4mm or 6mm vacuum line for BOV?

pysical

Sergeant
Jun 16, 2017
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I am going to have the valve cover and intake manifold off this weekend and want to get the line to the Tial BOV fixed. I have read a lot of people saying to go to a 4mm but Tony said a 1/4" line is what he recommends, which is 6.35mm. He recommends it here in this video..


I just want to make sure I get the right size ordered.

Thanks.
 

doublespaces

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Oct 18, 2016
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I run a 1/4" barb on my intake manifold and my bov opens very solidly, no squeaking/wheezing sounds.
 

Clean WHP

Corporal
Apr 24, 2017
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2010 135i Msport 6MT
im running a 5/16" barb and a 6mm line on my HKS. Amazing response. BOV blows off perfectly every time
 

19E92

New Member
Nov 20, 2016
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In the video, Tony says that the BOV should be open during idle, but I've read that (TiAL) BOVs should be barely cracked open in turbo applications during idle. I've read that only for supercharged applications should the BOV need to stay open during idle.

From what I understand, having the valve open during idle means that the turbos need to work harder to spool and actually introduce some turbo lag, since it's technically a boost leak.

Can someone please clarify, as I'm having trouble finding the right spring for my N54. Thanks!
 

doublespaces

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Oct 18, 2016
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In my opinion, that is just not true in the real world. There is not going to be any measurable lag. Have you seen a bov actuate? It's nearly instantaneous. Before you can get your foot down all the way I bet that thing closed. Even if that was true and it made a noticable difference... Would you rather have a little lag or ruin your turbos over time.

Remember you don't spool during idle while this valve just sits open. The moment you touch that pedal the vacuum is gone and you've got a 6-8 psi spring pushing that valve closed immediately. Long before any measurable boost builds up or escapes and its closed way before your car can down shift.

Now someone can come in here and argue about min/maxing and talk about transient response. But again, would you rather have the turbos operate properly or not. There's always some timing left on the table as well, it's just a matter of what do you consider healthy.
 

JDKruger4

Private
Aug 10, 2017
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335i LCI
I don't have any experience with BOVs but sitting open seems wrong to me. At idle the manifold is under vacuum (like any NA motor would be) this is the vacuum that holds the BOV open (since connected to manifold). It would follow that the chargepipe and intercooler would be under a slight vacuum as well at idle. If that's the case, and the BOV is sitting open then idle air is going to be pulled through the BOV as the path of least resistance. I personally don't want unfiltered air going into my motor at idle.

What am I missing here?
 

fmorelli

Major General
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Aug 11, 2017
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Wouldn't that imply that there is no manifold vacuum, if the turbos are flowing more air than being ingested at idle?

Filippo
 
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doublespaces

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Wouldn't that imply that there is no manifold vacuum, if the turbos are flowing more air than being ingested at idle?

Filippo
No, that's why there is a bov. To by pass that airflow. The throttle blade is the obstruction, and so it goes out the bov hole. Have you guys never held your hand near the bov at idle? Air comes out not in.
 
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Oct 24, 2016
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Tyler nailed it. Wrong is thinking that Tial is wrong. Leave 'er slightly open at idle. I have mine wide open at idle. Surge is bad. 30k+ on my old stage 2+'s still going strong.
 

doublespaces

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There is a difference between idle vacuum and driving vacuum so it's possible you are right. But have you measured how much vacuum your engine draws? Otherwise you are sayin that but don't know if it's true or not.