FAQs » Compressor Valves: BOV/DV/Radial R35/R55 (29)

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Category: Compressor Valves: BOV/DV/Radial R35/R55

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Individual Throttle Body Engine (ITB): Why won't by DV/BOV open?


If your engine is equipped with individual throttle bodies, you will need to make sure that the vacuum source you are using for the BOV/DV is NOT sourced from only one runner/cylinder.  Sourcing vacuum from only one runner will not have a high enough resolution for the actuating force needed to control the BOV/DV.

You will need to connect all 4 or more cylinders to a common tee, manifold or distribution block in order to get a proper actuating signal.  In most factory ITB configurations, there is a balance tube that connects all four intake runners, you should tap into this common port to use with your BOV/DV.

In some instances, I4, V6 and V8 engines with a common throttle body will also experience this same type of problem if the vacuum signal is sourced from only one intake runner instead of a common volume such as the plenum. 

Author: Peter Medina

Is it okay to run two dedicated vacuum lines for the BOV/DV?


Running 2 lines to the BOV/DV should be a non issue. Most cars do not have very many ports for connecting boost/vacuum lines, hence we suggest taking the one signal line from the Intake Manifold and split it into two. 


Is My BOV/DV/R35/R55 Kit Missing Components?


Our application-specific or vehicle-specific kits are built with the components tailored for that particular vehicle.  We frequently update the parts included in these kits to optimize their performance on the vehicle based on our R&D activities.


Beware that there are retailers that purchase separate components included in our kits and market them as the kit part number.  This is not authorized by Synapse Engineering, please report any retailer you believe is selling product in this manner.  When a retailer goes around the sytem and assembles their own kits there is a very high chance that their version of the kit is missing components from our designated kit number.  There are also system integrators in the marketplace that offer kits which we do not offer, and for which they have developed their kits independent of Synapse, Those systems integrators do not fall under this warning.  


We will only warranty missing components on Synapse assembled kits with accompanying labels to indicate it is as such.


My BOV leaks at idle or low load situations!


If you feel that there is something wrong with your Synchronic BOV being open at idle, do not be concerned. This is normal, and your BOV is functioning as intended.

By bypassing the restriction of the turbo, intercooler piping and all the surface area of the intercooler, you get better throttle response when coming off of vacuum, as well as fuel economy (if you care about that sort of thing). The engine does not have to work as hard to overcome those restrictions when the BOV is venting at idle or low load situations.

However, if the “hissing” sound is not to your liking you can switch to a different port configuration.  The Synchronic BOV has 3 configurations to allow for initial reaction (opening) of the valve.  The order of fastest to slowest initial reaction (opening) is B --> A+B --> A. 

Port Configurations:

1.)    Port B to intake manifold pressure, A to atmosphere:  If you have a very fast spooling turbo/sc, use this setup.  Allows for the fastest valve reaction resulting in fastest throttle response, as seen on our bench tests. 

2.)    Port A+B to intake manifold pressure: Recommended setup for most people.  The valve will stay closed longer than if using just port B. 

3.)    Port A to intake manifold pressure, B to atmosphere:  The valve will stay closed longer than if using port A+B.


My BOV/DV/R35/R55 is open at idle, what does this mean?


The Synchronic compressor valves mirror exactly the signal that it is being fed.  If it is given a vacuum signal, it will open, if it is given -4" to 100+ psi of pressure, it will be closed.

Although the valve may be open at idle, it is meant to unload the compressor when the throttle is closed.  Most turbo systems will be blowing airflow out of an open compressor bypass valve at idle and especially at cruising (under load) engine RPMs.  The level of discharge airflow will depend on the size of your turbo and the compressor housing to compressor wheel clearances of your turbo.  This means that when the compressor is cold it will flow more air out at idle than when it is hot.  This is normal for any system.  At almost any load, there should not be any air being sucked in, or ingested through the compressor bypass valve into the system since the throttle angle is nearly closed.

If the open valve is a significant concern to you, there are several solutions you can employ, with varying degrees of impact on performance and response:

  1. You can recirculate the discharge of the valve into the intake system.  If your valve is not designed to recirculate, you can purchase an SB or DV valve that is designed to recirculate.
  2. You can filter the discharge of the valve.  This would only apply to valves that have a recirculation flange available for mounting a filter.  Take great care in sizing the filter to be able to accomodate sufficient discharge airflow.  Too small of a filter and you will have a discharge flow restriction.
  3. In any of the latest generation of Synchronic valves, you can adjust the pre-load until the valve is closed at your desired vacuum level.  Please bear in mind that the more pre-load you add, the slower the valve will respond.

MAF-Equipped Systems:  In any MAF equipped system, we recommend recirculating the discharge of the valve between the turbocharger compressor and the MAF sensor.


My stock or XXX brand BOV had only 1 port going to vacuum. How do I connect the 2 ports on my BOV to my engine?


Many factory and aftermarket blow off valves have only one port to connect the blow off valve to the engine's intake manifold pressure. Because the Synchronic BOV and Synchronic DV have 2 ports that can be connected to the intake manifold pressure, a vacuum T fitting is required to have both these ports connected at the same time.

These 2 ports are separated for tuning purposes. By changing which ports are connected to the intake manifold pressure, it is possible to slightly alter the way that the BOV/DV reacts to pressure changes. 

Author: Chad

My valve opens after X amount of boost, how do I keep it closed?

  • If you take any of the Synchronic Compressor Bypass Valves and leave all signal ports to atmosphere with no boost pressure to the top of the valve, they will naturally open to boost pressure anywhere between 5-8 PSI depending on the spring installed and the pre-load adjuster level
  • If you have only Port A connected to boost/vacuum and Port B open to atmosphere and your boost pressure opens at 15 psi, you can change your boost/vacuum signal to Port B (Port A to atmosphere) and the pressure that the valve will open will go much higher, to 30 psi for instance
  • If you connect Port A+B to boost/vacuum, the valve will not open at any boost pressure

Proper Boost-Leak Test Procedure


If you are performing a boost-leak test on your system with the Synchronic BOV installed.  You will need to follow the instructions below in order to do the test properly.  These instructions assume that you have the throttle plate closed when pressurizing the system.

There are 2 versions of the Synchronic BOV in circulation today.  They are mechanically different on the outside.

1)  3 port BOV with Port C (Older model).  Two ways to test.

  • disconnect ALL lines to the BOV including Port C and leave them vented to atmosphere.
  • Or, connect all lines, Port A, B, C to the pressure source

2)  2 port BOV.

  • Pressurize ports A/B/A+B as well as the inlet of the BOV.

Tech Note:  If using a port combination like boost/vac to Port A only, or Port B only works well for your setup.  You can also use a regulator to determine at what pressure the BOV will naturally leak pressure.  If you are running the Synchronic BOV on a compounded turbo system, or anything higher than 70 psi of boost pressure, you will need to have both ports connected.  With both ports connected, the BOV will not open up to 250+ psi of working pressure.


SB & DV Adapter Flanges Compatibility List


The Synchronic Diverter Valve and Blow Off Valve are both designed to be backwards compatible.  However, there are certain flanges that are meant to be used only the Synchronic BOV and vice versa.  Before you choose the adapter flange need for your application, it is best to check the following compatibility list first.  If you have further application questions, please feel free to contact our office at +1 (858) 457-1700 or e-mail us at support@synapseengineering.com.  

Vibrant Part No.

Synapse Part No.






Aluminum Weld On Flange





Stainless Steel Weld On Flange





Greddy Adapter Flange





HKS SSQ Adapter Flange





Anti-Stall Kit





Tial 50mm Adapter Flange





Mitsubishi/Mazdaspeed 2 Bolt Adapter Flange





Volkswagon/Audi 1" Diameter Flange KIT (2pc)





Mitsubishi Evo 1.25" Diameter Flange





1" Diameter Inlet Flange





1" Diameter Outlet Flange





1.25" Recirculation Adapter





STI Inlet Flange





STI Recirculation Adapter




Things to Know Before Purchasing a Synapse Compressor Valve


Before purchasing a Synchronic valve there are several facets of the product that you need to know and understand that differentiate it from any other product on the market today.

  • There is no diaphragm.  There is only a stepped piston with o-rings.
  • When installed, the valve will perform according to the signal that is connected to the two ports A, B or A+B.  Vacuum will open it, boost will close it, and without a signal it will be closed.
  • There are two actuation ports on each valve, Port A (closest to center) and Port B (further from the center).
  • When the valve is installed, it will be open under vacuum.  This includes idle conditions.  This is by design, this is not a leak, the valve will close by -4" of vacuum and will seal over 100+ psi of boost.
  • If you have an MAF (Mass Air Flow) equipped vehicle, you will need to recirculate the discharge of the Synchronic valve.  This cannot be done with certain valves such as the Radial R35 and R55.
  • The valve will operate with any OEM control system.  It can be controlled by the OEM ECU and solenoid.

Author: PM
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