Explanation of MBC leak on boost leak tests

[burntheblobs]

Can someone explain to me how this ISN'T a boost leak? I have it set to 12psi, and it will leak heavily when there is about 1psi in the system. Everyone always says it's totally ok, but why? A leak is a leak in my mind...

 

[donmagicjuan]

Can someone explain to me how this ISN'T a boost leak? I have it set to 12psi, and it will leak heavily when there is about 1psi in the system. Everyone always says it's totally ok, but why? A leak is a leak in my mind...

If you have a bleeder valve type boost controller, it's just the way it operates. This setup introduces a "controlled" leak into the system, between the intake system and the wastegate actuator. By bleeding off air under boost, it requires a higher boost level before enough pressure builds up in the actuator line to cause it to open. That is why ball-and-spring type MBCs are considered to be superior. These valves isolate intake pressure from the wastegate actuator until the set pressure is exceeded. At this point, the valve opens, applying pressure to the actuator. There is still a small hole in a ball-and-spring type MBC downstream of the valve seat, which is designed to bleed pressure in the actuator line and allow the wastegate to close when coming off boost.

 

[burntheblobs]

I have the SBR ball and spring type that leaks a lot out of the small bleeder hole.

 

[donmagicjuan]

I have the SBR ball and spring type that leaks a lot out of the small bleeder hole.

In that case, it is because the operation of the valve is actually based on differential pressure between the intake and the actuator line. If your actuator opens at 11 psi, your boost controller will open at 1 psi differential when set to 12 psi. When performing a boost leak test, you need to either turn up the controller high enough to prevent opening, or remove it from the system entirely.

 

[EclipseTrbo420A]

I have the SBR ball and spring type that leaks a lot out of the small bleeder hole.

Its should only leak when the pressure you have it set to is reached. The ball should contain all pressure untill the spring is overcome. Disscinnect the MBC when doing boost leak tests. If you didnt have that bleeder your WG would get stuck open after you let off the throttle.

I did this earlier for someone else, its the same idea.

http://www.dsmtuners.com/forums/showpost.php?p=50454235&postcount=9

 

[burntheblobs]

So basically is my MBC shot?

 

[EclipseTrbo420A]

It should not leak at 1 if you have it set to 15

 

[donmagicjuan]

So basically is my MBC shot?

No, I thought I explained this. I have my boost set at 26 psi, which is 18 psi above my factory wastegate actuator setting of 8 psi for a 16G. When I do a boost leak test, my MBC will unseat and begin to leak out of the actuator pressure relief hole (which I described earlier) at an intake pressure of 18 psi. Thus, if you have your boost controller set for 12 psi, which I'm guessing is 1 psi above your actuator opening pressure, you will see a leak at an intake pressure of 1 psi unless you turn it up.

During normal operation at the boost level setpoint, the pressure downstream of the MBC stays equal to that required to open the wastegate actuator. The MBC valve cycles about the setpoint while intake pressure remains greater than the actuator pressure by the amount you have set it. When coming off boost, the actuator line relief hole in the MBC bleeds pressure from the actuator allowing the wastegate to close again.

You can always check this by turning up your boost controller while performing the boost leak test. If the leak stops suddenly, you have your answer.

 

[steve]

A gate valve MBC works by blocking the boost pressure from the WGA. It (the gate) may start opening before the WGA opens the wastegate but that's not it's primary design it's more a side effect of the hole that allows the WGA to close after the valve closes.

His leaking at 1psi is a sign that it has problems.

Steve

 

[donmagicjuan]

While it pains me to commit the cardinal sin of disputing the words of a DSM wiseman, I respectfully disagree. If I turn my boost controller all the way down, it will begin to leak as soon as I start to raise pressure during a boost leak test, yet boost is controlled at the stock actuator setpoint. If I set the boost to 18 psi, the MBC leaks at 10 psi, and so on. If the rate I am putting air into the system exceeds that of the leak from the actuator line relief hole, I am able to raise pressure in the system, but that doesn't change the fact that it's leaking. I agree that a ball-and-spring valve isolates pressure from the actuator until its setpoint is reached, but that setpoint is based on differential pressure and not necessarily intake sytem gage pressure.

 

[EclipseTrbo420A]

A gate valve MBC works by blocking the boost pressure from the WGA. It (the gate) may start opening before the WGA opens the wastegate but that's not it's primary design it's more a side effect of the hole that allows the WGA to close after the valve closes.

His leaking at 1psi is a sign that it has problems.

Steve

I was thinking its simple pressure vs pressure in 2 different forms. If you have a spring on the right side of the ball pushing 15PSI and air pressure on the left side also pushing 15PSI the ball isnt going to move...Now, if the air pressure goes up anything over what the spring is pushing then the ball will move in the air pressures favor. Also goes the other way around only the spring wont move the ball, it will pin it agains the opening, not allowing the weaker boostpressure in.

 

[donmagicjuan]

I was thinking its simple pressure vs pressure in 2 different forms. If you have a spring on the right side of the ball pushing 15PSI and air pressure on the left side also pushing 15PSI the ball isnt going to move...Now, if the air pressure goes up anything over what the spring is pushing then the ball will move in the air pressures favor. Also goes the other way around only the spring wont move the ball, it will pin it agains the opening, not allowing the weaker boostpressure in.

Yes, but the pressure on the other side of the valve comes into play here. In order for the valve to control boost at say 15 psi with an actuator setpoint of 8 psi, there must exist a 7 psid across the valve. This would indicate that the valve opens at 7 psid. During a boost leak test, however, the pressure in the actuator line is very close to 0, so the valve will begin to open at 7 psig in the intake system.

 

[EclipseTrbo420A]

Yes, but the pressure on the other side of the valve comes into play here. In order for the valve to control boost at say 15 psi with an actuator setpoint of 8 psi, there must exist a 7 psid across the valve. This would indicate that the valve opens at 7 psid. During a boost leak test, however, the pressure in the actuator line is very close to 0, so the valve will begin to open at 7 psig in the intake system.

Think of it like this, its all a one way train to the actuators head. If I put 7 psi in the actuators head it will open the flapper.

I need to now hold back pressure from that actuator with a mbc... The MBC is like another actuator. Just a, say 18 PSI actuator. Now, pressure comming to the MBC will have to exceed 18PSI to open/move the ball so it can rush to the Real actuator which will shoot open at anything from 7PSI and up. Thats it.

Boost is controlled by the WG, All a MBC does is eliminate the actuators authority and become the new "Boost pressure setter"

Putting on a 18psi WG actuator is the same thing as running a MBC at 18lbs only its not adjustable

 

[EclipseTrbo420A]

I also see what you car saying about the 7 psi pressure, but the thing is, the 7psi isnt pushing anything untill the car sees 8 Psi and even then it doesnt matter because it cant beat 8 psi. Know what Im saying?

 

[donmagicjuan]

I also see what you car saying about the 7 psi pressure, but the thing is, the 7psi isnt pushing anything untill the car sees 8 Psi and even then it doesnt matter because it cant beat 8 psi. Know what Im saying?

Here's something to think about. Imagine replacing your 8 psi wastegate actuator with an 18 psi one, but making no adjustments to your MBC (still set at 18 psi). If your explanation is correct, you would see no change in what pressure your MBC controls boost, but I'd be willing to bet a substantial amount that you would.

 

[EclipseTrbo420A]

Here's something to think about. Imagine replacing your 8 psi wastegate actuator with an 18 psi one, but making no adjustments to your MBC (still set at 18 psi). If your explanation is correct, you would see no change in what pressure your MBC controls boost, but I'd be willing to bet a substantial amount that you would.

Theres some good Info in this thread. Im going to say that if I do run a set up like you stated above that the only difference between running just the WG at 18 and MBC and WG at 18 is a minor spike. It would take a fraction of a second longer for the pressure that had to build up at the MBC to get to the WG. Where as the 18PSI would build right on the head of the WG untill it opened at 18 PSI.

The other variable there is how far from the actuator you have the MBC, but thats not what this is about. What do you think?

 

[donmagicjuan]

Theres some good Info in this thread. Im going to say that if I do run a set up like you stated above that the only difference between running just the WG at 18 and MBC and WG at 18 is a minor spike. It would take a fraction of a second longer for the pressure that had to build up at the MBC to get to the WG. Where as the 18PSI would build right on the head of the WG untill it opened at 18 PSI.

The other variable there is how far from the actuator you have the MBC, but thats not what this is about. What do you think?

I believe the only effect the line length will have is on the responsiveness of the controller.

If you have an 18 psi actuator with an MBC and you desire 18 pounds of boost, I think you'll agree that you can get there by turning the boost controller all the way down. So are you saying that turning the controller up however many turns it took to achieve 18 psi on the softer actuator will have no effect on boost? That seems very illogical to me. I'm not sure how to word it any differently than I already have, but I'm still sticking to my story.

 

[EclipseTrbo420A]

I believe the only effect the line length will have is on the responsiveness of the controller.

If you have an 18 psi actuator with an MBC and you desire 18 pounds of boost, I think you'll agree that you can get there by turning the boost controller all the way down. So are you saying that turning the controller up however many turns it took to achieve 18 psi on the softer actuator will have no effect on boost? That seems very illogical to me. I'm not sure how to word it any differently than I already have, but I'm still sticking to my story.

Hey when you say turn down you mean back it out, so theres no pressure on the ball from the spring, Right ? If so then YES your right....if the MBC is turned all the way down then its doing nothing at all..the boost, wether it be 1 psi or 35 will blow right by the ball. What I am saying is if you screw it back in to where it would take 18PSI to blow it open then nothing other than 18PSI or above is going to move that ball. Once 18PSI comes rushing past that ball, the actuator at 7PSI is dominated and blown open untill boost pressure lowers to 17PSI and the spring in the MBC shuts the passage for the air. See?

 

[donmagicjuan]

Hey when you say turn down you mean back it out, so theres no pressure on the ball from the spring, Right ? If so then YES your right....if the MBC is turned all the way down then its doing nothing at all..the boost, wether it be 1 psi or 35 will blow right by the ball. What I am saying is if you screw it back in to where it would take 18PSI to blow it open then nothing other than 18PSI or above is going to move that ball. Once 18PSI comes rushing past that ball, the actuator at 7PSI is dominated and blown open untill boost pressure lowers to 17PSI and the spring in the MBC shuts the passage for the air. See?

I definitely understand what you're saying, but I still contend that you're neglecting the effect of the pressure downstream of the valve. A relief valve (which is basically what a ball-and-spring valve is) is opened when a differential pressure is applied across the seat that overcomes the spring pressure holding it closed. So for a valve with its downstream side vented to atmosphere, changes in atmospheric pressure affect the (absolute) system pressure at which the valve opens. The pressure in the actuator line opposes the force generated by the pressure in the intake system. It is necessary that the pressure in this line equals the actuator setting when the boost controller is controlling boost effectively. Therefore, a different actuator will exert a different amount of force opposing the opening of the valve, and will thus require a different MBC setting to control boost at the same level. Does that help?

 

[steve]

While it pains me to commit the cardinal sin of disputing the words of a DSM wiseman, I respectfully disagree.

Not a problem with me.

If I turn my boost controller all the way down, it will begin to leak as soon as I start to raise pressure during a boost leak test, yet boost is controlled at the stock actuator setpoint.

Just like it would be if the MBC wasn't connected.

If I set the boost to 18 psi, the MBC leaks at 10 psi, and so on.

Yours might but not all gate valve MBC do.

If the rate I am putting air into the system exceeds that of the leak from the actuator line relief hole, I am able to raise pressure in the system, but that doesn't change the fact that it's leaking.

If the inlet flow is sigficantly larger than the vent flow rate the pressure change is almost instantious. The effect your addressing in these posts is more a defect caused by a vent that is larger than it should be. It causes the MBC to act like a bleeder valve in some modes of operation rather than as a gate valve.

I agree that a ball-and-spring valve isolates pressure from the actuator until its setpoint is reached, but that setpoint is based on differential pressure and not necessarily intake sytem gage pressure.

If the vent was removed would you still be making this claim? The vent hole should be as small as possible, it's job is to lower the pressure behind a closed gate so that the wastegate closes. There are ways to design the venting that wouldn't cause the valve to leak except after the gate closed. The only reason the setpoint is lower than the desired value is to get enough flow to swamp the vent on the common design and move the actuator. If you have a smaller vent all of the settings would also change just like they do in your example of a higher pressure WGA.

Steve

 

[EclipseTrbo420A]

I definitely understand what you're saying, but I still contend that you're neglecting the effect of the pressure downstream of the valve. A relief valve (which is basically what a ball-and-spring valve is) is opened when a differential pressure is applied across the seat that overcomes the spring pressure holding it closed. So for a valve with its downstream side vented to atmosphere, changes in atmospheric pressure affect the (absolute) system pressure at which the valve opens. The pressure in the actuator line opposes the force generated by the pressure in the intake system. It is necessary that the pressure in this line equals the actuator setting when the boost controller is controlling boost effectively. Therefore, a different actuator will exert a different amount of force opposing the opening of the valve, and will thus require a different MBC setting to control boost at the same level. Does that help?

I got your point...Like a boost leak the turbo is pushing 18PSI on the gauge, but because of the leak its actually working twice as hard 36PSI just to get 18PSI to the manifold and gauge.

Like the MBC, we adjust for a certain PSI reading...If we want to see 18LBS of boost the MBC ITS SELF may let 20 LBS of air in...but only 18 get to the actuator because of the vent, so while we make the turbo run at 18 PSI in actuality it takes more pressure to hold it there?

I dont think that makes sence now because if its set to 18PSI ( reading on boost ) that means the theory above requires a greater force than that the turbo makes....but the turbo IS what makes the force..

I see it like this the hole is to small to make a difference. Sure it leaks air, but not enough to de-pressureize the line any when the turbo is kicking all that boost into the Intake system see what I mean?

 

[kenamond]

Due to the hole in the WGA end of all worthy ball-spring MBCs, the pressure on the WGA end of the MBC when the MBC is closed is 0psig, not 8psig. The fact that the actuator opens at 8psig has nothing to do with the pressure in the WGA when the MBC is blocking the boost because the hole lets it leak down to 0psig.

If the MBC is set to 4psi, you'll get 8psi boost, not 12psi (ignoring spikes). At boost>4psi, the MBC is open, and the boost makes it to the WGA, but the WGA won't open until boost>8psi. The MBC is just a door that hides boost from the WGA until it reaches a specified level, then that boost is permitted to enter the WGA where the WGA will open IF that boost is also greater than the boost required to open the WGA. Adding the MBC boost level to the WGA boost level doesn't make any sense to me.

That's my .02

EDIT: And I type too slow.

 

[donmagicjuan]

If the inlet flow is sigficantly larger than the vent flow rate the pressure change is almost instantious. The effect your addressing in these posts is more a defect caused by a vent that is larger than it should be. It causes the MBC to act like a bleeder valve in some modes of operation rather than as a gate valve.

I'm not arguing that the vent should be preventing you from raising intake pressure above the setpoint of the MBC-- the OP simply wanted to know why his MBC was leaking when conducting a boost leak test. It didn't sound to me like he was having a problem getting pressure above 1 psi, so I had no reason to believe his controller was defective.

If the vent was removed would you still be making this claim? The vent hole should be as small as possible, it's job is to lower the pressure behind a closed gate so that the wastegate closes. There are ways to design the venting that wouldn't cause the valve to leak except after the gate closed. The only reason the setpoint is lower than the desired value is to get enough flow to swamp the vent on the common design and move the actuator. If you have a smaller vent all of the settings would also change just like they do in your example of a higher pressure WGA.

The actuator line vent hole is a necessary evil, and I agree that its size should be kept to the minimum necessary for adequate operation. If the vent were absent, the valve would open initially at a pressure lower than the boost setting, but only long enough for pressure to accumulate in the actuator line (assuming the actuator is leak-tight). This negligible amount of leakby aside, the MBC would not leak at all during a boost leak test, as there would be nowhere for the air to escape.

 

[EclipseTrbo420A]

, the pressure on the WGA end of the MBC when the MBC is closed is 0psig, not 8psig.

Yep, Good stuff there, I have been trying to say that all night.

I also see what you car saying about the 7 psi pressure, but the thing is, the 7psi isnt pushing anything untill the car sees 8 Psi and even then it doesnt matter because it cant beat 8 psi. Know what Im saying?

 

[steve]

I definitely understand what you're saying, but I still contend that you're neglecting the effect of the pressure downstream of the valve.

In our case that pressure downstream of the valve should be atmospheric except when the valve is open.

A relief valve (which is basically what a ball-and-spring valve is) is opened when a differential pressure is applied across the seat that overcomes the spring pressure holding it closed.

Correct.

So for a valve with its downstream side vented to atmosphere, changes in atmospheric pressure affect the (absolute) system pressure at which the valve opens.

The pressure on both sides of this system are referenced to atmospheric so it should drop out. It's the time when the differential pressure behind the valve is equalizing that's related to the behaviour you've been discussing.

The pressure in the actuator line opposes the force generated by the pressure in the intake system. It is necessary that the pressure in this line equals the actuator setting when the boost controller is controlling boost effectively. Therefore, a different actuator will exert a different amount of force opposing the opening of the valve, and will thus require a different MBC setting to control boost at the same level. Does that help?

Changing the parameters of the system and then trying to compare the settings doesn't really tell us anything useful at this point. Your going to adjust the system to get the desired end result. Who cares if it means one MBC needs 3 turns and another needs 5 to get the same boost level.

Steve