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

[JSCC]

So is it possible to get both of these situations out of a BOV?? both of these take different signals, just wondering if it was possible..??

Certain BOV can be adjusted to stay open at idle, and close with boost. Most I think...
There are some larger Bov's that have a line for vacume and a line for boost. One line to the top of the BOV, and one to the bottom.

Cheaper BOV's like the one provided in BD's kit, can be adjusted with different springs. I have cut them up trying to find that perfect spot, but that has not worked to well for me.

 

[Fack]

The BOV is a way to bypass pressure in a system that would normally "dead head" and stall the compressor. This allows the compressor to stay spinning or "spooled". Then when you hit the flipper and the pressure is equalized and the BOV closes whallah!!! instant boost again.

-Fack

 

[JHG]

The BOV is a way to bypass pressure in a system that would normally "dead head" and stall the compressor. This allows the compressor to stay spinning or "spooled". Then when you hit the flipper and the pressure is equalized and the BOV closes whallah!!! instant boost again.

-Fack

I know this is the theory but in riding several low boost pump gas kits w/o blow off valves I've never seen the boost pressure stall the compressor. It just doesn't seem like in a low boost application there is enough pressure to cause all the problems that are being discussed in this thread. I see the need in a RG application when boost levels are higher, like in the 12+ range but less boost than that I've never heard of a compressor stalling.

 

[Jkinzer]

I know this is the theory but in riding several low boost pump gas kits w/o blow off valves I've never seen the boost pressure stall the compressor. It just doesn't seem like in a low boost application there is enough pressure to cause all the problems that are being discussed in this thread. I see the need in a RG application when boost levels are higher, like in the 12+ range but less boost than that I've never heard of a compressor stalling.

not saying my set up is the way to go, i may just have gotten lucky for the last 3,000 miles but i ran my sled with no BOV at boost levels from 10-13lbs consistantly and a couple climbs even as high as 14-15lbs and never had any issues at all, always instant throttle response and never once felt like it was even coming close to stalling the compressor

 

[savedbygrace]

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

 

[Angermangement890]

Car and trucks don't come stock with BOV's because they are not street legal. You can run a bypass valve and that is pretty much what skidooin'it is talking about.

Incorrect information. The 90-99 Mitsubishi Eclipse and Eagle Talon all had BOV's from the factory, running 9-12 lbs of boost pressure on turbos sizes from a 14b (smaller than a Evo III) to a T-25.

The 03-2010 Mitsubishi Lancer Evolution also has a BOV stock.

The 01-2010 Subaru WRX/STI also have BOV/Diverter Valves stock.

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

Recirculate your Bov, closed when in vacuum is preferable to open under Vac, Use an adjustable BOV, such as the HKS which closes via a spring and is sealed via boost pressure. Pretty sure your results will improve.

 

[JHG]

Incorrect information. The 90-99 Mitsubishi Eclipse and Eagle Talon all had BOV's from the factory, running 9-12 lbs of boost pressure on turbos sizes from a 14b (smaller than a Evo III) to a T-25.

The 03-2010 Mitsubishi Lancer Evolution also has a BOV stock.

The 01-2010 Subaru WRX/STI also have BOV/Diverter Valves stock.

Are these not compressor bypass valve (CBV), or diverter valves? I just read the law and it said that any Blow off valve or Dump valve the released air into the atmosphere wasn't legal.

 

[savedbygrace]

Recirculate your Bov, closed when in vacuum is preferable to open under Vac, Use an adjustable BOV, such as the HKS which closes via a spring and is sealed via boost pressure. Pretty sure your results will improve.

I agree a BOV that is adjusted to be closed under vacuum at idle is ideal for off idle throttle response, and the recirculation of boost can help to minimize the loss of throttle response when getting out of the throttle, then right back in. Even still, every time the BOV opens it dumps boost, boost that the turbo will have to rebuild when the BOV is closed again. This will translate into a loss of throttle response to the end user every time.

Junior

 

[product tester]

Jr you are right I have never seen bearing damage from not haveing a bov just seal damage. Last season half way through the season Nate Zolenger bought a bunch of bov's from me and put them on there race sleds and Improved Throttle responce and after that cpc decided to add them to his kit.
On a two stroke with a garrett turbo you wont see compressor surge but you will see extreme blow by in both the intake and exaust seals.

 

[Vertical-Extreme]

I was also told that you can get what they call "turbo runon" that is when you let off the throttle but the turbo is still forcing air into the air box and the sled will want to keep moving like the throttle is still on for a few feet till the boost pressure decreases. I think that happens more often on carburated snowmobiles unless you install a BOV

This is also true. I rode with a buddy last year who war running a aero charged xp and the run on was horrible, even dangerous at times if you didn't know it was going to do it. This year he is having a BOV installed. you could get right off the throttle and there would still be boost pressure for 10-15 feet

 

[Angermangement890]

Are these not compressor bypass valve (CBV), or diverter valves? I just read the law and it said that any Blow off valve or Dump valve the released air into the atmosphere wasn't legal.

A blow off valve does not have to vent to atmosphere, I think this is where the confusion lies, there are multiple names for the same part that function basically in the same way, difference being whether the vented air is recirculated into the intake track or vented tot he atmoshpere.

I agree a BOV that is adjusted to be closed under vacuum at idle is ideal for off idle throttle response, and the recirculation of boost can help to minimize the loss of throttle response when getting out of the throttle, then right back in. Even still, every time the BOV opens it dumps boost, boost that the turbo will have to rebuild when the BOV is closed again. This will translate into a loss of throttle response to the end user every time.

Junior

This is why you recirculate the air, boost is not lost and is plumbed back into the intake of the turbo. Most people do not take the time to properly install the BOV due to space constraints or wanting the cool sound. The benefit of decreased wear/potential damage outweighs the other tradeoffs.

Another thing that may be affecting your spool with/without bov is placement of the airflow measuring sensor and BOV placement. If you run a Mass air flow meter pre turbo or in the intercooler pipe or a MAP sensor, and place the bov after that device and vent it to atmoshpere you will run rich momentairly everytime the BOV acutates as the computer has calculated fuel for air that has just been released into the atmosphere. (note this is not present on the carb'd sleds which may be why they see a perfromance increase with the BOV).

 

[Fack]

I'm with Product Tester on this one. I've tried both and with the blow off valve PROPERLY SET-UP I see better throttle response when getting in and out of the throttle while I'm making good boost. The compressor is mounted on bearings so even after the exhaust pressure is reduced it will still freewheel due to it's own inertia. As it does this it will still make pressure. Compare the volume of air your turbo is capable of making to the volume of your air box and charge tube. In a split second you've filled your intake. Where is the pressure suppose to go? How can the turbo keep spinning and not stall or at least slow down? Sure, when the blow off is open it could be considered a "boost leak" but if your not using that air what does it matter? The BOV is necessary to give the air a place to go in order to keep the compressor spinning. The torque load required for the compressor to displace air will stall/slow the compressor so it's not like your turbo shaft is maintaining full RPM while the BOV is open, but it does greatly reduce this load. Also, on my carbed ski doo tubo a BOV is absolutely necessary for things to work right in the bumps and jumps.

-Fack

 

[Himark]

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

^^^

Best explanatiion yet.

I have been watching this thread and have been talking to several turbo builders prior about this and all have told me to leave it off. The real question is. Will it hurt the turbo bearings? and if we can all agree it wont it makes perfect sense to not have a BOV.

 

[Racegun]

Are these not compressor bypass valve (CBV), or diverter valves? I just read the law and it said that any Blow off valve or Dump valve the released air into the atmosphere wasn't legal.

A lot of guys run catch cans on the BOV's to keep it legal.

 

[mountainhorse]

In the new Twisted D-8 prototype... the BOV also acts as a compressor bypass idle-air-supply. In this install... the BOV does more than one job.

I've been on many different Boondocker Turbo D-8's Without BOV's and they work VERY well...At low boost levels... compressor surge/chatter isn't an issue in ridablilty or even noticeable. JKinzer has over 3000 miles on his non intercooled, non-BOV equipped, D-8 without an issue... and he beats the tar out of that thing anywhere from 8 - 13 lbs of boost .

I would imagine that for the larger turbos on big engines making max boost... a BOV would be mandatory.

BOV's also have "lag time"... basic physics.

Proper sizing of BOV, location, counter-spring force, piston/diaphragm size and many other factors play a big role in weather or not a BOV system will work.

Even the I.D. and length of the sensing line play a role here.

As far a a recirc style BOV "repressurizing" the system "or not loosing boost"... Please tell me how that is possible?? The Blown off charge goes into the intake AREA of the turbo which is a low pressure area large area compared to the charge tract. This does not "boost" the output of the compressor. It is just as free to go into the atmosphere in the filter area as be sucked into the turbo. Once it leaves the recirc return line... it is relatively decomressed.

This has become kind of an urban myth.

If it did "compress" or "boost" the output of the compressor.. it would defete the purpose of the BOV venting in the first place.

Simply....Recirc valves are installed on efi setups where the MAF (mass air flow sensor) must account for the air that is "blown off" in order to run correctly and have acceptable, functioning, emississions systems.

Another reason for this is in situations where the BOV is open at idle...allowing some bypass air into the engine... this should come from a clean source. Drawing from a cold, filtered source would be better than hot, dirty under hood area.

What this all boils down to is that there is more than one way to "bake a cake"...

Different system designs will respond differently.... some will need BOV's some wont.... Field testing is the key.

GREAT TOPIC!!!

 

[mountainhorse]

Savedbygrace: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.

Partly true...

A BOV will remain open as long as the VACUUM in the sensing line is enough to overcome the spring/preload setting in the BOV. Once the sensing line does not have enough force to overcome the spring force... the valve will start to close.

There is transition time for this process to happen which is why proper selection and tuning of the BOV is pretty crucial for optimum performance.

 

[mountainhorse]

There are some electronic BOV's out there that deal with bleed off very well. They have very fast shuttle times and open/closed thresholds can be contolled precisely. But they are pretty expensive and add another part to the system that can possibly break. They must also integrate well with the ECU/fuel/ingnition management.

 

[Wheel House Motorsports]

I think a recirculating setup is definately the way to go and will enhance the performance, no questions asked, problem is, in most intake setups, the routing doesnt allow for this in any practical manner, nor to the benefits justify the effort to install.

also, there is also the question of, would you want your BOV to be open at idle? i think not, it would run you the risk of sucking in unfiltered air, say, snow, belt dust, whatever, that is under your hood floating around, seems like a risky situation.

I have the ability to dump my BOV back into my turbo intake, and am really interested in trying this as i have a way to plumb the BOV so it DOESNT open, opens to atmosphere, and resirculates, it would be really easy for me to test out and i think would be a good way to compare on a single day so most variables are null.

 

[Wheel House Motorsports]

BOV's also have "lag time"... basic physics.

Proper sizing of BOV, location, counter-spring force, piston/diaphragm size and many other factors play a big role in weather or not a BOV system will work.

As far a a recirc style BOV "repressurizing" the system "or not loosing boost"... Please tell me how that is possible?? The Blown off charge goes into the intake AREA of the turbo which is a low pressure area large area compared to the charge tract. This does not "boost" the output of the compressor. It is just as free to go into the atmosphere in the filter area as be sucked into the turbo. Once it leaves the recirc return line... it is relatively decomressed.

This has become kind of an urban myth.
GREAT TOPIC!!!

Agreed, probably one of the best technical discussions I have ever seen on here.. keep it up, now back to the debate

as far as recirculating goes. if your BOV dumps right at the inlet of your compressor, or VERY close, then this should net a gain.

in my intake tract, i have a 2' tube from the turbo up out into the free air for my intake setup. the turbo is pulling air down this tube, thus creating a lower pressure area, which is why you want less intake restriction, to aid in turbo spooling, less resistance= faster spool. so now on a sudden throttle chop from full boost, you have a LOT of air moving and is now flowing back into the turbo intake tract, NO, your not making boost in your intake tract, BUT, it will help lower the vaccum situation going on, theoretically helping the turbo to work less... then the turbo is sort of a recycling system for the air.

as a second or so passes, and your turbo begins to slow dramatically, the benefits are really not there as far as recirculating systems, It just seems to me as though say on a long WOT climb, your at max boost, and just chop the throttle on/off ever so quickly, you still lose a bit of boost, but this would help you lose less. maybe its not a noticable gain, but again, i see the little simple things being beneficial to the ridability of a setup overall. but what do i know!

 

[leisureexpress]

I'm with Product Tester on this one. I've tried both and with the blow off valve PROPERLY SET-UP I see better throttle response when getting in and out of the throttle while I'm making good boost. The compressor is mounted on bearings so even after the exhaust pressure is reduced it will still freewheel due to it's own inertia. As it does this it will still make pressure. Compare the volume of air your turbo is capable of making to the volume of your air box and charge tube. In a split second you've filled your intake. Where is the pressure suppose to go? How can the turbo keep spinning and not stall or at least slow down? Sure, when the blow off is open it could be considered a "boost leak" but if your not using that air what does it matter? The BOV is necessary to give the air a place to go in order to keep the compressor spinning. The torque load required for the compressor to displace air will stall/slow the compressor so it's not like your turbo shaft is maintaining full RPM while the BOV is open, but it does greatly reduce this load. Also, on my carbed ski doo tubo a BOV is absolutely necessary for things to work right in the bumps and jumps.

-Fack

You use the word "stall" referring to the compressor a couple of times and I thought I would clarify for others (it sounds like you do know what is going on).

With a compressor stall it is not that the compressor quits rotating it is just that it has interrupted / inefficient / not smooth airflow / greatly reduced CFM through the compressor which could theoretically lead to undesireable vibration and destruction especially if the turbo bearings are getting out of tolerance. It is kind-of similar to cavitation in a PWC or jet boat.

Here is more on compressor stalls: http://en.wikipedia.org/wiki/Compressor_stall