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Turbo Blow off valves

Austin

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I'm adding a turbo on my rig and the setup I bought has an internal wastegate. I've watched a lot of videos that talk about how external blow off valves are better for several reasons. Many people talk about ducting the pressurized air back into the non-pressurized intake side to help with lag. Something like this:

Screenshot 2023-01-30 at 3.57.01 PM.png


I'm trying to picture how that works. Seems to me the pressurized air would just escape from the air filter?!?! If not, it's trying to spin the compressor wheel, which would make the exhaust side almost create a vacuum in the exhaust manifold. No?

Is there a HP range that this set up starts to make sense vs the internal one?
 
wastegates and blow off valves do different things. Wastegates are for for limiting boost and preventing turbine overspeed. Blow off valves prevent your compressor wheel from stalling when you get off the throttle. Stalling the compressor can snap the turbine shaft, the big boost pulse can also blow out intake hoses and crack composite manifolds. Routing it back to the atmospheric side of the intake helps with systems that use a MAF sensor because you aren't dumping air that the computer is already expecting to be delivered to the cylinders. Just free dumping it can cause a very rich surge when the BOV opens up and vents. It can also help a little with spooling
 
Gotcha, I certainly didn't put that together when looking into them, I thought they essentially did the same thing from different sides. I'll have to look for it but I watched a video about removing the built in wastegate and adding a BOV, maybe I took it wrong or it was application specific.

IMG_7030.jpg

I was hoping to remove the wastegate setup because of the space it takes, sounds like thats a bad idea.

So if I don't have a MAF, do I need a BOV at all? This OM617 had the compressor side dumping straight to the intake manifold, not sure if they had BOV back then.
 
OM617, so a diesel, correct?

Diesel's don't typically have a blow off valve as they don't have a throttle blade in the air tract. BOV's function to exhaust excessive boost when the throttle is slammed shut, avoiding excessive pressure in the intake system (turbo keeps spinning, nowhere for air to go, boost increases). In a diesel, there is no throttle blade to shut and slow the air into the cylinders. At "normal" boost levels the pressurized air can continue to fill the cylinders, even with reduced rpm and fuel input. Diesel's can run lean, so extra air in the cylinders until the turbo slows don't doesn't hurt the engine.

So, the wastegate is the preferred method of boost regulating on a diesel as it keeps turbine speed in check as well. You could chop a hole in your exhaust manifold and run an external wastegate that vents into the post turbo exhaust, but it's not going to save room overall, would only be useful if you needed something exactly where at wastegate actuator is.

Hopefully that makes sense, there are some diesel's with BOVs, but mainly in extreme cases of big turbos and huge boost numbers.
 
Wastegate is designed to open and allow exhaust to bypass the turbine so it doesn't build more boost than the wastegate is designed to allow. The BOV is designed so once you close the throttle plate, engine side of throttle body gets vacuum which opens the BOV allowing the turbo boost pressure to release into the atmosphere or intake tube like you have above. Otherwise the turbo pressure would be pushing onto the throttle plate, cold side intake turbine and cause other issues. You want the BOV close to throttle body so it releases pressure asap so it doesn't cause backpressure on the intake turbine of the turbo. I'm also a newbie when it comes to turbo stuff since I'm doing a turbo on my 6.0L LS motor in my 1968 Ford Prerunner. I've done lots of reading and talked to a few dyno guys that are very experienced with turbos that have helped me along the way. Ask away any questions that you think are stupid and i'll post answers or links to things I've found that answered my questions over the years of research.
 
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OM617, so a diesel, correct?

Diesel's don't typically have a blow off valve as they don't have a throttle blade in the air tract. BOV's function to exhaust excessive boost when the throttle is slammed shut, avoiding excessive pressure in the intake system (turbo keeps spinning, nowhere for air to go, boost increases). In a diesel, there is no throttle blade to shut and slow the air into the cylinders. At "normal" boost levels the pressurized air can continue to fill the cylinders, even with reduced rpm and fuel input. Diesel's can run lean, so extra air in the cylinders until the turbo slows don't doesn't hurt the engine.

So, the wastegate is the preferred method of boost regulating on a diesel as it keeps turbine speed in check as well. You could chop a hole in your exhaust manifold and run an external wastegate that vents into the post turbo exhaust, but it's not going to save room overall, would only be useful if you needed something exactly where at wastegate actuator is.

Hopefully that makes sense, there are some diesel's with BOVs, but mainly in extreme cases of big turbos and huge boost numbers.

My 7.3 with an automatic has some sort of BOV that you can clearly hear dump back into the intake tube when it shifts gears.
 
I'm adding a turbo on my rig and the setup I bought has an internal wastegate. I've watched a lot of videos that talk about how external blow off valves are better for several reasons. Many people talk about ducting the pressurized air back into the non-pressurized intake side to help with lag. Something like this:

Screenshot 2023-01-30 at 3.57.01 PM.png


I'm trying to picture how that works. Seems to me the pressurized air would just escape from the air filter?!?! If not, it's trying to spin the compressor wheel, which would make the exhaust side almost create a vacuum in the exhaust manifold. No?

Is there a HP range that this set up starts to make sense vs the internal one?
You don’t have to have the wastegate on your turbo, you can move it. More of a race type setup.
 
I'm trying to picture how that works. Seems to me the pressurized air would just escape from the air filter?!?!

The engine is running and produces a vaccum behind the filter.
It's not going back out.
 
The engine is running and produces a vaccum behind the filter.
It's not going back out.
In every video I watched their diagrams, like the one shared, are crude and I took them as 'the setup' and it makes sense for a diesel setup. Yet most were on gas cars with a throttle body I was not picturing so it make sense now.

Even without it ... are you saying air filters are one way?
 
Bov isn't needed for diesel. You are probably hearing turbo surge.

Wastegate is mandatory for a diesel or any turbo application.

On gas engines the bov vents excess pressure during deceleration to prevent slowing the compressor wheel and allowing faster spooling when reapplying throttle.

BTW scout nissan sd33t diesels have bov that limit your pressure to 10 psi.
 
In every video I watched their diagrams, like the one shared, are crude and I took them as 'the setup' and it makes sense for a diesel setup. Yet most were on gas cars with a throttle body I was not picturing so it make sense now.

Even without it ... are you saying air filters are one way?

When the engine is running, it produces a vaccum behind the filter / inside the intake piping.
The air dumped back behind the air filter is not going back out. It's getting sucked in by the engine.
 
When the engine is running, it produces a vaccum behind the filter / inside the intake piping.
The air dumped back behind the air filter is not going back out. It's getting sucked in by the engine.
Right, I get that, but how much can it take? Using simple numbers for discussion, if the compressor wheel is taking in 5psi and the BOV dumps 10psi in front of it will that additional 5psi be forced through the compressor or go out the air filter? That was my train of thought ...
 
Bov isn't needed for diesel. You are probably hearing turbo surge.

Wastegate is mandatory for a diesel or any turbo application.

On gas engines the bov vents excess pressure during deceleration to prevent slowing the compressor wheel and allowing faster spooling when reapplying throttle.

BTW scout nissan sd33t diesels have bov that limit your pressure to 10 psi.
No wastegate on a Detroit two-stroke :grinpimp:
 
Bov isn't needed for diesel. You are probably hearing turbo surge.

Wastegate is mandatory for a diesel or any turbo application.
I agree, the 7.3 guy is hearing surge not a BOV.

Wastegates are not mandatory on all turbo applications. In addition to the 2 strokers, there are plenty of turbo diesels without wastegates. The early 12 Valve Cummins come to mind, my 91.5 didn't have a wastegate from the factory and many of them didn't. If the turbo has an exhaust housing sized for it, and it's not pushing a lot of fuel, such as a 160hp 12V, a waste gate isn't needed.

Variable geometry turbos negate the need for a wastegate as well, and those have been factory fitted for at least 20 years now.

The above doesn't apply to Austin, as a wastegate is appropriate when you're adding a turbo to something and not spending millions on OEM R&D. Plus he's probably going to increase the fueling side of the equation as well. Do portals tolerate burnouts?
 
Right, I get that, but how much can it take? Using simple numbers for discussion, if the compressor wheel is taking in 5psi and the BOV dumps 10psi in front of it will that additional 5psi be forced through the compressor or go out the air filter? That was my train of thought ...
This got long, but I'll try and show my thoughts on it.


In it's normal state, and especially when chooching, the compressor wheel is in a small vacuum. It is sucking air through a pipe and filter. It's not intake manifold on a gasser at idle levels of vacuum, but less than atmospheric pressure. For simplicity and not switching units to inches of vacuum, we'll say it's seeing approximately 0 psi at the intake side of the compressor wheel. Think of the air filter indicators on most heavy equipment and semis. They show the filter status based on the level of vacuum behind the filter.

While venting via BOV the turbo is still spinning, but slowing down so it is actively pulling air into itself. With a captured BOV, it's pulling that bypassed air from further up the cold side.

Could it go out the filter. In theory yes, but the engine is still consuming some air, and the BOV is only venting the excessive pressure. So it's not necessarily sending a full 10psi down the vent (to atmosphere or back to intake). When engine speed decreases the exhaust drive pressure decreases as well, slowing down the turbine speed from that side as well. Think of ricer movies, that BOV sound is very short, just that quick excess pressure burst when the throttle is quickly closed while the turbo is still full steam ahead.

In the case of using an intake returned BOV to limit total boost, then the engine is still pulling in plenty of air and energy isn't free, so that excessive pressure won't compound in the turbo, just less air will come through the filter. This is why MAF systems prefer to return the BOV to the cold side, as that air is already accounted for. If vented to atmosphere, the engine is not consuming as much air as the MAF has calculated and it'll be rich while the BOV is venting to atmosphere.

Air pressure also will dissipate when going into a larger volume, so 10psi through the relatively small BOV vent into the larger intake tract that was at a slight vacuum will be less than 10psi after entering the larger container.
 
Also just had a thought to help explain:

We refer to boost numbers as psi and that is actually psig, or psi gauge, with zero being atmospheric pressure.

In reality, zero boost (0 psig) is actually 14.7 psia (PSI absolute) because the atmospheric pressure is still there. Zero psia is a perfect vacuum, absolutely no molecules present.

So 10psi on your boost gauge (psig) is actually 24.7 psia.

So wastegate is sending venting 24.7psia into (edit: less than)14.7psia. If that scale makes it more understandable instead of 10 into zero.


Note: For simplicity, I used 14.7 psi for atmospheric pressure. This varies by elevation and other factors, but it's a simplified example.
 
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More thinking, based on bebop's correction.

Diesel filter restriction gauges via Google commonly go-to 25 in-hg. That would be with a dirty filter at full chooch. So I'll use say 8 in-mg as that's where those gauges start to move and that seems realistic. 8 in-hg (below atmosphere) converts to approximately -4psig, so 10psia.

That gets us to:

24.7 psia venting into an intake tube that is 10 psia while 14.7 psia is sitting on the other side of the filter.


And with a dirty filter full send showing red on the filter gauge at 25 in-hg (-12.3psig=2.4psia) it's:
24.7psia venting to 2.4psia with 14.7psia on the outside of the filter.


Note: I didn't realize in-hg was such a large jump in psia, haven't done that math in 10 years. I was thinking something like 14.65psia before bebop made me rethink it.
 
Diesel engines don’t need BOV.

Waste gate is good to have but can be made to work without it. Such as if the red line is low (3000rpm or less-ish) with max fueling dialed in and verified drive pressure not getting too high and max boost is not too high (sometime it still can get too high without dial back you max fueling compromising drivability).

Wastegate is desired if you’ll be seeing high boost for prolonged of period from such as on highway pulling trailer, pushing brick through air at 70mph, etc to bypass exhaust, otherwise there’ll be either undesired higher drive pressure or more boost than you need.

Worst case, you can try leave those waste gate components and install some kind of gauge or device to give you evidence on how often the waste gate open, as not all vehicles or applications react the same way. Lack of waste gate operation, then it can probably safely be deleted
 
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