How does it work?

Honda Importz01

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Im in this like big deal problem on figuring out the true technical answer on how a Blow Off Valve works. I searched How Things Work, but didnt really find anything.
Ok...
I know that when it builds up pressure it releases the pressure some how. But how does it know to blow off when you let your foot off the gas? and so on...
 

blacknight

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The blow-off valve is more commonly used to keep the turbocharger spinning
when the throttle plate is suddenly closed. When the turbocharger is
generating maximum boost pressure at full throttle and then the throttle
is suddenly closed, compressed air coming from the compressor slams
against the throttle, generating extremely high pressures that travel
backwards to the compressor stopping the compressor from spinning. When
the throttle plate is again opened, the engine must spool the turbocharger
shaft again. The effects of this high pressure can also be very damaging
to the turbocharger.
 

Wildman

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basically when you're on the gas the turbo is spooling up and building pressure. pedal on the floor of your car next to the brake pedal is actually a throttle pedal, not a gas pedal. the more you push down that pedal the more the butterfly valve in your throttle body opens up, and the more air it lets in. your car then adjusts for the increased air by adding more fuel.

when you let off the gas, the butterfly valve closes and prevents air going into the engine (not completely, obviously, but mostly). with the butterfly valve closed, all the air pressure that your turbo is building up has trouble finding a way out of the piping, and will then start to back up towards the turbo itself and can cause it to stall.

that's what the blowoff valve is for - when you let off the gas, and close the butterfly valve, it provides an escape for all the pressure inside your intake piping.

edit: okay Eric already basically said that lol :rolleyes:
 

Honda Importz01

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but what prevents from blowing off even when your throttle is open? cant some escape from it?
 

Totalimmortal

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A BOV is vacuum actuated. The nipple you see on top connects to the throttle body/IM area. When a vacuum is present, when the throttle plate is closed, it sucks the diaphram open. When your under boost, it pushes it closed. Now, there is also a spring that will open at a certain PSI or boost pressure. Helps prevent boost creep.
 

blacknight

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Honda Importz01 said:
but what prevents from blowing off even when your throttle is open? cant some escape from it?

A BOV is only engaged when pressure is being released (Blown through the valve) During vacuum suction the BOV is disengaged because the throttle plate is opened and sucking condensed air into the the intake manifold, to be mixed with the proper fuel to creat power.

When (and only when) the throttle plate is released to a closed position (It's never fully closed-even at idle) all the pressurized air that never got into the thrttle body, has to be released....through the BOV. It can't go back out through the intake filter via the turbo, because the turbine is already spooled. Doing so will desrtoy the turbo


Here's an example.

Imagine you're really hungry and you are eating your favorite meal and inhaling air through your mouth. (That's the turbo spooled and the throttle opened and sucking in air)

Now, when time to exhale, YOu can't do it through your mouth (the turbo) because you still have food in there (Turbo still spooled)
So you exhale through your nose...Blow Off Valve
 

Totalimmortal

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blacknight said:
A BOV is only engaged when pressure is being released (Blown through the valve) During vacuum suction the BOV is disengaged because the throttle plate is opened and sucking condensed air into the the intake manifold, to be mixed with the proper fuel to creat power.

When (and only when) the throttle plate is released to a closed position (It's never fully closed-even at idle) all the pressurized air that never got into the thrttle body, has to be released....through the BOV. It can't go back out through the intake filter via the turbo, because the turbine is already spooled. Doing so will desrtoy the turbo

Not quite. Vacuum is present when the TB plate is closed. That vacuum actuates the BOV, which vacates the excess boost created by the compressor.

To further get into compressor surge. If the air(boost) isn't vacated, it will cause whats called compressor surge. Your compressor wheel is spinning in one direction, compressor surge will actually stall the wheel and even make it spin the opposite direction. This destroys the bearings VERY quickly.

They're are two types of bearings in a turbo, thrust and ball bearing. Thrust bearings are the most common, and consist of what can be described as a ring that floats between the shaft and the housing. Lubricant is obviously used. They're are 270 degree and 360 degree thrust bearings, I think you can figure out what that means from the picture. http://www.newindia.com/kmp/pics/c3-3.jpg
Ball bearing is just what you think it is, a sealed ball bearing. Just like in a skateboard, just a lot nicer. You can get single and dual ball bearing turbos. Dual having a ball bearing on both the tubine wheel and compressor wheel. Single having a ball bearing on the compressor(cold) side and a shaft(thrust) bearing on the tubine(hot) side, most of the time. I've never seen it any other way.
http://www.airpowersystems.com.au/350z/turbo/ball_bearings.gif

Thats as far as Im getting into turbos tonight ;)
 

talontsiawd

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There are two types. Both work basically the same way. In a regular BOV, there is a spring that keeps the valve shut. There is equalibrium in pressure between the top, which seems vaccum or boost though vaccum lines, and the bottom which sees boost under boost. When you let off the throttle, the top no longer sees boost, but the bottom does, cause it to force the valve up, against the will of the spring.

The second type, usually used in high boost applications has the top and the bottom linked together. This creates and automtatic equalibrium. And then under boost, the vacuum hose on top gets more boost, actually forcing the valve down. Since the top and bottom sections are connected, it's harder for the valve to "blow off". That's why this is far less common and only used in high boost applications. And a normal bov will usually do fine with the right spring or adjustment in high boost, making this type fairly rare and pretty much obsolete.

But, the main idea is equalibrium in pressure. When the vaccum lines see the same amount of boost as the intercoooler pipes, the spring in the BOV keeps it shut. When the vaccum lines stop seeing boost, when there is still boost in the piping, the boost forces up the spring, opening up the valve.


Btw, valve and diaphram are used as the same term as they are the same principle. The valve is the actual piston, the diaphram puts pressure on the valve, in conjunction with the spring.
 

retroshark

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the ssqv bov's actually use the pressure from the turbo to seal it closed, they are the best bov's onbt eh market :)
 
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