What is the purpose of a Blow off valve(BOV)??? How does it work and why???

[genesmob]

A blow off valve is a pressure relief, it is designed to open, and relieve charge air pressure when the throttle plates are closed under boost. This pressure relief prevents turbo surge. Turbo surge occurs when the throttle plates are closed under boost, the air supply to the engine is cut off, and the exhaust supply to the turbo is cut off. If the engine can not intake air, it obviously will not have air to exhaust. When the exhaust pressure is removed from the turbo, and charge air pressure is still present the compressor wheel is thrust back into the turbo housing. In a turbo with a journal bearing when the compressor wheel is forced back into the turbo housing a thrust bearing bares this load. In a ball bearing turbo there is no thrust bearing, the ball bearings would bare the load. The purpose of a BOV is to protect these bearings. However out of the hundreds of turbos we have sold without a BOV I have yet to see one damaged by turbo surge.

In the snowmobile industry a BOV is also said to enhance throttle response, by allowing air to leak from the charge area at idle, or with a snap of the throttle. It has been said that if the pressure is allowed to leak under these circumstances, the turbo compressor wheel will maintain momentum, and recover boost quicker. A BOV used this way is typically adjusted to be extremely sensitive by using a light weight spring. A diaphragm style BOV is also preferred for this type of use, because of its ability to pull open at idle. In fact a BOV set up for this purpose will often open under part throttle conditions, such as backing off the throttle partly when making a climb, or cruising down the trail.

A BOV is closed by a combination of boost, and spring pressure. The boost pressure that closes the BOV is supplied through a boost hose located on the manifold side of the throttle plates. This means that boost can not assist in closing the BOV until boost is being built in the intake manifold, then this manifold boost has to travel through a boost hose, and fill the back side of the BOV before assisting the return spring, and closing the BOV. The heavier the BOV return spring the quicker the BOV will close, and the quicker the turbo will build boost again. A light spring will close the BOV slower, in turn the turbo will rebuild boost slower. Under either circumstance the turbo will start to build boost before the BOV can slam closed. As long as the BOV is open you have a boost leak. Boost leaks are bad for throttle response. Claiming that a BOV will enhance throttle response could not be further from the truth.

We have an R&D team of the best riders in the world testing this stuff, and in every circumstance we have come across with a two stroke turbo, from drag racing in the east, mountain boondocking, the hill climb circuit, big shoot pulling, to even just pounding down the trail. Rather we add a BOV to a turbo that does not already have one, or we remove a BOV from a sled already equipped, the answer is always the same, no blow off valve= better throttle response.

You don’t have to take my word for it. Take your sled out, when its running good pull your BOV, and replace it with a block-off plate, you’ll see.

Junior

Great post
Over the last two years Ive tried this numerous times. With bov and without bov , greddy and vortech bovs and a large Tial. soft springs that are open at idle and stiff springs. Ive come to the conclusion that, on my sled, a bov is unnecessary and does not help with throttle response.

 

[jijohans]

Nice discussion here.. But how long will the bearings and gaskets last in a non BOV setup? I've been with cars where the turbine axle snapped and turbin wheel vent out the exhaust...The sound is great, but the risk of ruin the turbo is not worth it. I'm going to test my m8 BD PG kit with and without a Tial valve...

 

[manowar]

in a two stroke i think the pressure disapates fast enough to not put to much strain on the turbine wheels and bearings .

 

[turboxp]

bov

The biggest reason that you will get run on when you have carbs is the under boost it will keep your slides from closeing.

 

[Boyko]

I always run one, I do not like the sound dead heading and surging the compresor makes (sounds like a big a$$ bird chripping). Even at low boost, 7psi. I feel guilty not running one, I can't see it will making a differance in responce.

 

[Dano702]

The first time i saw my 8 idle with the BOV wide open it caught my eye. The thing i did not expect was the volume of air rushing out of the valve . When the sled is just at idle the compressor is spinning fast enough to make air come out .

So when it is at WOT and i slam shut the throttle without a BOV where does all of that pressure go?

I think the compressor is spinning faster at idle because the valve gives it some where to go. Why else would there be a lot of air rushing out of it? It must be building more psi than the engine can draw in at idle.

I love a good debate , this reminds me of BigBull's private turbo thread . Good stuff.

 

[mountainhorse]

Sincere question... How many actual examples do you (people posting on this thread) know of, on a 2 stroke sled, where there was a turbo failure related to compressor surge or chatter?

 

[jijohans]

Non 2 strokes, but several cars with 4 strokes..

 

[powderlites]

Blowoff Valve

We always use a good blowoff valve that is wide open at 7 inches vaccum on 4 strokes.
A lot of blowoff valves are also junk and don't work right or never have been setup.
Diaphram we use has our own mount design and is big enough for our turbo kits.
Blowoff valve always gives better throttle response when built right.
Blowoff valve does not cause lag!!
www.powderlites.com

 

[BigAir]

Blowoff valve always gives better throttle response when built right.
Blowoff valve does not cause lag!!
www.powderlites.com

powderlites,

Do you believe the same to be true for 2 strokes or are you only talking 4 strokes?

 

[Super Dave3]

here is the biggest reason every one of my kits has a bov with out one even at 4#s at 500 miles the intake of the turbo is coated with oil a real mess witch does not affect performance , but where the performance is affected is on a long run where you get in and out of the throttle is starts to puff oil out the exaust witch means its going into the intake also witch creates loss in power and change in fueling.

X2 I will agree with Shain on this one. I did not notice a huge improvement on throttle response, maybe a little, but it did eliminate the oil mess that he is talking about. I am on the fence with the kit I am trying this year as to use a BOV, probably will try it without, but if I have the oil mess I wont hesitate to run a BOV.

 

[canucklehead]

Question for the experienced guys... if you were getting 5psi vacuum from one cylinder and T'ed it with the other cylinder would you go to 10psi or just a constant 5psi rather then the bouncing from 5-0-5-0-5-0-5...?

Can you go too light on the spring?

 

[devilrmk]

Yes you can go too light on the spring. The lighter the spring the slower its going to close.

I think the guys that think that the BOV adds lag don't understand how to set them up properly.

I just fiddled with mine for about 30 minutes getting the spring right and the valve closes immediately when the throttle is even touched, well before boost. Mine is closed at idle but opens anytime I back off the throttle. Not much seat time with it yet. Well see how well it works.

 

[10001110101]

quote "Question for the experienced guys... if you were getting 5psi vacuum from one cylinder and T'ed it with the other cylinder would you go to 10psi or just a constant 5psi rather then the bouncing from 5-0-5-0-5-0-5...?
"

connect it to both throttleplates then it will be more of a constant -5, rather than bouncing. On the turbo phazer 500 twin you have to run a small vaccum canister (basically a piece of 1 inch tubing 3 inches long) to dampen the pulses and give the map sensor a steady reading instead of a bouncing signal. Too big of a canister and the sensor sees a lag in signal.

quote "Can you go too light on the spring? "

in some bov's yes, if the piston area is greater than the spring pressure you will always have a boost leak due to the boost being able to push the piston up and overcome the spring, even with adding boost pressure to the top all you are doing is equalizing the pressure on both sides of the piston and if the spring is too light you will leak!

I personally like the Tial 50mm BOV the best due to the fact that the diaphram area is greater than the area of the piston itself. This design actually allows this bov seal 100 percent with no spring, but the problem is with no spring you have to build up pressure for it to actually close and it makes the engine very very laggy! So all you need is a very light spring just long enough to keep the valve closed when the engine is off and this laggyness goes away and makes a very responsive valve that has capacity to bypass as much air as you can possibly move, This valve works excellent in two stroke carb applications.

 

[milehighassassin]

Well for starters Diesels dont have throttle plates. New engines like the latest Duramax are an exception. I have seen lots of High performance diesels with a BOV. 6.0L fords stock in the right situation you can get them to stall the compressor. Even stock but you have to work at it. Long story short most truck diesel truck engines dont need them.

If you are running a really big fuel plate with bigger injectors your stock turbo on a 12 valve is possibly undersized. You might want to see how much boost you are running start getting up around 35 plus you could be looking for a falure.

Yes and no. Diesels do not have throttle plates for air flow is pulled in by the motor, no regulation is needed. Basically runs off of injection pulse.

The newer diesel that "have them", well they are there for emmissions reasons, to help on shutdown and to help prevent a "runaway" condition.

That plate closes when you turn the key off to keep the truck from shaking like old diesels did. It is also a safety device so if your turbo lets go, the motor doesn't consume all of it's own oil/blood and run it self dry.



THERE IS ZERO need to put a BOV on a diesel. You can do it but it serves no point other than to say you did it and to put one more piece that can fail.


Just for fun lets throw the word "Divertor Valve" out there. Basically the same as a BOV but it diverts the pressure back into the charge pipe so that you do not create a rich condition in the motor from venting metered (MAF) air out of the intake.


Edit... BOV are street legal, but like I said above OEM turbo cars/trucks don't use them because they create a RICH condition.

 

[chrisco]

Sounds good to me who can sell me a proper set up bov for a d8 pump gas kit at 12psi.i also just wanted to say thanks for this thread and keep up the useable info.also people running bd kits with the air intake under the hood fab up something to get fresh air wow what a difference in power and performance and charge tube temp no more hot wet air to me this is a flaw in the kit and needs to be done at install.the first time you plow into a drift or cross a creek (because that never happens)and your sled falls on its face from steam under the hood,not good after comming out of the hood with the filter no more inconsitant running.i had to turn my boost controler in 5 turns just to get back to 12 psi
best thing you can do to a bd kit.

 

[FLASH1970]

I saw some good info on here. I may have missed something but I'd like to give my 2 cents on BOV's. First off, they are legal. All my Eclipses and Talons have them from the factory. Even my VW has a Diverter Valve which is basically the same thing.

Whether it vents to the atmosphere or is recirculated depends on type of Air Flow Sensor setup you have.
If you have a "Draw through" type of turbo setup you want to have the BOV recirculated. The turbo is sucking air through the MAF so your ECU recognizes the amount of air taken in and delivers the correct amount of fuel. If air is vented to the atmosphere on a Draw Through setup, you will run rich for a second because your ECU is injecting fuel to compensate for air that is no longer in the system.
If you have a "Blow Through" setup there is no harm in venting to the atmosphere. Typically on these setups the Air Flow Sensor is closer to the throttle body after the BOV on the intake plumbing. So, if the compressed air is vented to the atmosphere via the BOV, it is just releasing air that hasn't been registered yet by the ECU.

I didn't see this aspect of BOV's covered so I apologize if it already was.