Where did you put your BOV??

[jc052685]

Yea lol. With a stock set up on a zx this was the spot I picked out after I relocated the coil. I responded because of the title question, for reference pouposes for others. I heard concerns about how far away from the T/B it should be, Mine works fine from this spot. .

Thanks.

 

I was not puting down the setup I was just stating that is pretty much the only place to put it. The BOV can pretty much go any where between the turbo and the throttle body.

[jayzrocket]

Thats kewl, I kinda figured that was what you were saying. I didn't take it wrong, but agreeing with ya-"lol". I guess the funky smiley I ended with may have been confusing. LOL! .

[jeffp]

I have read post after post about the "BOV". and I have yet to see many, or lets say VERY FEW people plumb in the BOV correctly to begin with.

 

What is the BOV designed to do? Well the standard answer is to stop turbo back spin, and that is correct in the extreme case, but tht is not all the BOV was designed to do. Also with the unit venting to atmosphere you are killing essentially half of the work the BOV was designed to accomplish. Sure you get the little cutsie PsssssT! when the valve opens but that is about all. Venting to atmosphere will relieve the initial pressure from the turbo when you close the throttle plate, but nothing more, in fact when the valve closes, the turbo again will try to produce pressure, and you will again need to open the valve to relieve the pressure. I realize that the valve will stay open X amount of time, but it will close and you will be again at boost levels with a closed throttle plate.

BUT, with the vent to atmosphere setup here is what happens to you. Now comes in the part you never hear about. The BOV is also designed to STOP the drag in the compressor, which is directly connected to the EXHAUST turbine.

So if you build 22 psi of boost, (lets say 1300 SCFM) when you close the throttle plate the BOV opens and relieves the pressure due to the closed throttle plate. BUT, the turbo is still spooled and compressing air. When the BOV closes you again have boost that effectively is going NOWHERE, and it is creating drag on the compressor, at the same time you are introducing drag on the exhaust turbine wheel and it spins slower. What do you get for all of your efforts to help out the turbo? You get a turbo that has stress on the shaft turning the compressor wheel, stress on the exhaust turbin wheel and it begins to spin slower. I ask you, what good is that? NONE! In reality what happens is that the compressor is blocked off from moving air, drag is introduced on the compressor wheel, the exhaust turbine wheel has drag (now keep in mind the turbo at this point is still spooled) the turbo RPM's start to slow down, the exhaust gases are now moving slower and you create back pressure on the exhaust side of the head, spent gases get backed up and you start to experience exhaust reversion into all of the other cylinders that have open valves (intake or exhaust valves) to equilize the air pressure. You slow down the exhaust evacuation to the exhaust system, pressurize the turbin housing, all so you can have your little PssssT noise and think that is cool, well it is not cool when you stop to think about what is really happening.

So for your cute little noise, what you get is a turbo shaft that really is still getting stressed, you get spent exhaust gases reversing flow path to the low pressure area, ANY valve that is open, you get crap air into the wrong cylinders, and a slightly less efficient burn for that cylinder, you get the turbo SPINNING SLOWER, that will take power and air flow to get back up to speed again, just so you can get your Pssst to begin the cycle all over again.

Now I don't know about you, not being the sharpest guy that has lived, but that is a good loss of throttle response, and additional stress on the turbo, as well crudding up the rest of the induction/exhaust system before the inlet of the turbo. I think that is most undesirable myself.

NOW, look at the recirculated BOV setup:

 

You build boost, the air has already been metered, by the MAF or the MAP sensors. You close the throttle plate. Air pressure begins to build up and slow down the turbo RPM, you open the BOV, the already measured air goes from the turbo compressor outlet, and is then plumbed back into the INTAKE of the turbo, where it again is compressed and pushed out of the turbo compressor to the BOV and into the intake of the turbo again. The compressor has little or no drag to flow air, the exhaust turbine is still spinning at high RPM's, the exhaust spent gases are still flowing into the exhaust system, the turbo maintains its RPM, or close to it, you have no air metering to speak of on either the MAF based system or the MAP based system, so the fuel management stops injecting fuel, you get the fuel cut that stops back fires.

When you tag the throttle, you have instant boost because the turbo is still effectively spooled. The BOV closes, and you build boost, the fuel is monitored again and the ECU injects fuel to make the engine run.

The BOV was designed to relieve compressor pressure, recirculate air that has already been metered, maintain the exhaust flow, and stop exhaust gases reversion into other cylinders where it is not welcome.

Which one seems better to you? BOV 101 class is over.

[jc052685]

Ummm, all that to say a recirc is better than a bov? I think most people already know that, but thanks for the write up on why its better.

[Nigel]

So if you build 22 psi of boost, (lets say 1300 SCFM) when you close the throttle plate the BOV opens and relieves the pressure due to the closed throttle plate. BUT, the turbo is still spooled and compressing air. When the BOV closes you again have boost that effectively is going NOWHERE, and it is creating drag on the compressor, at the same time you are introducing drag on the exhaust turbine wheel and it spins slower. What do you get for all of your efforts to help out the turbo? You get a turbo that has stress on the shaft turning the compressor wheel, stress on the exhaust turbin wheel and it begins to spin slower.

 

Wait a minute...

 

To summarize, you are saying that with a BOV that vents to atmosphere, once the blow-off valve closes, the compressed air from the turbo has nowhere to go and therefore, it's hard on the turbo and slows it down...

 

However, all else being equal, the exact same thing will happen with a recirculating system. The BOV is still going to close, and when it's closed, the air still has nowhere to go. Recirculating doesn't change that one bit.

 

NOW, look at the recirculated BOV setup:

 

You build boost, the air has already been metered, by the MAF or the MAP sensors. You close the throttle plate. Air pressure begins to build up and slow down the turbo RPM, you open the BOV, the already measured air goes from the turbo compressor outlet, and is then plumbed back into the INTAKE of the turbo, where it again is compressed and pushed out of the turbo compressor to the BOV and into the intake of the turbo again. The compressor has little or no drag to flow air, the exhaust turbine is still spinning at high RPM's, the exhaust spent gases are still flowing into the exhaust system, the turbo maintains its RPM, or close to it, you have no air metering to speak of on either the MAF based system or the MAP based system, so the fuel management stops injecting fuel, you get the fuel cut that stops back fires.

 

I will agree that with a volumetric or mass based efi system, recirculating is necessary to prevent the ECU from dumping in fuel for air that doesn't make it to the engine. However, with a MAP based system, this doesn't matter. When you close the throttle, the MAP sensor will instantly see vacuum and cut the fuel. So, with a BOV that vents to atmosphere, the ECU will never know that air was there.

 

Nigel

'73 240ZT

[jeffp]

Wait a minute...

 

To summarize, you are saying that with a BOV that vents to atmosphere, once the blow-off valve closes, the compressed air from the turbo has nowhere to go and therefore, it's hard on the turbo and slows it down...

 

However, all else being equal, the exact same thing will happen with a recirculating system. The BOV is still going to close, and when it's closed, the air still has nowhere to go. Recirculating doesn't change that one bit.

Hi Nigel, that statement all depends on where the vacuum is coming from to open and close the valve. The recirculated system is behind the throttle plate, so on decell the BOV is open until such a time comes that the actuator spring overcomes the vacuum on the valve. By that time, you are either going to come to idle, or there abouts, the valve closes. When you are in a situation where you are on throttle and off throttle, there is the difference. The turbo essentially is spooled or almost at spool depending on your rpm and cam, and you don't get the rich lean conditions you have with the vent to air setup.

So as I said it depends on how the system is setup, and for the recircukate setup the vacuum is always behind the throttle plate.

I don't know about you, but every car I have heard with the vent to air setup has a very short lived vent time. I could be wrong, but it is closing the valve, at which point you get the system building pressure again.

anyway.

Edited July 31, 2009 by Drax240z

[Nigel]

Hi Nigel, that statement all depends on where the vacuum is coming from to open and close the valve. The recirculated system is behind the throttle plate,

 

Where else would you get vacuum from other than behind the throttle plate? It's the restriction of the throttle plate that creates the vacuum. A BOV that vents to atmosphere is no different to a recirculating system in this regard. They both need a vacuum source to open, and there's only one place they're going to get it.

 

How much vacuum is required to open the valve varies from one BOV to another. I've tested a 1G Mitsubishi valve, a Subaru Legacy turbo valve and a Greddy Type RS. The Mitsu and the Greddy (depending on how tight you screw it down) both open around 15" and the Legacy valve opens at as little as 5".

 

The real problem that you can run into is that many designs will be open at idle and for part of the cruising range, like the three I mentioned above, and if they're left to vent to atmosphere, at the very least, on a MAP based system, this means that unfiltered air can be sucked in. With a volumetric or MAS system, this is unmetered air, which will result in the mixture running lean, although the O2 sensor will compensate somewhat. To get around this, guys that have vent to atmo setups will tighten the spring way down (depending on the design) to try and keep it closed at idle. But this results in the brief discharge you hear, and the other problems you talk about. I added a one-way valve to the discharge of my Greddy BOV to prevent this and allow me to run the spring with minimal compression.

 

I would argue that a recirc system is no better or worse than a vent to atmo system. Like anything, you have to understand the limitation of each and be familiar with the operating design of the BOV you are using.

[Six_Shooter]

I have read post after post about the "BOV". and I have yet to see many, or lets say VERY FEW people plumb in the BOV correctly to begin with.

 

What is the BOV designed to do? Well the standard answer is to stop turbo back spin, and that is correct in the extreme case, but tht is not all the BOV was designed to do. Also with the unit venting to atmosphere you are killing essentially half of the work the BOV was designed to accomplish. Sure you get the little cutsie PsssssT! when the valve opens but that is about all. Venting to atmosphere will relieve the initial pressure from the turbo when you close the throttle plate, but nothing more, in fact when the valve closes, the turbo again will try to produce pressure, and you will again need to open the valve to relieve the pressure. I realize that the valve will stay open X amount of time, but it will close and you will be again at boost levels with a closed throttle plate.

BUT, with the vent to atmosphere setup here is what happens to you. Now comes in the part you never hear about. The BOV is also designed to STOP the drag in the compressor, which is directly connected to the EXHAUST turbine.

So if you build 22 psi of boost, (lets say 1300 SCFM) when you close the throttle plate the BOV opens and relieves the pressure due to the closed throttle plate. BUT, the turbo is still spooled and compressing air. When the BOV closes you again have boost that effectively is going NOWHERE, and it is creating drag on the compressor, at the same time you are introducing drag on the exhaust turbine wheel and it spins slower. What do you get for all of your efforts to help out the turbo? You get a turbo that has stress on the shaft turning the compressor wheel, stress on the exhaust turbin wheel and it begins to spin slower. I ask you, what good is that? NONE! In reality what happens is that the compressor is blocked off from moving air, drag is introduced on the compressor wheel, the exhaust turbine wheel has drag (now keep in mind the turbo at this point is still spooled) the turbo RPM's start to slow down, the exhaust gases are now moving slower and you create back pressure on the exhaust side of the head, spent gases get backed up and you start to experience exhaust reversion into all of the other cylinders that have open valves (intake or exhaust valves) to equilize the air pressure. You slow down the exhaust evacuation to the exhaust system, pressurize the turbin housing, all so you can have your little PssssT noise and think that is cool, well it is not cool when you stop to think about what is really happening.

So for your cute little noise, what you get is a turbo shaft that really is still getting stressed, you get spent exhaust gases reversing flow path to the low pressure area, ANY valve that is open, you get crap air into the wrong cylinders, and a slightly less efficient burn for that cylinder, you get the turbo SPINNING SLOWER, that will take power and air flow to get back up to speed again, just so you can get your Pssst to begin the cycle all over again.

Now I don't know about you, not being the sharpest guy that has lived, but that is a good loss of throttle response, and additional stress on the turbo, as well crudding up the rest of the induction/exhaust system before the inlet of the turbo. I think that is most undesirable myself.

NOW, look at the recirculated BOV setup:

 

You build boost, the air has already been metered, by the MAF or the MAP sensors. You close the throttle plate. Air pressure begins to build up and slow down the turbo RPM, you open the BOV, the already measured air goes from the turbo compressor outlet, and is then plumbed back into the INTAKE of the turbo, where it again is compressed and pushed out of the turbo compressor to the BOV and into the intake of the turbo again. The compressor has little or no drag to flow air, the exhaust turbine is still spinning at high RPM's, the exhaust spent gases are still flowing into the exhaust system, the turbo maintains its RPM, or close to it, you have no air metering to speak of on either the MAF based system or the MAP based system, so the fuel management stops injecting fuel, you get the fuel cut that stops back fires.

When you tag the throttle, you have instant boost because the turbo is still effectively spooled. The BOV closes, and you build boost, the fuel is monitored again and the ECU injects fuel to make the engine run.

The BOV was designed to relieve compressor pressure, recirculate air that has already been metered, maintain the exhaust flow, and stop exhaust gases reversion into other cylinders where it is not welcome.

Which one seems better to you? BOV 101 class is over.

 

There is one huge flaw with this theory.

 

Every BOV/bypass valve/whatever you want to refer to it as, they function the same way, in that what causes them to open is the same for any system, including re-circulating and open to atmosphere.

 

There are two, and in most cases 3 points where "pressure" is applied to open or close the valve.

 

Lets look at the part that does most of the controlling of when the actual vent opens. That would be that top part where you attach the vacuum hose to. In that round part there is a diaphram. This diaphram has two sides. The top (farthest away from the valve) and the bottom (closest to the valve). The vacuum source is attached to to top side of this diaphram so that when vacuum is applied, it will create low pressure or vacuum in the space above the diaphram, this will make a pressure differential between the top and bottom of the diaphram allowing the dirphram to move, in a way that open the valve. Now the bottom side of the diaphram can be plumbed in a couple of ways, one way is to atmosphere, which most aftermarket BOVs I've seen are plumbed. So now we have approximatly 14.7 PSIA of pressure on the bottom side of the diaphram. Apply a vacuum to the top side and now the diaphram will want to move towards the vacuum. This is simple physics. Yes there will also be a spring above the diaphram, and this adds some resistance to opening, to work against the vacuum, and to ensure that the valve does indeed close when it's supposed to. The other way to plumb the bottom side of the diaphram is to the intake tubing, where we are pressurizing with the turbo. What this does is force pressure under the diaphram for a quick lift of the valve, to vent quickly. So as long as there is equal pressure on both sides of the diaphram which there will be with the throttle wide open (pressure is applied through that "vaccum" tube to the top of the diaphram), and now pressure applied below, the valve in theory will stay closed, because the spring that is on the top of the diaphram applies a little more force than pressure alone will.

Now there is a 3rd spot that pressure is applied, and that is under the valve itself. Pressure to the valve also has influence on when or how long a BOV will be open. It is because of this that some BOVs, such as the 1st gen DSM BOV will open at a certain pressure without applying vacuum to the top of the diaphram. The pressure on both the bottom of the valve it self and to the bottom of the diaphram will overcome the pressure above the diaphram.

It is because of the pressure applied to the valve itself in the BOV that they need to be installed in the correct orientation. Install a BOV backwards and it will have a hard time opening.

 

Basically as long as there is pressure "below" the BOV and vacuum above the diaphram, such as what would happen at idle or low RPM cruise conditions, the BOV will be open, and not cause the situation to describe.

[jeffp]

A compressor bypass valve (CBV), also known as a compressor relief valve or diverter valve, is a vacuum-actuated valve designed to release pressure in the intake system of a turbocharged car when the throttle is lifted or closed. This air pressure is re-circulated back into the non-pressurized end of the intake (before the turbo) but after the mass airflow sensor.

 

A blowoff valve, (BOV, sometimes hooter valve, not to be confused with a dump valve) performs the same task but releases the air into the atmosphere instead of recirculating it. The blowoff action produces a range of distinctive hissing sounds, depending on the exit design. Some blowoff valves are sold with a trumpet shaped exit that intentionally amplifies the sound. The TurboXS model RFL blow off valve is well known among tuners for this kind of design and some turbocharged vehicle owners may purchase a blowoff valve solely for the auditory effect even when the function is not required by normal engine operation. Motor sports governed by the FIA have made it illegal to vent unmuffled blowoff valves to the atmosphere. In the United States, Australia and parts of Europe cars featuring unmuffled blowoff valves are illegal for street use.[citation needed]

 

Blowoff valves are used to prevent compressor surge, a phenomenon that readily occurs when lifting off the throttle of an unvented, turbocharged engine. When the throttle plate on a turbocharged engine closes, the high pressure air in the intake system is trapped by the throttle and a pressure wave is forced back into the compressor. The compressor wheel slows rapidly and may even stall, and the driver will notice a fluttering air sound. The rapid slowing or stalling stresses the turbo and imparts severe turbo lag if the driver accelerates immediately after the surge event.

 

In the case where a mass airflow sensor is used and must be located prior to the blowoff valve, the engine control unit (ECU) will meter out excess fuel because the atmospherically vented air is not subtracted from the intake charge measurements. The engine then briefly operates with a fuel-rich mixture after each valve actuation.

 

The rich mixing can lead to hesitation or even stalling of the engine when the throttle is closed, a situation that worsens with higher boost pressures. Occasional events of this type may be only a nuisance, but frequent events can eventually foul the spark plugs and destroy the catalytic converter, as the inefficiently combusted fuel produces soot (excess carbon) and unburned fuel in the exhaust flow can produce soot in the converter and drive the converter beyond its normal operating temperature range.

 

One way to mitigate the problem is to reduce the boost pressure, which reduces the required venting volume and yields less charge overcalculation by the ECU. The air can also be recirculated back into the intake, a typical stock setup for cars with an upstream MAF sensor. The situation can also be corrected by switching the fuel metering system over to a manifold absolute pressure sensor, a conversion that usually requires a compatible aftermarket ECU or piggy-back fuel controller. The MAP sensor monitors the absolute pressure in the manifold at all times and will correctly detect the change that occurs when the valve vents, allowing the ECU to reduce fuel metering accordingly.

 

Dump valves are fitted to the engines of turbo charged cars and sit between the turbo intlet and the throttle body. When transitioning from a boosted state to a closed throttle state (as in between shifts), due to inertia, the turbo continues to pressurize air, but the closed throttle prevents the compressed air from entering the engine. In this case the pressure exceeds the preset spring pressure in the dump valve and the excess pressure is bled off to atmosphere.

 

Even with a dump valve the compressed air acts as a brake on the turbo (slowing it down), because the pressure on the backside of the turbo is at a higher pressure than on the front side (and the air actually wants to flow through the turbo backwards).

 

A blowoff valve is a more elegant solution to this problem by allowing the turbo to "freewheel" when the throttle is closed (equalizing the pressure on both sides of the turbo). Unlike a dump valve a blowoff valve can be used at multiple boost settings without reconfiguration.

 

Blowoff valves are sometimes incorrectly called dump valves because they serve a similar function, but they are very different solutions to the same problem.

 

The two have been discussed at length by god who knows how many people. Myself, I choose to recirculate the air instead of venting to air. I don't like the rich/lean conditions the BOV creates.

[jayzrocket]

Okay-thank you! I think that the most of us who have invested in this unit have done it for a better reason than the the neat sound that it makes. Personally it sounds like somebody wishes that they had a BOV, lol. I say that because everyone that Ive met that didn't have one says the very same thing, "the only reason that they have that on there is for the noise".

Well I could have bought a cheap knock off from a web site for 15 to 30 bux for that. I wanted one to try and increase the longevity of my compressor, I do a lot of hiway driving and try to make things easiest on my car as possible.

But thanks for the insight.

[Hugh]

If there is no pressure in the intercooler piping, what load is placed on the turbine while shifting? What extra work is the exhaust doing to spin a freewheeling turbo? The throttle plate is closed, the BOV is open, the injectors are essentially off... there is no power being produced anyway. All this talk of reversion and scavenging means nothing when my foot isn't even on the gas pedal.

 

The turbine slowing down a little when shifting doesn't hurt anything. At 7300rpm, my turbo was required to spin X RPM to maintain 1.6kg. At 5300 in the next gear, the turbo doesn't need to spin at X. It will be somewhat lower and maintain the required pressure, because the required flow is now less. If it were still spinning at that speed, it would spike the boost. The boost controller would run lower duty and bleed more pressure off to set the turbine spinning at the needed RPM.

 

It's not like the turbine spins at one speed all the time.

[Brad-ManQ45]

On 7/31/2009 at 8:29 PM, Hugh said:

If there is no pressure in the intercooler piping, what load is placed on the turbine while shifting? What extra work is the exhaust doing to spin a freewheeling turbo? The throttle plate is closed, the BOV is open, the injectors are essentially off... there is no power being produced anyway. All this talk of reversion and scavenging means nothing when my foot isn't even on the gas pedal.

 

The turbine slowing down a little when shifting doesn't hurt anything. At 7300rpm, my turbo was required to spin X RPM to maintain 1.6kg. At 5300 in the next gear, the turbo doesn't need to spin at X. It will be somewhat lower and maintain the required pressure, because the required flow is now less. If it were still spinning at that speed, it would spike the boost. The boost controller would run lower duty and bleed more pressure off to set the turbine spinning at the needed RPM.

 

It's not like the turbine spins at one speed all the time.

I kinda hate bringing threads back from the dead, but I would like to provide a more reasoned answer to Hugh's post...

 

If you are on the road not shifting out of gear and are in boost, slow down for a situation an then speed up, recirculating the bow-off will have the turbocharger providing air faster.  If you are accelerating through the gears at anything less than maximum boost, recirculating will provide a bit more kick during an upshift.

 

I see no disadvantage to either - only positives.

 

Without recirculation provides only negatives.

 

As far as open a bit during idle, manufacturers use this to provide more air immediately upon throttle tip-in for better response.

 

Just sayin'...