diverter valve or blow off valve?

Old 10-01-2006, 01:50 PM
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Default diverter valve or blow off valve?

whats the difference?
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Old 10-04-2006, 04:41 PM
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Both do the same job: release boost pressure. The blow off valve vents the boost from the turbo to the atmosphere, while the DV take it back to the airbox, creating a kind of closed loop system. Some like the "woosh" of the blow off valve, but our cars are design to work with a DV.
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Old 10-30-2006, 09:00 PM
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white fish

use dv or you will run rich = bad
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Old 11-17-2007, 09:10 PM
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Default Re: diverter valve or blow off valve?

can you explain how the car will run rich...since the blow off valves only release the extra boost when the throttle is closed... Just wondering. Dont really get why it would make it run rich.
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Old 12-25-2007, 09:56 AM
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Default Re: diverter valve or blow off valve?

Will the DV add and performace gains or anything becasue it it "closed loop"?
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Old 12-25-2007, 02:16 PM
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Default Re: diverter valve or blow off valve?

as far as i know the main difference between the 2 systems is that the Diverter Valve re-routes charged air into the intake pipe of the turbo, while a regular blow off valve is atmospheric and releases boost out of the system
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Old 12-29-2007, 05:09 PM
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Default Re: diverter valve or blow off valve?

From what I've been told is that our cars are tuned with the recirculation of the air. So if you want a blow off valve you have to tune it to adjust for that. So if you use a blow off valve and release all that air instead of recirculating you will lose power. (Thats what my mechanic said) I hope that makes sense.
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Old 12-30-2007, 12:15 AM
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Default Re: diverter valve or blow off valve?

Also isn't a blow off valve mechanical and a diverter valve electricaly controlled? I may be wrong!
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Old 01-04-2008, 11:49 AM
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Default Re: diverter valve or blow off valve?

They are both mechanical, our cars DV's are on a "closed loop" system, and the BOV releases air out of the system. You can run rich by having a BOV and have a heck of a lot more problems! I know guys that have converted successfully and a lot of guys that didnt! so here is some info from another site

This was lifted from another site but it will be great for this one
Originally Posted by Fourdoor View Post
All right guys,

The end all be all guide to how and why to use various types of diverter valves.


Diverter valve. A diverter valve is ANY valve used to divert a pressure wave from reflecting back to the turbo after slamming into a closed throttle plate. This pressure wave can be diverted back into the intake track between the MAF and the turbo (re-circulated) or vented out into the engine bay (vent to atmosphere). Common use alternate words are Blow Off Valve (BOV) and Bypass Valve.

A diverter valve consists of three main parts. The valve disk and shaft are one part, the pre-load spring, and the diaphragm assembly.

Now on to “how it works” 

During normal operation under idle conditions there is a vacuum applied to the top of the stock diverter valve, and the valve is partially open. Also during idle conditions the upper IC pipe has very slightly positive pressure in it (much less than 1 psi of pressure). If you take the hose that connects the stock diverter valve to the intake off you will feel a small amount of air coming out of the hose. This is perfectly normal.

Under heavy load conditions there is boost applied to the top of the diaphragm in the stock diverter valve helping to push it closed along with the pressure from the pre-load spring. When the throttle plate shuts you now have a vacuum on the top of the diaphragm and this pulls the valve open along with the boost pressure in the upper IC pipe pushing the valve open.

Diverter valve design is a balancing act.

The smaller a diverter valve the easier it is to package into the engine bay without interfering with other under hood parts, but this can limit the flow capacity of the valve and result in part of the pressure wave not being diverted from hitting the turbocharger. A larger valve can handle anything you throw at it, but takes up a lot of space and will be hard to mount in a cramped engine bay.

Another compromise is the valve disk vs. diaphragm size. There are two types of valves when it comes to this size of the issue. If the diaphragm is LARGER than the valve disk then the only purpose of the spring in the diverter valve is to help close the valve under IDLE conditions. If the diaphragm is SMALLER than the valve disk then the purpose of the spring is to help close the valve under BOOST conditions. The only valve I know of personally with a larger diaphragm than valve disk is the TIAL. I will never use anything other than a valve that has a larger diaphragm than the valve disk because unless there is a diaphragm failure it is impossible for it to leak no matter what boost pressure you throw at it. With a valve that has a smaller diaphragm than the valve disk to run high boost you need to run more spring pressure, and eventually you end up with so much spring pressure that the valve doesn’t open very far when vacuum is applied to the top of the diaphragm and this limits the ability to vent enough of the pressure wave to prevent the pressure wave from hitting the turbo. I just wish you could get a TIAL with a recirculation kit.... but the valve is gigantic even vented to atmosphere and would be even bigger with a recirculation setup.

If someone else wants to explain the ins and outs of vent to atmosphere diverter valves vs. re-circulation feel free.

Originally Posted by [email protected] View Post
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 )

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.

Boost Holding Capacity
Flow Volume
Adjustment Range
After-Sale Support

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.


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.
Originally Posted by [email protected] View Post
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)
Originally Posted by [email protected] View Post
Ok, now things are going to get a little more technical and I will also have to interject some more personal opinion. Some of the things I will explain will be specific to our valves, and some things won't be, so I'll try to be clear to make the distinction, and while I unfortunately do not have a lot of technical drawings or diagrams right now, I am working on having cutaways done of our valves to better illustrate how our specific valves work.

Firstly, it should be fully understood that a recirculating valve needs to be open at idle, and since it is a closed-loop system, no metered air escaping the system through the open valve as it is only open to the recirculating port on the intake drawing in already metered air.

Atmospheric valves need to be closed at idle otherwise the vacuum of the engine will draw in unmetered (and unfiltered) air causing a lean condition at idle.

Under load/boost, ALL valves are closed (or at least they should be assuming they are tuned/adjusted properly), thus retaining the boost pressure in the system.

At throttle lift, a combination of the residual charge pressure within the system and the return of the intake manifold back to vacuum causes the valve to open, thus venting the residual pressure. Either recirculating it back into the intake or venting it to atmosphere.

The terms "pull-type" and "push-type", and how they apply to a given valve are all dependant upon whether or not the valve will be "pulled" open by vacuum and/or whether or not the valve is "pushed" open by pressure.


The HKS valve is neither type. Most other valves are both types.

The HKS valve is unique in that it's spring is used to keep the valve closed at idle/under vacuum, preventing it from being "pulled" open. And while this is great when used in an atmospheric configuration, so as no metered air is lost from the system through the open valve, this principle does not apply when the HKS valve is setup to recirculate. When recirculating, the valve needs to be "pulled" open at idle, so the spring is subsequently too stiff to allow this to happen.

Additionally, in an atmospheric configuration, when under load/boost, obviously the HKS valve will be "pushed" shut from a combination of the pressure reference from the intake manifold and the positive pressure within the piping holding the valve's "disk" against its seat, but at throttle lift, when there is a residual amount of pressure needing to be released, that pressure within the intercooler piping is still "pushing" the valve's disk closed. And though the intake manifold has also returned to vacuum, at least momentarily, and the valve should be "pulled" open, as mentioned above, the spring used in the HKS valve is designed to prevent the valve from opening under vacuum or at idle, so the HKS valve only opens in response to a change in the pressure differential between the intercooler piping and the throttle body.

In my opinion, this change takes such a period of time to occur before opening the valve, that the residual charge pressure has already backed up into the compressor wheel on the turbo, thus slowing it down, and potentially even creating lag, or worse, possibly compressor surging.

Again, the above is my personal opinion of the operation of the HKS valve and is fully subject to argument, or HKS can speak for themselves.

OEM to HKS Comaprison:

HKS's valve, as can be seen in the diagram, has a separate chamber in which a diaphragm exsists. This is virtually idential in design to the OEM Evo valves, HOWEVER, there is one very important distinction related to my points above.

The intercooler piping pressure enters the HKS valve through the port at the bottom of the diagram, thus holding their valve shut when any amount of pressure or vacuum exists within the system until a certian pressure differential is reached.

The OEM Evo valves, however, have the intercooler piping pressure enter the valve through the port on the side of the diagram above, and this is better for three reasons.

One, the spring tension in the valve is not so stiff that the valve won't be "pulled" open at idle/under vacuum, thus it CAN open at idle maintaining proper function of the closed loop system.

Secondly, since the positive pressure enters the OEM valves from the port directly opposed to the main chamber, under load, the only pressure required to hold the valve shut is a combination of the spring pressure and the pressure reference from the intake manifold.

Lastly, at throttle lift, not only will the valve immediately open from the residual charge pressure "pushing" the disk off of its seat, but the return of the intake manifold to vacuum will also "pull" the valve open releasing the residual charge pressure before any lag or compressor surging can take place.


Forge Valves:

Now, on to our valves. And again, I apologize for not having diagrams handy, but I'll get some as quickly as I can.

It should also be noted that many other manufacturers valves use similar operating designs to ours and will work in the same way.


Our recirculating piston valves are only a two chambered design, as are our recircularing diaphragm valves. You will have a main body, that either uses a piston sealed with silicone o-rings against the walls of the body or a diaphragm that separates the two chambers within the body.

Under vacuum/at idle, the piston or the diaphragm will be "pulled" open by the vacuum from the intake manifold. As mentioned above, this is a normal occurance and not a problem on a closed-loop MAF system.

Under load/boost, the spring tension and the positive pressure from the intake manifold will "push" either the diaphragm or the piston against its seat, creating a seal within the valve body holding the pressure within the intercooler piping.

At throttle lift, both the residual charge pressure within the intercooler piping, AND the return of the intake manifold to vacuum will both "push" and "pull" the piston or the diaphragm open thus releasing the residual pressure from the intercooler piping back into the intake.


We have 3 types of atmospheric valves:

- Single Piston Atmo.
- Single Diaphragm Atmo.
- Dual Piston Atmo.

Single Piston/Diaphragm:

Both the single-piston and single diaphragm types would only be for those applications able to accept the use of an atmospheric valve that would be pulled open at idle and that will vent to atmosphere at throttle lift. Speed Density and MAP engine management systems cap use these valves.

At idle/vacuum, the piston or diaphragm is "pulled" open from the vacuum reference to the intake manifold. consequently, the valve will vent a small amount of already metered air, thusly only Speed Density and MAP based engine management systems can use these valves as these are the only systems that can compensate for such operation.

Under load/boost, the spring pressure and pressure reference from the intake manifold "push" the valve closed creating a seal, holding the boost pressure within the system.

At throttle lift, the diaphragm or piston is both "pulled" open by the return of the intake manifold to vacuum and "pushed" open by the residual pressure needing to vent from the intercooler piping.

Dual Piston:

Our dual-piston type valves are only for those applications which can accept the use a valve that will vent at throttle lift, however, it should be noted that, the dual piston design (which I will explain below) prevents the valves from being open at idle, yet does not suffer some of the same issues as those mentioned above for the HKS valve.

The dual piston valves, while being compatible with both Speed Density and MAP engine management systems, are also designed to aid in compatibility with those rare MAF applications that are tuned to allow for the use of an atmospheric valve, however, we make no guarantees to compatibility with all MAF applications.

At idle or under vacuum, only the main piston is "pulled" open by the vacuum from the intake manifold. Since the secondary piston, however, is counter balanced against the main piston with a smaller spring, it remains sealed preventing the minimal amount of already metered pressure within the intercooler piping at idle from escaping the system.

Under load/boost, both pistons are "pushed" closed by the main spring pressure and pressure reference from the intake manifold, thus creating a seal and holding boost.

At throttle lift, though, the return of the intake manifold to vacuum will "pull" the main piston open and the residual charge pressure will "push" the secondary piston open, allowing the air to be vented without any issue.

Last edited by 1move; 08-24-2012 at 01:53 PM.
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