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...

 

[EclipseTrbo420A]

It says "reach the WGA" not "open the WGA". Considering how complicated it is to try to explain this (and understand it), my guess is that A) the person who wrote the description doesn't know what's really going on or B) that person dumbed it down a bit.

Very good possibility and heres the rest of that, word for word.

"The boost pressure then presses against the Wastegate Actuator's diaphragm, causing its arm to move."

Im kinda tired , Im going to get some sleep now. I enjoyed reading your points and hope to see some more tomorrow. Thanks and sleep well, tomorrow is a new day and many more basketball and 402 WHP room scenarios to come.

 

[EclipseTrbo420A]

In my scenario, for 18psi boost, the MBC is leaking at 10psi boost, and it doesn't ever close again until you get off the gas. As boost increases, the leak gets worse and worse. Since the vent hole requires more pressure to increase flow out through that little hole, the pressure in the WGA begins rising from 0psi once the MBC first starts to leak. The boost is rising, the leak past the ball is getting worse, and the WGA pressure is rising because it only has a tiny hole to let that increasing inflow of air out. Eventually, boost is 18psi, and the WGA pressure is 8psi, and the flapper opens and boost goes no higher. You continue at full boost for several seconds, and the pressures and flowrate through the MBC are constant.

One last thing, say the MBC does "Leak" at 10PSI. Whats stopping it from seeing the full 18PSI all over the line, after the turbo is kicking? Im in awe of what your saying. This turbo is pressureizing a huge FMIC, piping and Intake manifold and a motor that counsumes this pressure at a HIGH rate....but it cant hold a VAC line and tiny chamber with a pin hole leak at 18PSI? Its to powerfull....the MBC and all lines and chambers associated with it become part of the Intake manifold in a way....just because out of the whole system that gets pressureized this is maybe 2% of it.

Boostcontrollers.com- The MBC is adjusted by turning a knob (or other adjustor), which varies the load on the spring inside the MBC. By adjusting it so there is more load on the spring, you are 'raising the boost" because more boost pressure is required to move the ball off its seat before the signal can pass to the Wastegate Actuator. By contrast, lessening the load on the spring allows the boost signal to more easily unseat the ball and continue on its voyage to the Wastegate Actuator, so by backing the adjustor away from the spring, you are "lowering the boost". The Joe P MBC, and all Hallman Manual Boost Controllers, are ball-and-spring type MBCs.

 

[kenamond]

One last thing, say the MBC does "Leak" at 10PSI. Whats stopping it from seeing the full 18PSI all over the line, after the turbo is kicking? Im in awe of what your saying. This turbo is pressureizing a huge FMIC, piping and Intake manifold and a motor that counsumes this pressure at a HIGH rate....but it cant hold a VAC like and tiny chamber with a pin hole leak at 18PSI.

Pressure is force divided by area. 10psi is 10 pound applied to 1 square inch. So the opening in the MBC may be 0.25 in^2, so that means the force would be 2.5lb pushing on the ball. Not too hard for a compressed spring to push 2.5lb.

 

[EclipseTrbo420A]

With bleeder types, a valve simply "bleeds" off some of the boost pressure that it receives. It always allows some boost pressure to reach the Wastegate Actuator, but the boost pressure that the Wastegate Actuator receives is always less than the level of boost pressure in the charged portion of the intake system (or the boost level delivered to the bleeder-type valve) because this kind of MBC basically is a controllable boost leak. Since the Wastegate Actuator does not receive the "full boost signal", it only opens the Wastegate when the amount of boost that gets past the "leak" is sufficient to force it open. The bleeder-type MBC is adjusted by changing the size of the leak. Closing the leak down lowers boost level, because more of the boost signal then reaches the Wastegate Actuator, opening the Wastegate sooner. Opening the leak wider raises the boost level, as more boost is released to the atmosphere, as opposed to being delivered to the Wastegate Actuator as a boost signal; so the opening of the Wastegate is delayed.

This is what you guys are saying?

TO above: thats why the spring is adjustable and not stationary. So you can make it take 18PSI to move.

Im going to bed for real now, Night !

 

[toojung2die]

If you agree that the flapper is open and that the boost is at 15psi, then that means the WGA pressure is 8psi (remember, the flapper is open, which can only happen if the WGA is at or above 8psi) and the boost is still 15psi...so the MBC has a pressure difference across it of...what?

The difference is 0psi, there's 15psi on both sides of the check valve, minus a negligible loss on one side due to the bleeder hole. This neglible loss is enough to keep the ball open and a small amount of air flowing through the MBC. This 15psi holds the 8psi WGA open.

We contend that if you want 15psi boost with an 8psi WGA, you must set the MBC to 7psi.

If you set the MBC to 7psi, it opens at 7psi (ignoring pop-off) and applies 7psi to the WGA. Since the WGA requires 8psi to move, nothing happens and it stays closed. This senario is the equivalent to having no MBC and connecting the WGA directly to the boost source; the WGA only opens at it's set pressure (8psi in your example).

I don't doubt what you say about ball-spring valve pop-off issues (that's good to know). But in our situation where the MBC opens at ~7psi, that popoff might happen at 8 or 9psi, and this will happen when the turbo first spools through those boost levels. The MBC will stay popped off from then on out until you let off the gas. So it shouldn't be an issue.

If you want a 8psi WGA to open later at a higher boost pressure, you need to have a check valve set at that higher desired pressure in the line to keep the WGA from seeing any pressure until the desired pressure is reached. Here's real life evidence: My MBC is set so I get 15psi on my boost gauge WOT. My MBC does not leak air during a boost leak test until the intake pipes are pressurized to 15 psi, nothing less than that will cause it to release.

 

[donmagicjuan]

Here's something to think about: during steady state operation, the MBC valve is barely open, so that flow into the line matches flow out of the relief hole. Since it is not fully open, it introduces a significant restriction in the path of the air from the pressure source to the actuator. Much like an orifice, this restriction results in a pressure drop across the valve ball, which explains the lower pressure in the WGA line than the intake manifold. It's called head loss.

 

[toojung2die]

Here we totally agree. That's what I described in my first response in post #80. The pressure drop is very small but enough to keep the valve open and a little air flowing.

 

[donmagicjuan]

Here we totally agree. That's what I described in my first response in post #80. The pressure drop is very small but enough to keep the valve open and a little air flowing.

The pressure drop has to be significant, otherwise the spring would shut the valve.

 

[toojung2die]

The pressure drop has to be significant, otherwise the spring would shut the valve.

No, it only has to be slightly larger than the pressure set point. Slight difference, slightly open. Large difference, largely open.

 

[kenamond]

No, it only has to be slightly larger than the pressure set point. Slight difference, slightly open. Large difference, largely open.

If the preload on the spring is greater than the boost pressure times the area of the ball which is exposed to that boost, the ball will not move. Now if there is not a good seal between the ball and seat, then it will leak, but a small leak will cause a HUGE pressure loss. If it didn't, then that leak at 9psi boost would open the WGA and no matter how tight you made the spring, it wouldn't close the leak and you'd see 9psi max boost.

So if there is a good seal, it won't leak, and the ball won't begin to leak until the pressure times the exposed ball area exceeds the spring preload. So you get no signal at all to the WGA until the boost reaches whatever D/P overcomes the spring. Then it will *start* to leak, but not too much, and the pressure will drop considerably as air leaks past the ball, so the WGA pressure will be much lower than the boost pressure.

 

[toojung2die]

If the preload on the spring is greater than the boost pressure times the area of the ball which is exposed to that boost, the ball will not move. Now if there is not a good seal between the ball and seat, then it will leak, but a small leak will cause a HUGE pressure loss. If it didn't, then that leak at 9psi boost would open the WGA and no matter how tight you made the spring, it wouldn't close the leak and you'd see 9psi max boost.

So if there is a good seal, it won't leak, and the ball won't begin to leak until the pressure times the exposed ball area exceeds the spring preload. So you get no signal at all to the WGA until the boost reaches whatever D/P overcomes the spring. Then it will *start* to leak, but not too much, and the pressure will drop considerably as air leaks past the ball, so the WGA pressure will be much lower than the boost pressure.

You're over complicating a simple concept. You can ignore the area of the ball and the WGA. Don't even think about leaky valve seats. All that you need to know is what psi you want the check valve to open at. Once that psi has been achieved even a very small amount of difference in pressure will keep the valve open. Ounces of pressure will keep it open, grams/sq cm are enough.
I have to get to work. It's been fun, check back later...

 

[kenamond]

You're over complicating a simple concept. You can ignore the area of the ball and the WGA. Don't even think about leaky valve seats. All that you need to know is what psi you want the check valve to open at. Once that psi has been achieved even a very small amount of difference in pressure will keep the valve open. Ounces of pressure will keep it open, grams/sq cm are enough.
I have to get to work. It's been fun, check back later...

I'll check this hypothesis out next time I do a boost leak test. I should see no leaks out my vent hole at anything less than 15psi boost test pressure. Then it should keep leaking out the vent hole until the intake pressure drops back to 0psi. If someone beats me to this test, I'd love to hear the results.

 

[EclipseTrbo420A]

I'll check this hypothesis out next time I do a boost leak test. I should see no leaks out my vent hole at anything less than 15psi boost test pressure. Then it should keep leaking out the vent hole until the intake pressure drops back to 0psi. If someone beats me to this test, I'd love to hear the results.

I think what he is saying is at anything under 15psi it will close again. So at 15 psi during a boost leak test it will then and only then start to leak and when the pressure is bled off untill 14.9PSI it will close and hold, untill more pressure is applied and the force is at/above 15 again.

 

[kenamond]

I think what he is saying is at anything under 15psi it will close again. So at 15 psi during a boost leak test it will then and only then start to leak and when the pressure is bled off untill 14.9PSI it will close and hold, untill more pressure is applied and the force is at/above 15 again.

That's not what I read in his post:

Once that psi has been achieved even a very small amount of difference in pressure will keep the valve open.

Anyway, I think I'm done debating this until someone does an experiment to prove it one way or the other.

 

[burntheblobs]

Holy lord, what have I created! =P. Can we at least come to an agreement to weather or not I need a new MBC?

 

[mobythevan]

Can we at least come to an agreement to weather or not I need a new MBC?

If it leaks at 1psi boost test, you either need a new one or take it apart and clean the ball and seat. My ball and spring does not leak whatsoever during boost leak test at or below 10psi so you must have bad ball and seat or totally screwed up spring tension.

 

[EclipseTrbo420A]

Make one at home depot. Its easy and cheap.

 

[kenamond]

If it leaks at 1psi boost test, you either need a new one or take it apart and clean the ball and seat. My ball and spring does not leak whatsoever during boost leak test at or below 10psi so you must have bad ball and seat or totally screwed up spring tension.

What boost are you running?

 

[kenamond]

Holy lord, what have I created! =P. Can we at least come to an agreement to weather or not I need a new MBC?

Sorry about this long-winded thread, but the answer to your question is yes if you believe toojung2die and 420a and no if you believe me and Don. Either pick which one you believe or hang in there. I think that it is functioning properly. Your WGA opens at 8-10psi and your MBC ups that a few psi.

It is a boost leak, however, but it's not one you're expected to fix. The stock BCS recirculated the bled air back into the intake pipe (after the MAS so the air isn't double-counted), but an aftermarket MBC vents that bled, metered air to atmosphere. You could, in theory, tap the MBC at the vent hole, thread a nipple in there and recirc the bled air back to the stock BCS nipple on the intake pipe and not lose metered air. Since nobody does this, I suspect that the minor leak is not something you should worry about. It's like ring blowby or leaking valve stem seals; they'll leak during a boost leak test, but it's not anything to worry about as far as boost leaks are concerned.

 

[EclipseTrbo420A]

What boost are you running?

I knew it , I think he was just throwing that out there because of previouse posts, but a good question. What boost are you running and are you implying that your MBC leaks at 10PSI for sure, or guessing it does that?

 

[mobythevan]

I run 17psi

My boost controller does not leak up to 15psi to be more accurate(I just said 10psi because I wanted to point out for sure that his should not be leaking at 1psi if it is a ball and spring type, I did not want to get into your argument, but you drug me in kicking and screaming anyway). I do not test it any higher than 15psi because I do not want to put any more pressure than that on the turbo compressor seal. I use a stethoscope to listen to MBC to know it is not leaking. My intake holds pressure in the 20 second time frame before it bleeds down to 8psi and then it holds there longer.

All I will say is that it is abvious several people in this thread are not under the hood testing these theories, so get out there and then let us know the results.

 

[EclipseTrbo420A]

I run 17psi

My boost controller does not leak up to 15psi to be more accurate(I just said 10psi because I wanted to point out for sure that his should not be leaking at 1psi if it is a ball and spring type, I did not want to get into your argument, but you drug me in kicking and screaming anyway). I do not test it any higher than 15psi because I do not want to put any more pressure than that on the turbo compressor seal. I use a stethoscope to listen to MBC to know it is not leaking. My intake holds pressure in the 20 second time frame before it bleeds down to 8psi and then it holds there longer.

All I will say is that it is abvious several people in this thread are not under the hood testing these theories, so get out there and then let us know the results.

I gotta work at 4 Thanks for that info though.

 

[donmagicjuan]

I run 17psi

My boost controller does not leak up to 15psi to be more accurate(I just said 10psi because I wanted to point out for sure that his should not be leaking at 1psi if it is a ball and spring type, I did not want to get into your argument, but you drug me in kicking and screaming anyway). I do not test it any higher than 15psi because I do not want to put any more pressure than that on the turbo compressor seal. I use a stethoscope to listen to MBC to know it is not leaking. My intake holds pressure in the 20 second time frame before it bleeds down to 8psi and then it holds there longer.

All I will say is that it is abvious several people in this thread are not under the hood testing these theories, so get out there and then let us know the results.

If you look at one of my first posts, you'll see that I have tested my theory. I've done numerous boost leak tests at various boost levels, and each time the MBC started leaking at my boost level minus 8 psi. Weird, huh? Obviously that wasn't convincing anyone, so I moved on to more in-depth explanations.

 

[EclipseTrbo420A]

If you look at one of my first posts, you'll see that I have tested my theory. I've done numerous boost leak tests at various boost levels, and each time the MBC started leaking at my boost level minus 8 psi. Weird, huh? Obviously that wasn't convincing anyone, so I moved on to more in-depth explanations.

Mine doesnt leak man, that is funky and is probably the reason were all still in this. My post count is like 123 a day now.

 

[toojung2die]

I think what he is saying is at anything under 15psi it will close again. So at 15 psi during a boost leak test it will then and only then start to leak and when the pressure is bled off untill 14.9PSI it will close and hold, untill more pressure is applied and the force is at/above 15 again.

Yes, that's what I should have added.
Today I was thinking, "This ain't exactly rocket science, what's so hard to understand?". Then I remembered my friend, Hank, who IS a rocket scientist. He's a retired engineer from the JPL in Los Alamos. He helped design the first succesful Mars probe. I called him and he agreed to look at the problem and give me his answer. I'm believing whatever he says. I've sent the email and I'm waiting for his response. Can we agree to agree on the opinion of a rocket scientist??? I'll post my question and his response.