BOV/DV : Useless or useful?

[bimeur]

Hi guys,

I was reading at this article :

http://www.autospeed.com/cms/A_1457/article.html

and get some doubts......

Useful or useless?

Except the noise it does (if not recirculated), in a pure technical point, with or without.......

if you argue, please, make it with proof (mathematical or whatever)

It seems that many tuners in Australia (especially on wankel's engines) don't use them at all, with a lot of miles on there set-up, without any turbo issue.

THX

[boost_boy]

With 10psi or less, you can pull it off without a BOV. But when you start running high boost pressures, you are creatin havoc for the turbine and shaft when the throttle plate closes. All that excess pressure is creating a counter clock-wise effect on the turbine while the fresh exhaust pressures are still attempting to flow the turbine in a clockwise direction. With all that action taking place and when throttle plate closes, it creates wastegate flutter which varies in sounds because of turbo sizes and boost pressures. Flutter is not so bad on small turbos running under 10psi of boost. Flutter is not so bad on big turbos running 10psi or less, but it's not helping them either. The CA18DET in stock form did not come with a BOV from factory. The little CA18DET cannot be compared to a Wankel engine when it comes to exhaust pulses and flow. The Wankel produces way more flow than the CA. Remember, the turbo is in the middle being sandwiched by positive and possible negative pressures. A bov is the relief for the turbocharger because it creates an avenue for the unused pressure and vent it to the atmosphere, which also keeps the turbo spooling in its clock-wise direction. Without the BOV, the turbo's turbine will be going back and forth and have to basically restart after the negative pressure creates the flutter and slows down or completely stops the poor turbine. In an engine with an automatic transmission, the gearshifts are so consistent and since the throttle plate is usually open upon acceleration, you may be able to pull it off in higher boost ranges. BOVs are supposed to not only be insurance mechanisms for your turbocharger, but keeps the turbo spooling as well. Just my $.01 on the matter. I'm not the expert, so don't sue me if I'm not totally accurate or not on par with the information you guys find on the web. This is straight out of my brain-housing group with no food in my stomach

[float_6969]

To correctly answer this some explanations about what's actually happening inside the turbo need to be explained first.

Lets start here:

These maps can be complicated and confusing to understand. I'm not going to explain everything associated with them as that will take too long and is superfluous for this conversation, but here are the basics. On the vertical axis of the map is the pressure, in Bar, as a pressure ratio. Kind of confusing, but bascially, the higher you go up, the more boost pressure that is being created. On the horizontal axis is the air flow in lbs/min. The important part of this map (for this discussion) is located on the left side of the map. What you see is a dotted line that says "Surge Limit". This is defined as an area where the pressure ratio is too high for a given air flow for the compressor to maintain stable air flow. What we mean by stable air flow, is air moving in one direction. Under all other circumstances, the air is drawn into the inlet of the compressor housing, and is forced out of the outlet of the compressor housing. What happens when the pressure is too high is that a situation is created where the compressor's ability to prevent the pressure on the outlet of the housing from going backwards through the compressor is overcome. Now I want to clarify something here. The compressor doesn't spin backwards when it happens! This is a common myth all over the internet. I don't know where it started, but it's not true. The force required to achieve that would shatter the compressor impeller and probably brake the turbine shaft inside the center section. What does happen is that as the air moves the wrong way for a moment, a sound is created. That sound is from the air molecules inside the compressor bouncing off of each other, much like a whistle does. As this reversion occurs, the pressure on the outlet of the compressor drops. Now the compressor is operating on the RIGHT side of the surge limit again. Now it is able to produce positive pressure again. The pressure builds until the Surge LImit is reached, and then process starts over again. Now, there are 2 types of compressor surge. "On Throttle" and "Off Throttle". We're concerned about "Off Throttle". This is what occurs when you are on boot, and you lift the throttle. The compressor is now trying to move air into a closed throttle. As it does this, the volume and pressure of air in the system after the turbo, before the throttle is increased. At the same time, the compressor is spinning more slowly because the throttle is shut. This means that the air flow is decreasing. If you look at the compressor map, you will see that we are in the area where the air flow is low, and the pressure is high. This is the area of surge and the situation I described above will occur. This will repeat until the compressor slows down enough that it is out of the surge area and stable air flow is regained.

So that's what's happening inside the turbo, but what does that mean to us in this discussion? The issue with the above situation is damage to the turbocharger. When the turbo operates on the surge line, the loads placed on the compressor can be more than what it was designed for, and damage can occur. This damage is worse the higher you go on the surge line. In our "off throttle" situation, the operation point is decreasing, as so the potential damage to the compressor is decreasing. At low boost levels, the loads placed on compressor turbine are below it's damage limit. This is why older turbocharged systems (like the CA18) didn't have bypass/blow off valves. They weren't needed. The issue is when the boost level is increased. If you're running 25psi of boost, the operating point moves straight to the left, right into the surge line, and the loads in this area are VERY high and damage definitely occurs. This damage is something that shortens the life of the turbo every time it happens. It's also cumulative. THIS is why you need/should run a bypass/blow off valve. To prevent damage to the turbocharger assembly at high boost levels.

As far as why to choose a bypass versus a blow off valve is varied as well. If you have a BOV, and a MAFS you can have engine stability issues as the metered air is released into the atmosphere, a rich condition occurs. This is why all OEM MAFS equipped engines run bypass valves. The issue with a bypass valve is that while in boost there are more forces acting on the valve, trying to open it. Not only is the pressure in the intake system trying to force it open, but once the outlet of the valve is connected back into the inlet of the turbo, you now have the low pressure before the turbo trying to open it as well. This generally means that you are limited on the amount of boost that you can run before the valves ability to stay closed is overcome. You can run a stronger spring to correct this, but this only works to a certain point. If the spring is too strong, the valve won't open at lower boost levels and you will encounter surge again.

There is more that I could discuss, but I'm out of time and that is enough to get started. I hope this helps.

[cbh148]

Cheers for some quality replies up in this thread.

[float_6969]

I was in a hurry when I wrote that. I just went back and re-read it. I'm a long winded bastard! Sorry about that, LOL

[louiswun]

The issue with a bypass valve is that while in boost there are more forces acting on the valve, trying to open it. Not only is the pressure in the intake system trying to force it open, but once the outlet of the valve is connected back into the inlet of the turbo, you now have the low pressure before the turbo trying to open it as well.

Not 100% true, try to think this .....when going down hill in gear, engine reving but throttle closed, the vacuum force is higher than idle....
when vacuum force pull the bypass valve crack open, the air can flow from the inlet of the turbo, but all air passed through the MAF and the filter, it did not create problem.
If the same situation on the BOV, the air is the same way go inside the engine, but it is from atmosphere and did not passed via MAF and the filter, this is the problem.

when on boost, no matter the bypass valve or BOV, in the theory, both internal port also receive same amount of positive pressure helping the spring to push close the valve.
but usually aftermarket BOV is adjustable, the adjustable bolts is not tight seal, the tiny little leak make the BOV internal port hold less pressure than the intercooler pipe, so the the pressure inside the intercooler pipe push open the BOV, and create leak. this problem was found on every JDM brands BOV, like HKS, Blitz, ARC....etc
The main reason BOV leak is the internal valve operation direction, most JDM BOV internal valve move the same direction as the air release when opening, the higher intercooler pipe pressure, the more force to push open.
The Bosch bypass valve commonly found on VW and Audi, it is cheap and made from plastic, but it hold the pressure tight, I have tested it up to 2 bar, zero leaking.
the reason it can hold pressure so tight is it's internal valve operation direction, the valve closing direction is the same as air release direction, the pressure only can push close but not push open, the higher pressure in the intercooler pipe, the tighter it seal, when it release pressure, the valve move opposite direction to the release air.
Same design can be found on Forged bypass valve, it hold pressure so tight and zero leaking, tested was done under a big bucket of water, no bubble goes out.

Forged bypass valve test
[youtube]
https://www.youtube.com/embed/SXZAKV8SFfk
[/youtube]

Some more HKS BOV leak test
http://www.facebook.com/photo.php?v=101 ... 988&type=3
http://www.facebook.com/photo.php?v=101 ... 988&type=3

[float_6969]

It's true that some of these issue are dependent on the type of BOV. I was referencing "typical" BOV's. Ideally they should all be "pull type" valves like the HKS. I know you were showing issues with the HKS leaking, but I'm running that same valve and it doesn't have an adjustment on it like your does and it has held plenty of boost and never leaked.

Ideally, they would all be setup so that the pressure in the intake pipe keeps them closed and they are pulled open against it. Unfortunately, most of them AREN'T of this design and so the statements I made previously are true.

Also, Synapse makes a really good valve as well.

Regardless, the original questions was in reference to whether you needed a BP/BOV or not. And regardless of the type of valve you're using, for higher boost levels, they are needed to prevent damage to the turbo.

[blownhemi]

Whew, there are some things wrong or partially true in that article.
The article talks about a dump valve which is a non-recirculated blow-off valve, and mentions, that without it, they gained 2 second on a 1:44 lap. I guess this is, where your confusion may have come from. But this is a race car they are talking about, and don't forget, everything on a race car is a wear item, so they did not need to take care of their turbocharger. If it lasts 3-4 races, that's good enough for them. And if that's what it takes to gain 2 seconds, then to hell with the *DUMP* valve.

Instead of a DUMP valve, with a proper, recirculated BOV installation, the high amount of pressurized air is fed back right in front of the turbo, allowing it to kind of "freewheel", propel itself, and it will not lose spool speed (at least definitely not as fast as by DUMPING the air), nor go into the surge territory. (Plus, as others have said, you will not toss out metered air, and make a fool of the ECU, but this one, I do not quite understand yet). Install a recirc. BOV, you get the best of both worlds. Well, maybe not the best, but you get as much good from both worlds as you can possibly ever need.

You'll never hear a World Rally Car going pssshhht because they don't use a dump valve - you get the woof-woof-woof noise instead."

I think that's about the best one. So, yeah, high-strung rally guys are not doing it, then why should you, the street tuner do it? They, again, are not interested the slightest about turbo longevity. They use ALS (at least they do now, I don't know about 2002, when the above article was born), and replace turbos every race. It's pocket money for them compared to all the other wear items, fees, and other costs.

By the way, your definitive answer:
http://www.turbobygarrett.com/turbobyga ... off_valves
And, well, Garrett knows some things about turbochargers, and they don't sell BOVs, they sell turbochargers, so if they are saying, "this is how you take care of your turbocharger", then I don't think it's just some marketing BS.

I'm not a Garrett fan, but I'd take their point over the OZ guys you mention, who "got away with it", under unkown conditions. (Boost, turbo sizes, typical usage, etc.)

Also, just in case you were wondering, various anti-surge methods on compressor housings, ported shroud (Garrett), Map Width Extension (Holset), etc., are not replacements for a proper recirculated BOV, or even a DV. These do not, can not prevent the amount of sudden surge that the "Off Throttle" condition produces.

Oh, yeah, forgot to mention, I'm running a Holset HX35 at 15 psi, no BOV. (temporarily, at least)
But it pains me every single time when I hear the surge. So I try to shift quick, and when I'm done accelerating, I get off the pedal quite slowly.