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BOV location

ercp98

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northlake in illinois
i'm in the process of redoing my intercooler and the its pipes. my question is, can i have the BOV in between the turbo and the intercooler? or does the BOV need to be in between the throttle body and the intercooler? keep in mind that the piping is very short. i'm using a syclone intercooler. thanks for any advice.
 

ktmrider

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The BOV can go anywhere so long as it relieves the "reverse" pressure before reaching the turbo. Closer to the TB the better.
The thing to consider is whether you are recirculating air from the BOV. Stock location is right next to the intake tube for easiest connection.
Not a problem if you are tuned for atmospheric vent, can cause idle and other issues if not.
 
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Dialcaliper

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If the BOV has a significantly large port probably won't make too much difference, but between intercooler and TB is better - the less air that is flowing backwards in the system, the better. If you have a lot of pressure in the system, that means mass and inertia as well. Surging is caused by a pressure wave traveling backwards in the system. It also means all that air in the intercooler is going to be stalling, or flowing backwards between shifts, meaning that when you open the throttle again, the response might suffer as the flow reverses again.

Another potential problem is that you run water/meth, the combination of pressure dropping (which also causes temperature to drop) and stall in the intercooler can cause the heated water to condense inside the intercooler instead of venting out, which may make the intercooler heatsoak the same way as if you inject meth between the turbo and the I/C.

That's the theory anyway - whether it will make any noticeable difference, who knows.
 
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When you pop a balloon with a needle does it matter where you poke it? Same principle, it all just gets squeezed out. Its under pressure remember. I think people think of it like water sloshing in a tube back and forth and when the throttle body closes, all that air sloshes back and wait! lets put the bov here so we can stop it before it sloshes back and hits the turbo!

Not how it works. I have had them all over and if they are responsive enough it doesn't matter.
 

beaner

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I've had my BOV about 6" from the compressor outlet on my 1g for months now. It works fine. It was always my theory that the factory puts the BOV near the TB because it was easier to recirc and quieter because the intercooler acts as a muffler in a way. The whole point of the BOV is to relieve pressure on the compressor wheel between shifts, so I wanted that release point pretty close to where the action is happening.

Doesn't the SRT-4 have a BOV on the compressor housing? I know WRX has the BOV on the intercooler. I say put it where ever you want so long as it's doing its job.
 

Dialcaliper

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Air has plenty of inertia - not nearly as much as water, but it's there. When a balloon pops, the rubber actually tears and retracts around the sphere, making a huge opening compared to the volume, and the air is not moving, so it doesn't matter, so that's a bad analogy.

I think surging isn't really the main issue in this case, but rather flow reversal - high velocity air has plenty of energy, and air exiting from the bov is moving at pretty high speeds.

Think more like letting a balloon loose to fly around the room, hitting you in the face - a little compressor wheel that weighs a few ounces has to do all the work to get air going the same speed in the opposite direction!

Also don't forget that at part throttle with a recirculated BOV open, air is being drawn through the BOV into the engine, partially bypassing the turbo and intercooler, and the pressure drops caused by each.

You may never notice the subtle difference from the driver's, but flow reversal through a restriction like the intercooler is generally a bad thing the place it will hurt is boost recovery on a quick upshift. The less air traveling backwards in the system the better.
 

Quoting Beaner:
The whole point of the BOV is to relieve pressure on the compressor wheel between shifts, so I wanted that release point pretty close to where the action is happening.



The action is happening everywhere in the charge system. Nissans have theirs all over the place. Right before the TB, on the i/c, wherever. It really doesn't matter, i'm telling you.
 

14u2nV

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Balloon won't pop if you put tape on it first /ubbthreads/images/graemlins/tongue.gif
 

jepherz

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I'd go with Dialcaliper's advice and, if you can afford to, put it as close to the TB as possible. As others have said "it doesn't matter", though, I think it's just one of those things where the closer you put it to the TB the better, but the overall loss from putting it pre-intercooler wouldn't be HUGE, but just there.
 

turbowop

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Does the airflow really flow backwards when you close the throttle plate? I've never understood why people say that. I always thought of it as just stopping and then the compressor wheel trying to spin against stopped airflow, not backwards airflow. With that said, putting the BOV anywhere after the compressor would work as it keeps the compressor wheel spinning freely, and that is the most important thing.
 
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Dialcaliper

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It doesn't flow backwards when you close the throttle plate, it makes a pressure wave just like throwing a rock into a pond, which is what hits the compressor wheel and makes it surge. The wave is essentially like a very low frequency sound wave, and mostly independent of any sort of flow going on. A very close analogy is water hammer in old plumbing when you shut the water off.

The backwards flow is a separate issue - if the BOV is pre-intercooler, when the BOV opens, the pressurized air expands, and as it exits the BOV, you end up with a backwards flow out the valve. (the air coming out of the turbo is still moving forward at this point, out the BOV as well)
 

14u2nV

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Unless your running huge pipes or huge pressure, there's not going to be enough noticable difference to matter.
 

GVR-4

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In addition to the pressure wave I always considered having to repressurize the intake plumbing. If the BOV releases pressure closer to the throttle, wouldn't that mean the compressor wheel has less volume to fill before getting back up to the desired boost pressure?
 

CutlassJim

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Quoting Beaner:
I know WRX has the BOV on the intercooler. I say put it where ever you want so long as it's doing its job.



They have it mounted on the intercooler because the intercooler attaches directly to the throttle body. This way it's only about 4" away so when it opens all the air in the system continues in the same direction with only the small amount in the endtank changing direction.

Air is a fluid and should always be considered such, I don't understand how Dialcaliper's explanation doesn't make perfect sense to everyone. /ubbthreads/images/graemlins/dunno.gif
 
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89coltgt

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I believe it would work out like this(arrows pointing where the pressurized volume of air would travel when bov vents):

bov close to the t-body:

t/body>>>>>bov
 

turbowop

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In theory it may be better to have the BOV closer to the throttle body, but in practice it really doesn't matter. Convenience would be more important to me when dealing with something like this.
 

Rausch

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^ +1

The pressure concern (refilling the pipes) is negligible, as the pressure will constanty equalize over the entire length. So no matter where you release the air, the volume would be the same remaining, at the same pressure.
 
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