure, in my personal opinion.
(Yes, everything below is a personal opinion and subject to argument, but I’d like to think I know what I’m talking about.)
While they are designed to accomodate those with the desire for an increase in the "blow-off valve sound" from their car, they do so in a way that is still venting metered air, still causing a somewhat richer fuel condition, and still potentially leading to the above mentioned problems. While they may appear to work on any given vehicle, they are only doing so within a window that is not yet necessarily a largely detrimental problem to the OEM ECU at that time and it's ability to alow for the venting of metered air.
The difficulty lies in that there is no precise way to measure, at least cost effectively for aftermarket companies anyway, the PRECISE amount of air that can "safely" be vented out of the system and not cause a problem for the ECU, in whatever it's current state of tune may be, and it's ability to correctly add fuel to the system in the appropriate ratio. There are innumerable factors to consider that will never be perfectly "tuned" in a single 50/50 valve design to suit all applications.
50/50 valves are essentially trying to "trick" the ECU into believing that enough air is still being recirculated to maintain a proper air/fuel ratio under any given load condition, when, in fact, there is no way to effectively know what ratio should be used, if any at all.
The safe bet is to just use a recirulating valve where recommended and enjoy the security that you know that your ECU is not struggling to maintain a proper air/fuel ratio.
If the added sound is REALLY the most important thing to you, however, you must be willing to accept a level of risk that some problems MAY occur.
While it may be nice to have that blow-off valve (whooshing) sound, I personally feel that it's significantly more important to have a valve that will perform and operate properly for the given application regadless of the amount of noise it makes. I do not feel that valves should be designed to make a particular sound, nor to trick their engine management system, whether factory or aftermarket, into thinking the car is operating properly. Valves are meant to perform a specific function that should be done in a manner best suiting the specific application.
Valve Tuning:
Once you have made your valve choice, regardless of which manufacturer’s valve you select, the same basic tuning principles will apply.
If you are finding that your valve is dumping more air than required, venting too soon, or it is leaking boost before the redline of your application is reached, it is normally an indication that the valve is adjusted or tuned too softly and that a stronger spring or more spring tension is required. Conversely if the valve is failing to dump boost pressure, or you are experiencing valve fluttering at full boost throttle lift, it may be necessary to install a weaker spring or tune the valve to a softer setting.
If you have a valve that uses different springs for tuning and, during the tuning of your valve, you are faced, for example, with a given spring being to strong and another spring being too weak, you can add spacers/shims/washer (which are typically included) to the softer spring to increase the tension by small increments, thus achieving a setting between the two springs.
If your valve uses some sort of adjustment ****, screw, or bolt at the top used to adjust the amount of pre-load on the spring to increase or decrease its tension and, subsequently, its boost holding capacity, unfortunately, there are other things, you must consider first, so read carefully.
First, you must consider what type of spring is used in the valve.
Does it use a cylindrical spring in the shape a can of food, or a conical spring in the shape of a funnel?
A cylindrical spring is the most commonly used type of spring, and while great for most applications, has limited tenability. Cylindrical springs can only be compressed to a certain point to which all of it’s coils are stacked; resting on top of one another. At this point, the spring has reached its full range of travel and can compress no further. In tuning a valve with a cylindrical spring, adding pre-load through an adjustment **** or adding spacer/shims/washers must be done carefully so as not to add to much pre-load that the valve’s piston then has limited travel. Limited travel can result in limited airflow which can inhibit the valve’s ability to perform optimally for the application.
A conical spring offers a much larger range of adjustability than a cylindrical spring within a given valve. A conical spring of the same height as a given cylindrical spring can be compressed much further as each smaller spring coil will rest inside the inner diameter of the next largest coil, preventing a “stacking” effect of the coils. This typically means that more pre-load can be added without limiting piston travel allowing for unobstructed airflow at all pre-load levels.
Also, consider how the spring used in the valve of your choice is manufactured, regardless of which type it is. Cold-winding of springs is a practice used to ensure the strength of the spring and to prevent any relaxing or sagging of any individual coils as the spring is heat cycled through daily usage. This will ensure that the spring retains its rate and boost holding capacity for an extended period of time. Not all manufacturers use cold-winding techniques, so keep that in mind.
With these things in mind, tuning of the valve can commence with caution given to how much pre-load is added, and an understanding that while there may appear to be more adjustment available from the ****, screw, or bolt, the amount of pre-load may already be the maximum allowable amount to not inhibit piston travel, and subsequently airflow volume.
Valve Fluttering:
Valve fluttering is commonly thought to be an automatic indicator of compressor surging, and I would like to put that rumor to rest right now. This could not be further from the truth.
Compressor surging (caused by a bypass valve) implies that the bypass valve is not opening fully to allow the optimal amount of airflow required to keep the compressor wheel of the turbo spinning at an optimal speed.
Bypass valve fluttering will occur under various circumstances, so please consider under what situations you are experiencing fluttering before you presume that compressor surging is taking place, or more importantly, before it is assumed that a problem even exists.
Valve fluttering under wide open throttle or full boost throttle lift, again, typically means that a valve is tuned or adjusted to stiffly, and while this can lead to compressor surging and potential damage over an extended period of time, if the user fixes the issue quickly, no significant damage will occur. It would only be after prolonged use of a valve in an improperly tuned configuration that compressor surging MAY lead to damage or excessive wear on the turbocharger.
Valve fluttering under partial throttle or partial boost throttle lift, on the other hand, is a completely normal occurrence and IS NOT an indication of compressor surging by any measure.
Partial throttle or partial boost valve fluttering is solely an indication that the valve is directly responding to the inconsistent pressure differential on either side of the throttle plate (throttle body).
An internal combustion engine naturally creates a vacuum effect during the intake stroke of a given cylinder. When boost pressure is built from the turbocharger, it will reach a certain level inside the intercooler piping, but as it enters the intake manifold, it is almost instantly reduced by a given amount of vacuum created by the intake stroke of the engine, thus resulting in a marginally lesser amount of boost pressure inside the intake manifold compared to inside the intercooler piping.
Since the bypass valve sees references from both of these pressure sources, the sealing surface of the valve, be it a diaphragm or a piston, will respond to these differences in pressure, as minor or severe as they may be. This sealing surface response is what is creating the fluttering effect at partial throttle or partial boost throttle lift.
It may be more pronounced on some applications than others, and as mentioned above, the OEM Evo valves incorporates a feature to combat this issue, and while it will always be present to a small extent, it is not a problem for the vehicle in any way whatsoever.
Mike@Forge
Jul 3, 2006, 11:33 PM
Bypass Valves and Boost Controllers:
The bypass valve of any forced induction application must always have a intake manifold pressure reference. You should NEVER reference the bypass valve from the turbo outlet nipple nor from the intercooler piping at any point before the throttle body.
With that said, if the boost controller and bypass valve share the same reference from the intake manifold, you can figure that they are essentially sharing the same volume of air (pressure/vacuum).
When the boost controller (lock-ball-and-spring type) actuates, meaning when the spring allows the ball to come off of it's seat and send the pressure signal to the actuator or external wastegate, some of the "shared" pressure helping to hold the bypass/blow-off valve closed under boost is now momentarily diverted to the actuator or wastegate potentially causing a "flutter" in the seal surface of the bypass valve and a slight loss of boost pressure altogether within the system.
When the boost controller and bypass valve each have their own individual reference from the intake manifold, the possibility for this flutter is greatly decreased, however, it may still exist to some extent because both references are still sharing a given volume of intake manifold pressure.
It is always highly recommend to ensure that any boost controller is referenced from the turbo outlet nipple where available, or from the intercooler piping before the throttle body if no turbo outlet nipple exists.
This is also important in that the boost controller only ever needs to see a positive pressure reference, and never vacuum.
Boost Controller
Positive Pressure Only
(Turbo Outlet)
Bypass/Blow-Off Valve
Positive Pressure AND Vacuum
(Intake Manifold)
07-24-2007, 02:04 PM
#3
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Wow! Where do I submit my nomination for the longest post ever? To be honest, I didn't have time to read this whole thing...could all be jibberish...but I'm impressed nonetheless! +10 for Toddk!
07-24-2007, 03:17 PM
#4
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It is not Jibberish!
What Todd has written is exactly correct. I have dealt/designed pneumatic logic circuits for a large part of my professional career and he is right on the money!
Todd, specifically I am seeing some issues with my BOV system that I hope you can help me resolve:
I have a TURBO SX "Dome Loaded" Blow off valve (the pressure/vacuum line that connects to the top of the BOV to use another term) with recirculation, i.e. I am returning the blown off air back to the intake downstream of the MAF to perserve A/F ratio to the ECU. Originally I plumbed it so that it had as it's reference the Intake manifold pressure, i.e. downstream of the throttle plate. This I knew intuitvely to be the correct source of pressure since the idea is that as boost builds with the throttle plate open that more load is created on the top of the piston on the BOV and will hold it down even tighter, then as you close the throttle plate when shifting it goes into vacuum and helps open the BOV. I used one of Lausts multiple port manifolds so the BOV had it's own dedicated line (see photo).
The problem that I have found was that the BOV was really blowing off a lot all through the rev band thus wasting pressurized air that was supposed to be making it's way into the combustion chamber. I only have one spring that was in the BOV but tried stacking several washers up inside the piston to increase the pre-load on the piston. This had minimal effect.
Then someone suggested that I change the reference line tap to the turbo hard pipe (instead of from the intake manifold). Intutively this did not make sense since then the reference would never go into vacuum and help open the BOV when you lift on the throttle. But I said what the heck I'll give it a try. This did in fact help the BOV stay closed better through the rev rane and it did relieve when I lifted. I say better because it still is blowing off throughout the rev range just less. So while this is working better it is still not right and I still think that as you said in your summary that the right place to tap the pressure for the dome load is the Intake Manifold not the hard pipe for the reasons stated earlier.
Upon reading your post it is now clear to me that this change, i.e. moving the refernce from the intake manifold to the hard was not the correct approach. My thought is that the problem could be in one of two areas:
1) The piston in the BOV is not properly sized, i.e. it is too small in that the force generated by the boost acting on the differential area of the BOV piston (top side area - bottom side area) is not sufficient to hold the valve closed. Both sides of the piston should in fact be seeing the same pressure under boost conditions, i.e. boost pressure. In other words the top side area needs to be increased relative to the bottom side area so that more force can be generated to hold it closed (he he a new product idea for a guy who can calculate and has a machine shop at his disposal, me for example any one interested... I digress). This however requires a complete redesign of the BOV which is a last resort IF I can make up for it with a band aid...
2) The band aid being to either increase the pre-load on the spring or increasing the spring rate if I run out of travel with the current spring rate (Lee Springs or Century Springs have an entire catalog full of off the shelf springs that can do the job I'm sure). I would prefer to use a low a spring rate as possible that I can generate enough preload with to hold the BOV closed when in boost vs. using a spring with too high of spring rate which could hinder the BOV from blowing off properly when the throttle is lifted and it is supposed to open fully.
Todd, I am assuming that you don't want ANY air blowing off ideally until you lift the throttle and the intake manifold goes into vacuum, at which point you want it to really open up and flow very well so as to minimze any loss of turbo rpm's, correct?
Then, are you familiar with the TUROB SX BOV and how best to approach the fix using this model? Is it a pooprly designed BOV for this application, i.e. is the top vs the bottom area ratio incorrect ? I would think that that would apply to any turbo system that is using boost pressure as the motive force.
By the way the piston relies on a closely lapped surface to provide a VERY close fit between the BOV housing and the piston itself to prevent leakage from the top of the piston to the bottom. The surfaces in mine are very finely polished and in great condition however the housing is made from aluminum and the piston is made from brass and I wonder if as the BOV heats up that the greater coefficient of thermal expansion of the aluminum relative to brass might be causing the housing to walk away from the piston causing a lot of blow by resulting in a loss of force to hold the valve closed? Maybe I could machine a gland and install a small Viton FKM lip seal to eliminate this possibility. What do you think?
Currently I think my approach will be to:
a) Move the BOV reference line tap back to the intake manifold (vs the hard pipe).
b) Increase spring pre-load using washers to see if I can generate enough pre-load to hold the BOV closed until the throttle is lifted.
c) If I cannot generate enough pre-load then move to a spring with a higher spring load so that I can.
Do you agree with this approach or do you have any other comments? They would be greatly appreciated. While I understand the physics I have not tuned BOV's before and lack practical experience. You have both and may be able to save me some time.
Thanks in advance!
Tom
PS I have attached a picture of how my BOV is plumbed in. You can kind of see Laust's manifold (chrome) in the upper left portion of the picture.
Todd, specifically I am seeing some issues with my BOV system that I hope you can help me resolve:
I have a TURBO SX "Dome Loaded" Blow off valve (the pressure/vacuum line that connects to the top of the BOV to use another term) with recirculation, i.e. I am returning the blown off air back to the intake downstream of the MAF to perserve A/F ratio to the ECU. Originally I plumbed it so that it had as it's reference the Intake manifold pressure, i.e. downstream of the throttle plate. This I knew intuitvely to be the correct source of pressure since the idea is that as boost builds with the throttle plate open that more load is created on the top of the piston on the BOV and will hold it down even tighter, then as you close the throttle plate when shifting it goes into vacuum and helps open the BOV. I used one of Lausts multiple port manifolds so the BOV had it's own dedicated line (see photo).
The problem that I have found was that the BOV was really blowing off a lot all through the rev band thus wasting pressurized air that was supposed to be making it's way into the combustion chamber. I only have one spring that was in the BOV but tried stacking several washers up inside the piston to increase the pre-load on the piston. This had minimal effect.
Then someone suggested that I change the reference line tap to the turbo hard pipe (instead of from the intake manifold). Intutively this did not make sense since then the reference would never go into vacuum and help open the BOV when you lift on the throttle. But I said what the heck I'll give it a try. This did in fact help the BOV stay closed better through the rev rane and it did relieve when I lifted. I say better because it still is blowing off throughout the rev range just less. So while this is working better it is still not right and I still think that as you said in your summary that the right place to tap the pressure for the dome load is the Intake Manifold not the hard pipe for the reasons stated earlier.
Upon reading your post it is now clear to me that this change, i.e. moving the refernce from the intake manifold to the hard was not the correct approach. My thought is that the problem could be in one of two areas:
1) The piston in the BOV is not properly sized, i.e. it is too small in that the force generated by the boost acting on the differential area of the BOV piston (top side area - bottom side area) is not sufficient to hold the valve closed. Both sides of the piston should in fact be seeing the same pressure under boost conditions, i.e. boost pressure. In other words the top side area needs to be increased relative to the bottom side area so that more force can be generated to hold it closed (he he a new product idea for a guy who can calculate and has a machine shop at his disposal, me for example any one interested... I digress). This however requires a complete redesign of the BOV which is a last resort IF I can make up for it with a band aid...
2) The band aid being to either increase the pre-load on the spring or increasing the spring rate if I run out of travel with the current spring rate (Lee Springs or Century Springs have an entire catalog full of off the shelf springs that can do the job I'm sure). I would prefer to use a low a spring rate as possible that I can generate enough preload with to hold the BOV closed when in boost vs. using a spring with too high of spring rate which could hinder the BOV from blowing off properly when the throttle is lifted and it is supposed to open fully.
Todd, I am assuming that you don't want ANY air blowing off ideally until you lift the throttle and the intake manifold goes into vacuum, at which point you want it to really open up and flow very well so as to minimze any loss of turbo rpm's, correct?
Then, are you familiar with the TUROB SX BOV and how best to approach the fix using this model? Is it a pooprly designed BOV for this application, i.e. is the top vs the bottom area ratio incorrect ? I would think that that would apply to any turbo system that is using boost pressure as the motive force.
By the way the piston relies on a closely lapped surface to provide a VERY close fit between the BOV housing and the piston itself to prevent leakage from the top of the piston to the bottom. The surfaces in mine are very finely polished and in great condition however the housing is made from aluminum and the piston is made from brass and I wonder if as the BOV heats up that the greater coefficient of thermal expansion of the aluminum relative to brass might be causing the housing to walk away from the piston causing a lot of blow by resulting in a loss of force to hold the valve closed? Maybe I could machine a gland and install a small Viton FKM lip seal to eliminate this possibility. What do you think?
Currently I think my approach will be to:
a) Move the BOV reference line tap back to the intake manifold (vs the hard pipe).
b) Increase spring pre-load using washers to see if I can generate enough pre-load to hold the BOV closed until the throttle is lifted.
c) If I cannot generate enough pre-load then move to a spring with a higher spring load so that I can.
Do you agree with this approach or do you have any other comments? They would be greatly appreciated. While I understand the physics I have not tuned BOV's before and lack practical experience. You have both and may be able to save me some time.
Thanks in advance!
Tom
PS I have attached a picture of how my BOV is plumbed in. You can kind of see Laust's manifold (chrome) in the upper left portion of the picture.
07-24-2007, 03:20 PM
#6
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Sorry guys, but I can not take credit. 
"From Forge rep: Mike@Forge"
Right from the source of DV's.
"From Forge rep: Mike@Forge"
Right from the source of DV's.
07-24-2007, 03:28 PM
#7
very good info
and just in time for my question
i have Valve Fluttering at wide open throttle and around 18 psi
i did´t have it before i installed my maf-kit
i have the 993 dump (brass).
is the spring suddenly to strong?
or is my 993 dump to weak?
and just in time for my question
i have Valve Fluttering at wide open throttle and around 18 psi
i did´t have it before i installed my maf-kit
i have the 993 dump (brass).
is the spring suddenly to strong?
or is my 993 dump to weak?
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07-24-2007, 03:38 PM
#8
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"Todd, I am assuming that you don't want ANY air blowing off ideally until you lift the throttle and the intake manifold goes into vacuum, at which point you want it to really open up and flow very well so as to minimze any loss of turbo rpm's, correct?"
So we need to run an EDV???? Electronic Diverter Valve??
So we need to run an EDV???? Electronic Diverter Valve??
07-24-2007, 03:55 PM
#9
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07-24-2007, 09:49 PM
#10
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Originally Posted by ENGINEERMAN
Then, are you familiar with the TUROB SX BOV and how best to approach the fix using this model? Is it a pooprly designed BOV for this application, i.e. is the top vs the bottom area ratio incorrect ? I would think that that would apply to any turbo system that is using boost pressure as the motive force.
One problem that this DV has it that there are no o-ring seals between the DV body and piston (the Forge unit has the sealing o-rings). This means that not only can boost pressure leak, but vaccuum pressure can also leak when it tries to open the valve. This results in rough on/off boost transitions as the valve doesn't open as much as it should. I experimented with different mounting/amounts of shims, but was never happy with its operation. I got the Forge and noticed an immediate improvement in the car during acceleration and when lifting the throttle.
07-24-2007, 10:52 PM
#11
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Thanks 76 911S!
That's exactly what I wanted to hear. Anyone have a good Forge Motorsports Diverter Valve for sale?! I'm switching because I see exactly the same shortcomings on the design and stiffer springs will not fix the problem.
Thanks again!
Tom
Thanks again!
Tom
07-25-2007, 05:50 AM
#14
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Originally Posted by ENGINEERMAN
That's exactly what I wanted to hear. Anyone have a good Forge Motorsports Diverter Valve for sale?! I'm switching because I see exactly the same shortcomings on the design and stiffer springs will not fix the problem.
Thanks again!
Tom
Thanks again!
Tom
07-25-2007, 01:02 PM
#15
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Well, what prompted me to find that info was that it I just developed a boost leak. No hoses loose or leaks that I can see so I thought maybe the DV. I have the Evo Billit unit. I am only able to make and hold about 8-9psi and then put in some washers/shims and was able to make and hold about 12psi. When I had it apart I checked the o rings and they seem fine and the piston moves freely.
So the leak/issue has to be the DV right?
So the leak/issue has to be the DV right?