12 replies to this topic

[ChrisBruggemann]

Now i'm sure this has probably come in the past.. If so i apologize.

I am in the process of completing my fourth year as a motorcycle mechanic and as apart of my training I get to do the turbos and blowers training book. (most of my training books are car related anywho)

Now i have sort of always been a believer in the blow off valve but my trainer begs to differ with me, he sees no use for a bov other then 'wank factor'.

Now my understanding is that with an intercooler and piping and what not the compressed air has to be relieved somewhere when the throttleplate or butterfly is shut off. Ie blow off valve. But my trainer seems pretty confident that unless you are running 25+ psi boost then the wastegate is sufficient.

Now where i get confused is the fact that a wastegate regulates boost pressures by bleeding off exhaust gases past the turbine. Now I have looked at some diagrams in my training book but not done too much investigating into it, and one diagram i came across has the compressor housing with the hose fitting running into the wastegate actuator as normal. But it also shows a bleed to atmo type setup from the actuator. I cannot understand how an actuator can work if the diaphram has to hold pressure in order to work, but if my trainer is right can also bleed of boost when the butterfly is shut off?

Anyone shed any light? Sorry if i've blabbed on and could have mad things simpler.

Chris.

[ChrisBruggemann]

My trainer seems pretty damn certain that i wont have compressor surge if i remove my bov. Btw, this is on a 4g61t cyborg. I just can't comprehend how such a small vacuum style fitting can release so much boost before the turbo surges.

[JiMi]

the wastegate is in no way gonna releive pressure from your cooler/pipes etc...

there has been arguments time and time again about compressor surge doing damage and i'm not gonna go into that... the truth is.. with out a BOV it IS going to happen to some degree in any setup...

at the point when the throttle closes there is a closed area between the compressor and the butterfly.. there is no where for the compressed air to go other than back thru the compressor in the turbo or out a bov

if you want to get away from the wank factor plumb back FTW

Edited by JiMi, 15 October 2008 - 01:18 PM.

[to4garret]

My trainer seems pretty damn certain that i wont have compressor surge if i remove my bov.

correct, you are confused, your trainer isnt...

by removing your bov you will increase the chances of "reverberation" through the inlet track, air is like a spring... as you mentioned, TB closes, pressure builds up, pulse wave flow backwards giving that "full sik uleh tutututttuuututuuu" sound

which also has the lovely affect of trying to pass through a spinning compressor wheel, its not the air actually passing backwards through the compressor, rather the "pressure wave", which still has a rather nice "stalling" effect and infact can get so violent as to spin the nut off the end of the turbine shaft.

side note, older WRC cars (not sure about the new ones) dont run BOV's (but my tutor told me that too)

compressor surge, is a different kettle of fish, generally occurs with what could be considered "mismatched" turbine and compressors (generally large compressor, small turbine)

pretty sure you couldnt get a 16G surging

do some googling into "anti surge compressor covers", you should also be able to find out what conditions surge happens as well.

[JiMi]

pretty sure you couldnt get a 16G surging

ive got a cordia with a vr4 conversion and no bov... its has a massive surge through the 14b

by removing your bov you will increase the chances of "reverberation" through the inlet track, air is like a spring... as you mentioned, TB closes, pressure builds up, pulse wave flow backwards giving that "full sik uleh tutututttuuututuuu" sound

which also has the lovely affect of trying to pass through a spinning compressor wheel, its not the air actually passing backwards through the compressor, rather the "pressure wave", which still has a rather nice "stalling" effect and infact can get so violent as to spin the nut off the end of the turbine shaft.

cool.. never thought of it that way.. makes sence.. surely there has to be a point where the pressure needs to acutually escape and it will start going through the compressor though (on systems with big coolers etc)... there is going to be some air getting through the butterfly even if it is completly closed... but in a big system there is going to be backpressure on the turbo for quite a while if your relying on the little bit of air getting through the butterfly to equalise the pressure

[bazeng]

ive got a cordia with a vr4 conversion and no bov... its has a massive surge through the 14b

as answered,

by removing your bov you will increase the chances of "reverberation" through the inlet track, air is like a spring... as you mentioned, TB closes, pressure builds up, pulse wave flow backwards giving that "full sik uleh tutututttuuututuuu" sound

They are different things.

As for needing a BOV. You do not need a BOV, but it is ideal to have one.
A wastegate and a BOV are totally different things. A BOV is located on the intake side, a wastegate is located on the exhaust side. Both units are very similar in design.

The wastegate acts as a bypass for the exhaust gasses to pass once the turbo has reached a level or pressure. They can be integrated into the turbine housing or external. They simply apply pressure to a valve / flapper via a spring (tensions vary, most internal wastegates open at about 7psi). Once that pressure is reached via the hose from the inlet (turbo outlet), it starts to open up and allows the exhaust gas to bypass, which regulates the amount of gas passing the turbo.

A Blow off valve basically does a similar thing but instead of opening at a certain pressure, it stays closed on positive pressure/boost (you have a spring pressure with the assistance of your manifold pressure pressing down on the valve/piston). This keeps the blow off valve closed when making pressure and opening only when the blow off valve recieves vaccum from the inlet manifold (when the throttle body is closed)..... This allows the pressure before the throttle body to vent and allows the turbo to maintain its momentum.

A blow of valve is not required for certain applications. Some race cars do not use them as they have a tendancy to leak. Also in some drag applications, throttle release is not required (WIDE OPEN THROTTLE SHIFTING) therefor it is of no use.

Blow off valves can also be used to regulate boost, but this is not ideal.

Hope that makes some sense...
It's a little early in the morning to be thinking straight...


Now my understanding is that with an intercooler and piping and what not the compressed air has to be relieved somewhere when the throttleplate or butterfly is shut off. Ie blow off valve. But my trainer seems pretty confident that unless you are running 25+ psi boost then the wastegate is sufficient.

Now where i get confused is the fact that a wastegate regulates boost pressures by bleeding off exhaust gases past the turbine. Now I have looked at some diagrams in my training book but not done too much investigating into it, and one diagram i came across has the compressor housing with the hose fitting running into the wastegate actuator as normal. But it also shows a bleed to atmo type setup from the actuator. I cannot understand how an actuator can work if the diaphram has to hold pressure in order to work, but if my trainer is right can also bleed of boost when the butterfly is shut off?

[rob323]

The wastegate is on the exhaust side of the system, there is no way it could ever relieve the pressure on the intake side of the system. Trainer got head up bum.

[Evo-00x]

^^^ lol... BOV's are not required but they do serve a purpose as explained above by keeping the compressor wheel spinning at a faster momentum between shifts which in turn improves the amount of time it takes for the turbo to come onto boost. That's it... the sound is a wank factor as a plumb back unit will do the same thing.

[peasoup]

I've seen cars on decent boost (20psi) actually decrease lag by removing the BOV. This is when changing gears quick.

Have you noticed some turbos have those holes all the way around the mouth of the compresor cover, its to direct the surge.

I can't stand the sound of BOVs, whats the deal with every WRX driver living in 1999? Plumb back the valve!

[ChrisBruggemann]

As you have all pretty much stated. bov = inlet wastegate = exhaust they have nothing to do with each other and i just cannot explain that to my trainer :S

Thanks for the interest
Chris.

[mattrat]

TUTUTUTUTU killed my 14b turbo, bov all the way

[JiMi]

i'm planing on it... only bought the car a month before we went away.. drove it home on a permit and thats about it... FUN to drive.. got some repairs to do as it has had a minor front ender and then she will be reged within a couple of weeks from getting home from london

[kayarubin]

tututututtut killed my 16g.. spun the nut off the wheel... and in turn chewed my compessor wheel to shiz... plumb back bov's for the winnah!