BOV question

[chrisg]

so i'm going to be doing an engine build on a 1.6td, my plan is to run at 23psi with an intercooler. i've been doing alot of researsh on here, and everyone says block it off. i have an aftermarket one off a friends old honda turbo build.

So my question is, running 23psi, is it even safe to block off the BOV or should i use the aftermarket one?

[rodpaslow]

you need to block it off as it will blow at about 12psi.  The reason you don't need it is there are no throttle plates that shut closed with a diesel to shock the turbo like it would with a gasser if you didn't have a BOV.  This piece in the intake really isn't a BOV, it's was simply a over pressure valve that opened after 12-14 psi.  There are lots of posts about how to do it.

[chrisg]

okay so even the aftermarket one will blow at low psi?

where does that built up pressure go when you let off the throttle?

[rodpaslow]

I'm sure others can explain better, but because there is no excessive pressure created (like a gasser) that a BOV is not required.  No throtlle plates, no excessive back pressure.  I'm sure there is some pressure, but not enough to affect the turbo.

[theman53]

You do not need a BOV for diesel. I think VW called it and overboost protection limiter or something like that. The T3 and K24 that are stock on the 1.6 engines have internal wastegates and might be something your looking to deal with. An aftermarket boost valve will work for your desired psi if you block that overboost thing. Or you can turn the wastegate screw in until it is about flush and it will yeild you around 25psi YRMV

[Thezorn]

I could be way off track here but,

First off you dont need a BOV on a diesel, like everyone else has said

Second, the reason a gasser needs this is becasue of the back pressure, also like what everyone else has said.
but, (this is where I could be way off track) the reason the back pressure is so dangerous to the turbo on a gasser is that, when the throttle plate slams shut so abruptly, it stops huge amounts of air flow that is spinning the turbo. When this happens there is almost like an internal air lock inside the engine which will in turn will not be producing any air to keep the turbo spinning, which will stop the rotation of the turbo immediatley. This immediate stop in rotatation is extremely hard on the bearings and the impeller shaft itself and can even bend the shaft if there is enough pressure.

The gasser BOV work on possitive (boost) and negative (vacumm) pressure, this is another reason there isnt a BOV on a diesel, since diesels do not create a vacumm. When the throttle plate is open there is possitive MAP (manifold absolute pressure). There is a line running from the intake manifold to the top of the BOV, this line transfers the possitive pressure from the manifold to the BOV holding the BOV closed. As soon as the throttle plate closes, gasser engines create a vacumm, (at an idle gassers make a vacumm) as soon as that vacumm is created that line from the intake manifold to the BOV changes from possitive pressure to negative (vacumm) pressure and pulls up on the inside of the BOV opening it up, and voila, no more dangerous back pressure.

This really has no use on our diesel forum, but you know. Its cool

[bugnut]

If diesel's dont need a BOV then why does BD make a BOV for diesel's?
http://www.stillen.com/product.asp?id=BDTURGRD1&c=PE&year=&make=&model=

Letting off the throttle in a diesel when the turbo is loaded will cause the turbo to bark or surge with air because the air that the turbo was producing has no where to go now. The engine doesn't need it so it backlogs into the turbo and out the air cleaner.  I have had this happen a few times with my dodge.  It's not a sound you like to hear.   Heck I even had it happen to the rabbit when I was power braking and let off the pedal too quick. 

[Henchman]

Nothing changes when you let off the throttle in a diesel engine except for fuel being reduced/stopped to the injectors.  The engine continues to spin, valves open and close, combustion occurs (if there is fuel).  The only way that the condition you describe 'could' happens is if at full boots, one were to drop out of gear and shut down the engine.  Even then the engine should continue to rotate for a bit, giving any remaining charged air plenty of ways to dissipate.  The turbo, unless massive will very quickly stop making any real boost.

If you want to place an absolute limit on boost to then engine, then a BOV will do that.

Letting off the throttle in a diesel when the turbo is loaded will cause the turbo to bark or surge with air because the air that the turbo was producing has no where to go now. The engine doesn't need it so it backlogs into the turbo and out the air cleaner.  I have had this happen a few times with my dodge.  It's not a sound you like to hear.   Heck I even had it happen to the rabbit when I was power braking and let off the pedal too quick. 

[rabbid79]

Let's set the record straight.  The proper term for what is commonly being called a "blowoff valve for gassers" in this thread is in fact called a pop-off valve.  To call a pop-off valve a blow-off valve is a mistake, but unfortunately that's all you ever see anymore on the internet.  It's frustrating, even the Ford Taurus SHO website calls the pop-off valve a BOV in their sticky FAQ thread.

The engineers at VW aren't idiots.  They didn't install something that wasn't necessary.  They installed a blow-off valve on their early diesel engines.  The BOV is designed to prevent over-boosting of the engine by bleeding off excess intake manifold pressure in case the wastegate in the turbo fails.  It's a simple valve with a spring that's usually installed in the intake manifold.  It doesn't rely on any other factors to operate.  It simply opens at a preset pressure to bleed off boost.  If you want to go over 12 PSI on your VW diesel, you need to modify it or eliminate it, because it will start dumping boost.  (Of course in VNT-turbo applications, the boost is controlled by the position of the vanes in the turbine housing of the turbo, but we don't need to get into that.)  If the engineers had actually installed a pop-off valve, then yes, that would be a mistake.

The boost pressure is a turbocharged gasoline engine is also controlled by a wastegate.  The purpose of the pop-off valve, as explained in previous posts, is to dump excess boost pressure that is trapped between the turbo and the throttle plate when the throttle plate closes.  It does this by detecting a pressure differential ("vacuum" in the intake manifold, and some pressure greater than vacuum in the intake tract between the turbo and the throttle plate).  If this pressure isn't relieved it will either go backward through the compressor of the turbo (which is bad for the turbo, and also causes it to slow down, which hurts turbo response when back on the throttle again), or it will find a weak spot and blow the end tanks off of your intercooler (actually had this happen to me between 1-2 shifts on an Audi 5000 turbo with modified boost pressure and no pop-off valve), or it will find some other weak spot, such as a hose that isn't securely attached.

Since a diesel engine doesn't have throttle plates, the "excess boost" will exit through the cylinders just like it normally would, and there won't be a harmful boost spike in the intake tract.  Therefore, there's no practical purpose for putting a pop-off valve on a diesel engine.

If diesel's dont need a BOV then why does BD make a BOV for diesel's?

Because people will buy anything they're told to buy, and becuase it's cool it hear a popping sound between shifts.  And again, it's a pop-off valve.

[745 turbogreasel]

With my modified wastegate, I make 3-4 PSI and 200F EGT going downhill at 4500+ RPM, no cool pssht sound.
Excess boost my a**, it helps me engine brake