300zx bov placement

[Twinz]

heres the situation...i have blitz bov's on my Z and im told by other people that have the same bov that they are loud, however on my car they are very quite so i looked at some online and i found that different pics had the bov's mounted in different places......mine are on the top pipes coming off the turbo to the intercooler (outside pipes if looking at it, the inside pipes go from intercooler to throttle body)...now online ive seen them mounted on the inner pipes, the ones from intercooler to intake manifold. But ive also seen a few mounted like mine, so my question is basically is it possible that my bov's are mounted on the wrong pipes...and is it bad / will it cause any damage to my turbos or nething like that

[300zTT]

send me a pic cuz i dont understand where u have them.....

[highmilehatch]

No, your not going to have any problems either way you have them mounted. I would personally stick to how you have them now.

[Twinz]

send me a pic cuz i dont understand where u have them.....

ill post a pic tommorow...sorry if my desciption was confusing

[Twinz]

http://i79.photobucket.com/albums/j122/94ttZ/umm.jpg
here is a pic of another one whihc has same set up as mine with bov's on outside pipes rather than ineer charge pipes

[wgknestrick]

Where you have them mounted should be fine.

I typically like to mount them right before the throttle body because that is what they are "bypassing". This way the rapid release of air is still flowing the same direction as it would in the car normally, and you minimize the volume of air that has to reverse itself out of the intake system into the BOV.

The way yours is installed, the air has to stop, then reverse out through your intercooler and other 4 ft worth of pipes to the BOV. This sometimes (sometimes it's just fine) can cause compressor surge on throttle lifts which is clearly audible. With the larger turbo setups is normally where it becomes critical. This reversing of the airflow causes a delay in how fast the BOV can vent, and remember you always want the fastest venting BOV system to keep as much kinetic energy in the turbo wheel(s) on shifts.

[Twinz]

Where you have them mounted should be fine.

I typically like to mount them right before the throttle body because that is what they are "bypassing". This way the rapid release of air is still flowing the same direction as it would in the car normally, and you minimize the volume of air that has to reverse itself out of the intake system into the BOV.

The way yours is installed, the air has to stop, then reverse out through your intercooler and other 4 ft worth of pipes to the BOV. This sometimes (sometimes it's just fine) can cause compressor surge on throttle lifts which is clearly audible. With the larger turbo setups is normally where it becomes critical. This reversing of the airflow causes a delay in how fast the BOV can vent, and remember you always want the fastest venting BOV system to keep as much kinetic energy in the turbo wheel(s) on shifts.

alright yea thats what i thought about putting it near the throttle body and that was my question because i sometimes here the surge and i realized the part about how its not letting all of the air out and it goes back to the turbo...i have plans for bigger turbos so i wanna clear up the bov issue first...my idea was to get new pipes and mount them closer to the intercooler somewhere halfway between turbo and throttle body

[zerioustt]

alright yea thats what i thought about putting it near the throttle body and that was my question because i sometimes here the surge and i realized the part about how its not letting all of the air out and it goes back to the turbo...i have plans for bigger turbos so i wanna clear up the bov issue first...my idea was to get new pipes and mount them closer to the intercooler somewhere halfway between turbo and throttle body

that spots fine but better off putting them in the stock recirc valve location..just a suggestion

[Wiisass]

Where you have them mounted should be fine.

I typically like to mount them right before the throttle body because that is what they are "bypassing". This way the rapid release of air is still flowing the same direction as it would in the car normally, and you minimize the volume of air that has to reverse itself out of the intake system into the BOV.

The way yours is installed, the air has to stop, then reverse out through your intercooler and other 4 ft worth of pipes to the BOV. This sometimes (sometimes it's just fine) can cause compressor surge on throttle lifts which is clearly audible. With the larger turbo setups is normally where it becomes critical. This reversing of the airflow causes a delay in how fast the BOV can vent, and remember you always want the fastest venting BOV system to keep as much kinetic energy in the turbo wheel(s) on shifts.

It's a pressurized system. So the air at the tb will be at the same pressure as the air at the intercooler inlet (ignoring pressure losses through the IC and such). The blow off valve vents pressure. Having it anywhere on that pressurized section should be the same. The air waiting at the tb doesn't have to be the first molecules of air that escape from the bov. The main concern with bov placement is the fact that it's a hotwire maf car. So you're venting metered air and have no way to account for that on the ecu then you're going to run into some stalling issues.

[wgknestrick]

It's a pressurized system. So the air at the tb will be at the same pressure as the air at the intercooler inlet (ignoring pressure losses through the IC and such). The blow off valve vents pressure. Having it anywhere on that pressurized section should be the same. The air waiting at the tb doesn't have to be the first molecules of air that escape from the bov. The main concern with bov placement is the fact that it's a hotwire maf car. So you're venting metered air and have no way to account for that on the ecu then you're going to run into some stalling issues.

This isn't enitrely correct in theory or my real world experience. The intake system is not a "pressure vessel", it is a flow based system with direction. The turbo is always spinning when the engine is running and you ALWAYS have a flow of air from the intake to the heads. This air has mass and therefor inertia and will want to flow in that direction based on newton's laws.

If the BOV just "vented pressure" then the BOV dia would not matter and you could have very tiny BOVs. It is in fact venting a volume of air and needs to do this in a split second to keep the turbo wheel from stalling out. The turbo wheel can spin (which you want) as long as there is room for more air in the system.

Putting the BOV right at the compressor outlet normally causes compressor surge on throttle lifts with any large turbo car. This is because (like my post above) the air has to stop it's direction of flow and reverse back out thru the BOV. The air in the intercooler and charge pipes has pressure and will travel back thru the BOV.

If you were driving on on a desolate back road and a bridge was out 2 miles ahead. Would you want the detour sign to be located at the turn where you could bypass it, or right at the bridge itself causing you to turn around and travel back to the alternate route? Which case would cost you less time?

The flow is the key. This is why bigger turbos make more hp at the same psi as smaller turbos. Inlet pressure means very little as it is just a measurement of flow restriction thru the engine.

[Wiisass]

You need to consider how a blow off valve works. There are two pressure references on based on the pressure in the pipe it is attached to and the other from that vacuum line on the top to the intake mani. When the throttle plate closes the pressure in the engine falls into vacuum and the pressure in the piping will start to increase. The pressure does not increase uniformly throughout the pipes, but starts at the throttle body and builds up until it reaches the turbo. This pressure will keep building until some of it is vented through the blowoff valve. So once there is a great enough pressure differential to overcome the spring in the bov, it opens up and vents out that pressurized air.

The reason you may have problems with it when the bov is closer to the turbo, is the amount if time it takes to build pressure all the way from the tb to the bov point. It's not because the flow has to reverse. What would be driving that reversed flow, would the engine be pushing air out of the intake mani or would there be such a great pressure drop at the turbo that it causes the air to get sucked back. I don't think you realize how much energy it would actually take to reverse flow.

And your examples aren't very good. I hope you understand this now, I can try and look up some other sources that agree with me if you want. But I'm sure that if you think about it, you will realize that this is how it really is.

[markley02]

Maybe this is just the "ricer" side of me talking here, but I wonder what two different brand BOV's would sound like going off at the same time.

[wgknestrick]

It's not because the flow has to reverse. What would be driving that reversed flow, would the engine be pushing air out of the intake mani or would there be such a great pressure drop at the turbo that it causes the air to get sucked back. I don't think you realize how much energy it would actually take to reverse flow.

And your examples aren't very good. I hope you understand this now, I can try and look up some other sources that agree with me if you want. But I'm sure that if you think about it, you will realize that this is how it really is.

Well I tried to explain this civilized manor, but you still choose to argue over something you don't really understand:

The flow reverses simply because of a pressure differential. It is pressurized inside the pipes, it is not outside. When the BOV opens, well the pressurized air has to vent to the lower pressure. Please explain out your ass to me why this doesn't happen. Please shine your knowledge down on us ol great engineer. Please explain a situation where higher pressure just stays in one spot and doesn't "flow" to a lower pressure area. My examples are "no good" because you aren't intellegent enough to understand them. Look it up. You might want to take the word engineering out of your sig until you learn this.

[Wiisass]

Did I make you mad?

The air is always flowing into the engine. Even when you let off the throttle, there is still air entering the engine, either because the throttle plate is slightly open or throw some type of idle air control valve. So if the flow reverses, the air will be sucked out of the engine, the engine will stall. So the overall flow can not reverse anywhere in the intake track. Does this make sense to you?

Now when the throttle closes, the amount of the flow going into the engine is much less the amount of flow entering the intake from the turbo, so pressure start to build. Pressure begins to build at the throttle body and essentially keeps getting stacked on until it gets to the spot where the blow off valve is. At this point the pressure acting on the bottom of the blow off valve in the tube is high, while the reference pressure on the top of the blow off valve is taken from the intake manifold and is in vacuum at this point. So now the blow off valve opens. But air is still entering the intake from the turbo. So instead of that air continuing to pressurize the air in the intake, it is allowed to escape through the blow off valve. Since the air after the bov is not getting compressed like before it is allowed to expand. This does not mean it flows backwards, it is still flowing into the engine, but the volume of the air flowing into the engine is increasing. So some of this air may expand away from the throttle body, but the net flow is still going in the same direction as it always was.

So the blow off valve will stay open until the pressure in the intake tubes is equal to the pressure in the intake mani or it has dropped to a given pressure based on the spring in the bov and the amount of preload. Or it will stay open until you get back on the throttle and start building boost again. Either way, the air is always flowing into the engine.

I hope all of this makes sense, I don't like arguing about this stuff and turbos and engine stuff really isn't my strong suite, but I do know a little.

And I'm sorry my comment about your examples sucking offended you so much. I'll explain why they were wrong for this situation and maybe you will feel a little better. The first one about the car driving and having on way to get out but it was behind him a couple miles. That makes it sound like there is only one ball of air, not a continuous flow.

The second one and saying how the amount of power a turbo can make is depentent on the flow and not the pressure. That's not wrong, I just don't see how you're tying that into this discussion. I mean maybe if we were discussing the cross sectional area of the outlet of the blow off valve or something like that, but in this case, it's not important.

So now if you would like to continue this discussion, please actually pick out the parts of my posts that you don't agree with and we can clarify them together. It would be a lot more beneficial for everyone that just calling me names and not providing counter points to my arguments.

[moorefire]

hahaha

[zerioustt]

lmao...like i said put them in the location of your stock recirc valves..problem solved

[anthracite93]

tim you always have that special way of mentally diffusing a situation...and you're right...as *almost* always ha

jim

[D Money]

hmmm....

Play nice kids.

[wgknestrick]

The air is always flowing into the engine. Even when you let off the throttle, there is still air entering the engine, either because the throttle plate is slightly open or throw some type of idle air control valve. So if the flow reverses, the air will be sucked out of the engine, the engine will stall. So the overall flow can not reverse anywhere in the intake track. Does this make sense to you?

Now when the throttle closes, the amount of the flow going into the engine is much less the amount of flow entering the intake from the turbo, so pressure start to build. Pressure begins to build at the throttle body and essentially keeps getting stacked on until it gets to the spot where the blow off valve is. At this point the pressure acting on the bottom of the blow off valve in the tube is high, while the reference pressure on the top of the blow off valve is taken from the intake manifold and is in vacuum at this point. So now the blow off valve opens. But air is still entering the intake from the turbo. So instead of that air continuing to pressurize the air in the intake, it is allowed to escape through the blow off valve. Since the air after the bov is not getting compressed like before it is allowed to expand. This does not mean it flows backwards, it is still flowing into the engine, but the volume of the air flowing into the engine is increasing. So some of this air may expand away from the throttle body, but the net flow is still going in the same direction as it always was.

1. You are correct in the statement that air is always flowing into the engine, no matter the state of the throttle body. I never stated that air flows "out" of the engine.

2. You are wrong in the statement that no air reverses flow from the throttle body (what I think this whole argument is all about). Here is some information why I think this is the case.

Here's a pic of my throttle body from my STI. It is closed in what would be considered the "idle" position. Note how small or little area is actually open to flow air. I've estimated the air flow at idle for my engine to be around 20 CFM based on 800rpm and normal vol eff at idle (which is very low).

http://users.rcn.com/wgknestrick/Throttle%20Body%20001.JPG

Here is a diagram with estimated/calculated data from my STI with regards to flow and pressure (NOTE this is the ideal BOV location with regard to flow):

http://users.rcn.com/wgknestrick/turbo.JPG

From the diagram above, you can see that the entire volume (20l=5 gal) of pressurized air (around 19-20psi) has to be evacuated from the intake tract by the BOV minus the flow into the engine in about .5s for the average BOV to vent down to vac. This is my "ideal" setup and keeps the flow moving in mostly the same direction.

If the BOV was located between the IC and the turbo, the entire 20l of 20psi air would have to be flowed through the throttle body (take another look at that pic) in .5 sec for the flow not to be reversed between the TB and the BOV.

So the question is can the little opening left in the throttle body outflow a completely open BOV (55mm dia) in order for no air to reverse flow out the BOV? There must be a reason that every OEM turbo car I've seen has the BOV right before the throttle body and it is for this reason.

There is very little air flowing through the TB with respect to a turbo that "was" and still is spinning at full clip (20CFM out vs 675+CFM in) when the TB is closed quickly. This extra flow (655CFM) PLUS the air that is already compressed (5g @ 20psi) has to be evacuated through the BOV in .5s

Yes, the engine does help move some air, but the 20CFM is so minute compared to the 580CFM the BOV has to move. So if you compare these ratios you will see that only 3% of the air is flowed through the TB, while the remaining 97% of air has to exit out the BOV. This will "reverse" the flow depending on where in the intake system it is located. Basically you want as little VOLUME between the TB and the BOV as that volume of air will travel back thru the BOV when the TB is closed.

I admit there are slight errors in the quantitative data in the example, but they are close enough to clearly draw a conclusion.

[Wiisass]

Well this is a much better argument than before.

So we can agree that the net flow is always in the same direction, into the engine, correct?

I think we're a lot closer to agreeing on this now. I just don't like how you say reverse flow, I tend to think that the overall direction of flow has changed and that just can't be true.

Your example is pretty good, I'm not going to check any of the numbers or anything, but it seems to make enough sense. I would just think that the flow estimate for the throttle body might be low because unless you shift like a little old lady the engine won't be at idle. The only thing that it seems to be missing is the large amount of air still be pushed into the engine by the turbo. I think that it is mainly this air that is being released through the blow off valve. I also think that the air in the intake is allowed to expand, which means that it has to move backwards, but it is still flowing into the engine. So if that's what you want to call reverse flow, then you can and this whole thing can be settled. But the flow direction still doesn't change.

I would be very interested to see the pressure variation through the intake tube during this whole process. I don't think the whole system goes back into vacuum. And if it does, you should be shifting faster so that the blow off valve hardly has to open.

There are also other things to consider which I don't really want to get into. Like pressure waves and other air flow through a pipe dynamics.

About the throttle body placement, I think that most OE's will put the bov closer to the throttle body so that it will open sooner, based on what I said before and how the pressure builds in the pipes.

Bottom line, a wise man once told me, "Real men don't run blow off valves."

That is all.

PS. I will not argue that last statement, the person who said that is infallible so it has to be true.

[wgknestrick]

Well this is a much better argument than before.

About the throttle body placement, I think that most OE's will put the bov closer to the throttle body so that it will open sooner, based on what I said before and how the pressure builds in the pipes.

Bottom line, a wise man once told me, "Real men don't run blow off valves."

That is all.

PS. I will not argue that last statement, the person who said that is infallible so it has to be true.

Yeah we both realize that the moment you step off the throttle, the engine is still at around 6-7K RPM, but the TB is closed. The air is only being drawn thru the TB at little more vac (but still less vac than -14psi). The main variable in flow rate however is area which is small in the closed TB case. My example doesn't account for that and is flawed, but the order of maginitude is what I was after. I still don't think the airflow with the TB closed will ever eclipse 40CFM regardless of RPM. You can only flow so much thru such a small area (which is exactly the point of a TB).

I think we just got mixed up in our frames of reference and I wasn't able to explain my point very well in just text.

It is good we came to a conclusion on this and I think we both taught each other something (and hopefully other people) in the process.

As far as the no BOV statement, I point back to my example as the prime reason they are necessary in turbo charged, and centrifugal SC engines.

[Wiisass]

I'll drink to that.

[Twinz]

wow thanks didnt mean for this to turn into a physics debate but very useful information guys...and as for the others...placing in the stock place is probably best bet...just gotta find a way to mount it...just easier to have a hard pipe welded to mount bov...and for the other guy...2 different bov's would be interesting haha like a high pitched hks and low pitched rfl...sound like 2 cars haha