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...

 

[steve]

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.

Thanks for bringing this back to the problem. I won't disagree in principal, but I would suggest that in practice the controller in question is either defective or a very poor design.

The vent is a boost leak and the closer the opening point of the valve is to the desired boost level the less that leak will effect the air fuel ratio. This is after all measured air.

Steve

 

[donmagicjuan]

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?

This is EXACTLY what I've been getting at. Since the hole can't outflow the turbo or the MBC valve, the pressure in the line is actually at the actuator setpoint, not zero, meaning the boost controller is set to open at a DIFFERENTIAL pressure equal to desired boost level minus actuator pressure. It's impossible to grasp the concept of how a relief valve works if you continue to think of it in terms of what boost pressure the MBC is set to. During a boost leak test, the actuator line starts at zero pressure. This means the MBC will START to leak by at a pressure corresponding to the differential pressure setting of the controller, which is what I believe the OP was asking about.

 

[donmagicjuan]

It's the time when the differential pressure behind the valve is equalizing that's related to the behaviour you've been discussing.

Yes, I think I just realized at this point we're merely arguing semantics.

 

[toojung2die]

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?

Not for me. Not intending to be confrontational, but I think you're complicating something that is simple: I have a ball held by spring pressure and that spring is set to allow the ball to move when the air pressure pushing against it is 15 psi above atmospheric. It doesn't matter what is after the ball and spring as long as there is a way to relieve the air passing the ball. If I have a WG actuator or disconnect the WG actuator altogether the ball will still move when 15psi above atmospheric is applied. The only thing that changes the pressure at which the ball moves is a change in atmospheric pressure. It is not affected by the actuator or the bleed off hole when doing a boost leak test. If I disconnect the MBC from the actuator during a boost leak test the MBC begins to leak at the same psi as when the actuator is hooked up.

 

[EclipseTrbo420A]

This is EXACTLY what I've been getting at. Since the hole can't outflow the turbo or the MBC valve, the pressure in the line is actually at the actuator setpoint, not zero, meaning the boost controller is set to open at a DIFFERENTIAL pressure equal to desired boost level minus actuator pressure. It's impossible to grasp the concept of how a relief valve works if you continue to think of it in terms of what boost pressure the MBC is set to. During a boost leak test, the actuator line starts at zero pressure. This means the MBC will START to leak by at a pressure corresponding to the differential pressure setting of the controller, which is what I believe the OP was asking about.

So your saying when the acutator is open its still throwing 7 psi back at the MBC? Ill think of it like this...The 18PSI screams past the ball and hits the actuators 7 psi door..before it opens that door it pressureizes the MBC chamber at the sametime leaking out boost from thebleeder. The bleeder hole gets QUICKLY over run and then the 18PSI bombards its way through the actuator opening the flapper.....your also saying now, the actuator is pusing back on the pressure causing it to only give 11 psi to actual "push down". Since the pressure has nowhere to go there its like this....

Dont think of it as ratios This will give this, but this-this = this... 18PSI will Move 7 PSI Thats all, once that actuator sees anything above 7 its opening...ya know?

 

[EclipseTrbo420A]

If I disconnect the MBC from the actuator during a boost leak test the MBC begins to leak at the same psi as when the actuator is hooked up.

Nicely said right there.

 

[kenamond]

the pressure in the line is actually at the actuator setpoint, not zero, meaning the boost controller is set to open at a DIFFERENTIAL pressure equal to desired boost level minus actuator pressure.

You have two pressures: an "upwind" pressure on the intake side of the MBC and a "downwind" one on the WGA side of the MBC. The MBC only knows about these two pressures. It will open at a certain difference between the upwind and downwind pressures. The WGA only knows about the downwind pressure and will open when that pressure is greater than, say, 8psig. When the MBC is closed, the downwind pressure is 0psig, because the vent dumped everything and equalized with atmospheric pressure. So the MBC sees 0psig downwind and boost pressure upwind. When it opens, the upwind and downwind pressures equalize (assuming the MBC valve significantly outflows the vent hole). So the MBC opens at some differential (say 12psi). Since we know the downwind pressure is 0psig, then it will open when the upwind pressure (boost) is 12psig. Then the downwind pressure is 12psig and the 8psi WGA opens. And you limit boost to 12psig, not 20psig.

 

[donmagicjuan]

If I disconnect the MBC from the actuator during a boost leak test the MBC begins to leak at the same psi as when the actuator is hooked up.

This doesn't dispute anything I've stated so far. I already stated that the actuator line pressure is zero at the onset of a boost leak test.

I'm getting a little weary of arguing this tonight. I think I'll just have to go collect some theory-to-practice data to strengthen my argument.

 

[toojung2die]

This doesn't dispute anything I've stated so far. I already stated that the actuator line pressure is zero at the onset of a boost leak test.

I'm getting a little weary of arguing this tonight. I think I'll just have to go collect some theory-to-practice data to strengthen my argument.

Cool, then later we can argue the pop-off pressure variable inherent in different check valve designs and their merits!

 

[EclipseTrbo420A]

I think we should all get 18 PSI actuators and save the stress. Whats weird is I know and see how everyone is making their point, then I lose it figures haha. This is getting bad...I know that when I hit it....I see 18LBS on that little gauge and it feels good.

The DSMLink factory tune on my injectors has my car running better than I did on my AFC. I love it.

 

[donmagicjuan]

...I know that when I hit it....I see 18LBS on that little gauge and it feels good.

Hah! Just wait till you feel 26 on that 18G!

 

[steve]

My point is that the setting has more to do with the pressure loss cause by the vent than the actuator. If the vent didn't exist or if the air entering the column was much greater than the air leaving the column the pressure in the column would equal the pressure of the air entering. Once the pressure in the column exceeds the WGA setting the wastegate will open.

The problem is that while the gate is opening the flow rate is low and the vent can lower the pressure in the column. To counteract this the gate setpoint is lower to increase the flow rate entering to swamp the vent.

Steve

 

[EclipseTrbo420A]

My point is that the setting has more to do with the pressure loss cause by the vent than the actuator. If the vent didn't exist or if the air entering the column was much greater than the air leaving the column the pressure in the column would equal the pressure of the air entering. Once the pressure in the column exceeds the WGA setting the wastegate will open.

The problem is that while the gate is opening the flow rate is low and the vent can lower the pressure in the column. To counteract this the gate setpoint is lower to increase the flow rate entering to swamp the vent.

Steve

I understand that, technically we could recirc the vent back to the intake and have no problems, untill some one argues which is more powerfull, the pressure blowing out the bleeder or the vaccume created by the turbo, and now we have a whole other argument What will the side effects be !! tune in later to find out.

 

[EclipseTrbo420A]

Hah! Just wait till you feel 26 on that 18G!

Yea buddy...I gotta say bye to the injen intake now ahh well on to bigger better things.

 

[donmagicjuan]

Alright, after thinking about this for way too long, I think I've come up with a scenario that might convince some of you to rethink your beliefs on the subject. I think we're all in agreement that the controller valve operates according to the differential pressure applied across its seat, but the point of contention seems to be what the pressure is in the actuator line when the valve is actually controlling boost.

Consider the hypothetical case where a wastegate can flow exactly enough to prevent creep at a specific boost level, say 16 psi on an EvoIII 16G. This is an equilibrium condition for the MBC. In other words, the wastegate must be held open 100% of the time in order for it to control boost. Then it must be the case that there is pressure continuously applied to the actuator, and that pressure must be equal to or greater than the actuator setpoint of 8 psi. However, the leak in the actuator line requires that air must also be entering the line in order for pressure to remain above the setpoint. In comes the MBC.

This all makes sense if the valve for the MBC is open just enough so that airflow from the intake system exactly matches that lost through relief hole. This establishes two static pressures on either side of the valve seat: the intake pressure at the desired boost level of 16 psi and the actuator set pressure of 8 psi. The MBC valve can only be open if the differential pressure exceeds the value set by the operator. So it must be the case that the valve cracks open at a differential pressure of the boost level minus the actuator setpoint (16 - 8), or 8 psid. Anything larger than this value, such as the 16 psid suggested by others, would not allow for the valve to be open in this situation, and would therefore not control boost at this level.

 

[EclipseTrbo420A]

Alright, after thinking about this for way too long, I think I've come up with a scenario that might convince some of you to rethink your beliefs on the subject. I think we're all in agreement that the controller valve operates according to the differential pressure applied across its seat, but the point of contention seems to be what the pressure is in the actuator line when the valve is actually controlling boost.

Consider the hypothetical case where a wastegate can flow exactly enough to prevent creep at a specific boost level, say 16 psi on an EvoIII 16G. This is an equilibrium condition for the MBC. In other words, the wastegate must be held open 100% of the time in order for it to control boost. Then it must be the case that there is pressure continuously applied to the actuator, and that pressure must be equal to or greater than the actuator setpoint of 8 psi. However, the leak in the actuator line requires that air must also be entering the line in order for pressure to remain above the setpoint. In comes the MBC.

This all makes sense if the valve for the MBC is open just enough so that airflow from the intake system exactly matches that lost through relief hole. This establishes two static pressures on either side of the valve seat: the intake pressure at the desired boost level of 16 psi and the actuator set pressure of 8 psi. The MBC valve can only be open if the differential pressure exceeds the value set by the operator. So it must be the case that the valve cracks open at a differential pressure of the boost level minus the actuator setpoint (16 - 8), or 8 psid. Anything larger than this value, such as the 16 psid suggested by others, would not allow for the valve to be open in this situation, and would therefore not control boost at this level.

Goor morning, well all most morning. Look at it like this, The MBC is whats in control. If it where up to actuator, it would open at 7PSI, but the actuator sees NOTHING untill the MBC lets the pressure through. Now when that happends there is a constant 18LBS comming through the line, becase the MBC regulates that....its let in NO MORE OR NONE just 18 and whats what keep the turbo flowing that amount of air. So now since the source is 18LBS and the ball is "out of the way" Its just a big line to the actuator...pressureized at 18LBS.

In the pic below the RED indicates pressure below 18LBS and the spring holding it back. The Light blue indicates NO pressure in/behind the Ball. If that Pressure becomes enough to move the ball...18LBS then everything behind will now be at 18LBS seeing as its all one line. ( pic 2 ) The turbo flows to much for the bleeder to be a factor and as stated before, to be less a factor have the hole as small as possible.

 

[toojung2die]

That's it! ^^^^ Once the pressure stabilizes it's exactly the same on both sides of the ball.

 

[EclipseTrbo420A]

That's it! ^^^^ Once the pressure stabilizes it's exactly the same on both sides of the ball.

Thats what I was trying to say, its hard though because evrything everyone is saying seems like it works out. We need some MIT memebers in here haha

 

[donmagicjuan]

That's it! ^^^^ Once the pressure stabilizes it's exactly the same on both sides of the ball.

Wrong. Once pressure in the downstream side of the valve exceeds the D/P setpoint of the valve, the spring will return the valve to the shut position, and pressure on the downstream side of the valve will begin to fall again due to the relief hole. Think about this: the spring does not care about the flowrate of the fluid through the valve. D/P is the only factor.

The valve cannot be open with pressure equal on both sides of the seat unless the knob is turned all the way out.

 

[EclipseTrbo420A]

Wrong. Once pressure in the downstream side of the valve exceeds the D/P setpoint of the valve, the spring will return the valve to the shut position, and pressure on the downstream side of the valve will begin to fall again due to the relief hole. Think about this: the spring does not care about the flowrate of the fluid through the valve. D/P is the only factor.

The valve cannot be open with pressure equal on both sides of the seat unless the knob is turned all the way out.

Right, but now theres the issue of directional flow. Its comming at the ball...the "before line" has nowwhere to go accept at the ball. On the back side, sure it could push the ball closed, but it doesnt because the bleeder is there. Know what Im saying? Also I said this before.. If you ge 18PSI on each side of the ball it wont move...but before that happens the ball is blown BACK opening the valve..so then when the pressure is equal at 18PSI on each side of the ball it will hold open and not move closed. Untill throttle is closed and the source of the 18PSI is beat by the spring. You cant factor in the post ball chamber because it doesnt have its own pressure source and its not sealed off.

This is some sick $hit

 

[donmagicjuan]

On the back side, sure it could push the ball closed, but it doesnt because the bleeder is there. Know what Im saying? Also I said this before.. If you ge 18PSI on each side of the ball it wont move...but before that happens the ball is blown BACK opening the valve..so then when the pressure is equal at 18PSI on each side of the ball it will hold open and not move closed. Untill throttle is closed and the source of the 18PSI is beat by the spring.

No, I think you're misunderstanding the fundamental principle of operation for a spring type relief valve. Again, the valve cannot be open with pressure equal on both sides. Sure, there may be a dynamic transition period where the rapid increase in intake pressure generates some momentum in the ball, but the force of the spring is constantly opposing it. In a relatively short amount of time, the spring will reverse the direction of the ball and return it to its closed position, provided the pressure on the downstream side is sufficiently high (i.e. the D/P across the valve is sufficiently low). The valve will "hunt" around the boost setting by cycling open to maintain the D/P across it at the value of boost pressure minus actuator pressure as I stated earlier.

 

[EclipseTrbo420A]

No, I think you're misunderstanding the fundamental principle of operation for a spring type relief valve. Again, the valve cannot be open with pressure equal on both sides. Sure, there may be a dynamic transition period where the rapid increase in intake pressure generates some momentum in the ball, but the force of the spring is constantly opposing it. In a relatively short amount of time, the spring will reverse the direction of the ball and return it to its closed position, provided the pressure on the downstream side is sufficiently high (i.e. the D/P across the valve is sufficiently low). The valve will "hunt" around the boost setting by cycling open to maintain the D/P across it at the value of boost pressure minus actuator pressure as I stated earlier.

Maybe were adjusting the spring so that it lets only the right mount of air in to equal 18PSI all around and hold the actuator open. The spring rate can be...what ever Im not sure of the spring rate.

What your saying is....18PSI on the gauge could be a lower, but higer than 7 in the post MBC ball line?

 

[donmagicjuan]

What your saying is....18PSI on the gauge could be a lower, but higer than 7 in the post MBC ball line?

Yes, the successful operation of the boost controller, whose objective is to raise boost pressure above the stock actuator setting, hinges on this fact. There has to exist a way for the spring to shut the valve, and in turn the wastegate, in instances other than when you let off the throttle, while boost pressure remains at the desired setting; for example, when transitioning from the wastegate fully open at a certain RPM (preventing creep) to the wastegate fully closed at the same boost level at a higher RPM (turbo airflow is maxed out).

 

[donmagicjuan]

I got bored, so I constructed a theoretical step-by-step timeline to explain the operation of a ball-and-spring type MBC. I’m making the following assumption to simplify this discussion: the controller is set to maintain boost at 16 psi on an EvoIII 16G with a stock actuator setting of 8 psi.

Initial conditions: intake manifold pressure is at 0 psig, the throttle has just been fully opened, pressure in the actuator line is 0 psig, and the MBC valve and wastegate are shut.

Step 1: Boost pressure begins to rise, but it is still less than 8 psig, so the MBC valve is still shut – no surprises there.

Step 2: Boost pressure barely exceeds 8 psig. Since the actuator line pressure is still zero, the MBC valve cracks open. Pressure downstream of the valve attempts to equalize with intake system pressure, but as soon as it starts to rise, D/P across the valve drops and the spring shuts the valve. Therefore, pressure begins to bleed back to zero again. The pressure in the actuator line never reaches the level required to open the wastegate.

Step 3: Boost pressure is at 9 psig. Once again, the MBC valve opens, and pressure downstream of the valve attempts to equalize with boost pressure. When the actuator line pressure rises above 1 psig, the spring shuts the valve and pressure bleeds off. When pressure bleeds below 1 psig, the valve opens again and the cycle repeats. Thus, the valve will cycle to maintain an equilibrium pressure of 1 psig in the actuator line. At no point does the pressure in the actuator line rise high enough to open the wastegate.

Step 4: Boost pressure continues to rise between 9 and 16 psig. The MBC valve continues to cycle as outlined in step 3, maintaining actuator line pressure 8 psi lower than intake manifold pressure.

Step 5: Boost pressure exceeds 16 psig. Now the pressure in the actuator line is high enough to accelerate the actuator arm and open the wastegate. As the wastegate opens, D/P across the turbine is reduced, which reduces the output of the turbo, as we all know. Therefore, boost pressure is lowered. As this happens, the D/P across the MBC drops and the spring shuts the valve. Pressure once again bleeds from the actuator line and the wastegate is permitted to close.

Step 6: With the wastegate closed, boost pressure will rise again until the setting of 16 psig is exceeded, and the process repeats.

I hope this clarifies the point I’m trying to get across about this whole D/P thing. If you have any disagreements with what I’ve said, I’d love to hear them. I love this stuff, and I’ll never rule out the possibility that I could be missing something.

 

[EclipseTrbo420A]

Step 2: Boost pressure barely exceeds 8 psig. Since the actuator line pressure is still zero, the MBC valve cracks open.

What were all trying to say here it this this wont happen, The spring its self hold the pressure back untill 18PSI or in this story 16. I do see what your saying though, and I, as well could be worng. Lets join and E debate team