some questions about blow off valves

[ahmedsalem89]

hi guys,
for long I thought that blow off's are just a simple spring valve system that opens under enough pressure to release excess air.
but what I still do not understand is why does it have a vacuum nipple?

I'm wondering so because something that happened to me while driving: the adjustment screw of my Greddy BOV went airborne while on the highway (talking about cheap replicas :S) and with no screw it vented to the atmosphere, what I noticed is that the BOV acted like it's 90% shut (I kept hearing the comp. surge after every rev) and only opened under very heavy pressure.

and acceleration improved noticeably how is that??

after that I put duct tape where the screw used to be for testing and now the BOV is opening properly and acceleration declined.

my question is, what the use of vacuum for BOVs? and how does it affect acceleration ?

thanx in advance

 

[dac69er]

in simple terms the vacuum port on the bov is used to push the valve shut when under boost and then draw the valve open when you let off so it can dump the excess boost pressure.

do you have any kind of boost controller fitted?

 

[ahmedsalem89]

in simple terms the vacuum port on the bov is used to push the valve shut when under boost and then draw the valve open when you let off so it can dump the excess boost pressure.

do you have any kind of boost controller fitted?

thanks for your reply
but I believe this also happens if the vacuum is not connected to the BOV vacuum port and venting to the atmosphere instead, the BOV holds shut until you take your foot of the throttle.

and no I don't have any boost controllers

 

[dac69er]

it does to a degree as it just relies on the spring pressure to hold it shut, then the valve will open when you close the throttle as the pressure will overpower the spring and push it open. without the vacuum line connected the valve works more like a pop off valve or pressure relieve valve where it just releases the pressure when the spring is overpowered.

 

[ahmedsalem89]

it does to a degree as it just relies on the spring pressure to hold it shut, then the valve will open when you close the throttle as the pressure will overpower the spring and push it open. without the vacuum line connected the valve works more like a pop off valve or pressure relieve valve where it just releases the pressure when the spring is overpowered.

thanks it's getting clearer now, but I still have a couple of questions: 1-is the vacuum that strong to hold it shut/ how does it work?
2- how did it affect acceleration in my case ?

 

[dac69er]

its the boost pressure in the manifold that pushes it shut. it just aids the spring.

when you let off the boost pressure goes, changes to a vacuum and it will aid the valves response time to open.


i cannot see how it would affect the boost in your case though. unless your vac line is taken off the actuator???

 

[ahmedsalem89]

its the boost pressure in the manifold that pushes it shut. it just aids the spring.

when you let off the boost pressure goes, changes to a vacuum and it will aid the valves response time to open.


i cannot see how it would affect the boost in your case though. unless your vac line is taken off the actuator???

it's all clearer now, I too cannot understand how it affects acceleration I noticed that when the vacuum is venting to the atmosphere the car is more aggressive specially when coming from a stand still.

I'll put the stock BOV this weekend and notice if there's any difference.

"unless your vac line is taken off the actuator???" could u please elaborate more on this part

 

[Rev]

it's all clearer now, I too cannot understand how it affects acceleration I noticed that when the vacuum is venting to the atmosphere the car is more aggressive specially when coming from a stand still.

One possibility is you are going back on the boost while the turbo is still spinning the turbo will feel more responsive and pressure will build much quicker when it is already spinning.

 

[ahmedsalem89]

One possibility is you are going back on the boost while the turbo is still spinning the turbo will feel more responsive and pressure will build much quicker when it is already spinning.

even if it's from a standstill? I just put it in first gear, press the pedal to the metal and I instantly feel the difference. and there's much improvement when the vacuum is plugged to the BOV but the screw port is open sucking air

 

[corofin12345]

even if it's from a standstill? I just put it in first gear, press the pedal to the metal and I instantly feel the difference. and there's much improvement when the vacuum is plugged to the BOV but the screw port is open sucking air

trying to get my head around what your saying???

1. if theres no vacuum line going to BOV, the BOV wont operate.. so it will be closed all the time..

2. if the vacuum line is connected to BOV, it will open...

3. if your getting what seems to be more power with vacuum line off, then your BOV is faulty "opening under load/boost = loss of boost pressure"

 

[dac69er]

sorry, didnt quite get what you meant to begin with. you are creating a vacuum leak which can increase throttle response. in effect you have made you own ecotek valve

 

[ahmedsalem89]

trying to get my head around what your saying???

1. if theres no vacuum line going to BOV, the BOV wont operate.. so it will be closed all the time..

2. if the vacuum line is connected to BOV, it will open...

3. if your getting what seems to be more power with vacuum line off, then your BOV is faulty "opening under load/boost = loss of boost pressure"

thanks for ur reply, the vacuum line is connected to the BOV which is Greddy type RZ, it has a screw to adjust it and that screw went off and left it's hole sucking air. ( so it's like the vacuum is not connected to the BOV)

the BOV now acts like it's nearly shut and only opens under very heavy pressure. and I can feel more power from the moment I push the pedal,

sorry, didnt quite get what you meant to begin with. you are creating a vacuum leak which can increase throttle response. in effect you have made you own ecotek valve

it'll clear out what's the reason of this improvement if I use a trusted blow off like the stock one right??

 

[dac69er]

the vacuum will have some affect, but not as much as it is leaking out of the hole. it wont have any more power, you will possibly get more throttle response from idle and the extra power you 'feel' is most likely the extra noise you are getting from the vac leak making it sound more powerful. or your bov is defective in some way and not working as it should!

if you have a vacuum leak the car wont run as it should. get a screw put back in he hole.

 

[ahmedsalem89]

the vacuum will have some affect, but not as much as it is leaking out of the hole. it wont have any more power, you will possibly get more throttle response from idle and the extra power you 'feel' is most likely the extra noise you are getting from the vac leak making it sound more powerful. or your bov is defective in some way and not working as it should!

if you have a vacuum leak the car wont run as it should. get a screw put back in he hole.

no it's definitely power I can feel it, and when I used something to cover the hole in the BOV this extra power disappeared, so It's 90% a leaky BOV right?

 

[ahmedsalem89]

last weekend I put the stock BOV back on and to my surprise it sounded very loud even louder the the Greddy and the car now is pulling greater the only issue is the stock BOV sometimes is stuck open when the boost gauge is showing between 0 and 0.2 under, any ideas?

 

[dac69er]

its not stuck open, unless it has a real stiff spring or is a valve specific for a car with an air flow meter the valve will open when on idle. its working as i should

 

[ahmedsalem89]

its not stuck open, unless it has a real stiff spring or is a valve specific for a car with an air flow meter the valve will open when on idle. its working as i should

I remember when I first did the engine swap it was working fine, it was still recirculating to the intake pipe before I replaced it with a short Intake.
so it's supposed to do so? I mean shouldn't it stay closed all the way till the throttle is shut?

 

[dac69er]

on idle/tickover and possibly slight acceleration it wont. it should stay closed when you apply the throttle, so yes, generally it will stay closed unless the throttle is shut.

 

[ahmedsalem89]

on idle/tickover and possibly slight acceleration it wont. it should stay closed when you apply the throttle, so yes, generally it will stay closed unless the throttle is shut.

I tried slight acceleration and different pressure on the pedal to see exactly how it behaves, I found that no matter what, the BOV won't open unless my foot is completely off the pedal. EXCEPT when I keep the pedal slightly pressed that the car will keep it's speed and won't accelerate more and the boost gauge stays between 0.2 vacuum and zero, the BOV will stay open no matter how long I drive on this state until I press more on the pedal.

 

[dac69er]

I tried slight acceleration and different pressure on the pedal to see exactly how it behaves, I found that no matter what, the BOV won't open unless my foot is completely off the pedal. EXCEPT when I keep the pedal slightly pressed that the car will keep it's speed and won't accelerate more and the boost gauge stays between 0.2 vacuum and zero, the BOV will stay open no matter how long I drive on this state until I press more on the pedal.

that is exactly how it should behave. its fine