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Turbo Crossover Pressure

So there is!

However, I can't see how that can work properly.[8|]

Let me explain why I don't see this so someone can educate me....

The MBC is simply a pressure regulator which sets a limit to the amount of pressure that is passed downstream. To the WG in this case.

The single port wastegate will open when it receives sufficient pressure in its cold port to compress the spring in it's hotside.

So, the WG needs to open when pressure rises ABOVE a certain point and the MBC is designed to keep the pressure signal BELOW a certain point. Surely this is a fundamental missmatch......

Lets say for example that the WG will open when it receives a boost signal of 7psi.

If the MBC is set at 5 psi and you spool up to boost of 2 psi, the full 2 psi gets passed on. WG stays closed.

At 4 psi, 4psi gets passed on. WG stays closed.

At 5 psi, 5psi gets passed on. WG stays closed.

At 7 psi, STILL only 5psi is passed and the WG STILL stays closed......

Boost now rises to 20 psi, the WG still only sees 5 psi and so STILL stays closed........[:-]

For an MBC to work properly on a standard / single port wastegate, the above must be incorrect. Why???

Confused of Barkby.
 
An MBC probably provides a variable pressure drop. So effectively you are setting the pressure sent to the wastegate to be lower than the manifold pressure. E.g if you set a 5 psi pressure drop the boost level will be 5 psi higher than the spring pressure.
 
No, an MBC is a regulator just like you find on air compressors and industrial compressed air systems.

As far as I understand, they are designed to simply reduce the pressure down to the safe working pressure that devices or tools downstream require.

They set an absolute limit so that your 10bar compressor can be used with a tool that is only rated up to 5 bar say.
 
So the wastegate position doesn't move once the set boost pressure is reached then. I always assumed it varied its position with engine speed.
 
Oh yes, I'm sure it does vary because the pressure signal to it will (should) vary as the exhaust flow to the turbo is modulated.
 
But if the pressure to the wastegate is constant as long as the manifold pressure stays above the set pressure of the MBC then the pressure in the diaphragm of the wastegate will be constant and the wastegate wont move?
 
So, getting back to the question of how a MBC opens the Wastegate when the desired boost level is obtained"¦ When using the MBC, the user (that's you!) sets the MBC's adjustor to the desired setting. Then, when the car is accelerated, the boost pressure rises, and air pressure builds in the charged portion of the intake system. Because one of the MBC hoses is attached to this charged portion of the intake system, the rising boost pressure is also delivered to the MBC.

What the MBC does with the pressure that is delivered to it, and how it goes about causing the Wastegate to open, depends on what type of MBC is being used. There are two basic types of MBCs: ball-and-spring types, and bleeder types.

With ball-and-spring types, a spring-loaded ball is used to block this delivered boost "signal", until the desired boost level is attained. It is at this point, that the delivered boost pressure is strong enough to push the spring-loaded ball toward the spring and out of it's seat, allowing the signal to pass, and reach the Wastegate Actuator. The boost pressure then presses against the Wastegate Actuator's diaphragm, causing its arm to move, so that the Wastegate is opened. The opened Wastegate then allows the exhaust gases to divert away from the spinning turbine, thus preventing the turbo from boosting higher than the desired level.

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.

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.
Click to expand...

So the first type, the ball type keeps the wastegate close until a set boost level is reached and then full manifold pressure is sent to the wastegate. The bleed type allows a percentage of the full manifold pressure to bleed off so assuming its linear the pressure at the wastegate will always be say 80% of the full manifold pressure..
 
The first type, the ball & spring, is what is usually referred to and marketed as a Boost Enhancer.

I'm not sure that the second type sounds quite like the normal MBC either though. I thought that bleeder valves were different. I think they are also called Granger Valves and do indeed bleed a proportion of the pressure away as described above. But a pressure regulator, as commonly used for the MBC nowadays does not.
 
Sounds like you just need to be aware of what the MBC you are buying actually is. While we are on the topic of boost control, how about a little electronic boost control demo. I said I am varying the frequency when I meant varying the pulse width ;). Frequency is actually constant, we had to find the right frequency for the boost solenoid to give time for the valve to open and close effectively.

http://www.youtube.com/watch?v=Wia5vSNzRWM&feature=youtube_gdata_player
 
Tom, that development is maturing nicely...[;)], I am a bit curious. Do you rate the Apexi controller you have, can you moderate the rate of boost with it. Will your controller provide the same performance as the Apexi - or superior??
sorry for all the questions, - am just a bit conscious of the advantages of accurate control.
My car has the MBC sited under bonnet, and I see a 1psi variation in boost - possibly according to temperature there.
Perhaps I will upgrade this one day.
appreciate any comments on the subject - thanks
George
951
 
Its a pretty decent kit but it doesnt have closed loop operation which I though was a strange omission. Its a bit like an early car heating system where u just set the dial to *hot* and depending on how hot a day it is you get temperature X inside the car. Compared to a modern air con where u set the temperature you want and it just makes it happen. The Apexi is a bit the same you cant set a required boost level and leave it to get on with it. You have to tune it to get the results you want. It does have a learning mode but I didn't find that it worked very well. I just went back to the manual boost controller in the end.

As far as accuracy goes I am sure I can achieve the same accuracy as the Apexi controller.
 

ORIGINAL: GPF
Don't know if this will work but here is a video I uploaded to my photobucket........
Click to expand...

Interesting video Graham.

Took me ages to work out the scale on your boost gauge. You said 19psi yet it looked all the world like 15psi to me until I saw the scale wasn't 5, 10, 15 etc
 
Tom, thanks - it strikes me the MBC is fine but I will relocate it. I dont doubt you will achieve the same control or perhaps better. The Oil warning thru the ECU is more hassle than its worth IMO.

I enjoyed that video too Paul, Grahams car sounds very like my own - except for the dump valve...[8|]. Mine is only a 2.5L but there is quite a purposeful thump off it under load with good mods and a 3" D/P & exhaust.

George
944T
 

ORIGINAL: George Elliott
I enjoyed that video too Paul, Grahams car sounds very like my own - except for the dump valve...[8|]. Mine is only a 2.5L but there is quite a purposeful thump off it under load with good mods and a 3" D/P & exhaust.
Click to expand...
[:D][:D][:D] Yes, I must get my finger out and make up a proper inlet so I can tie that back in! Mine is still a 2.5L btw.


ORIGINAL: diver944
Took me ages to work out the scale on your boost gauge. You said 19psi yet it looked all the world like 15psi to me until I saw the scale wasn't 5, 10, 15 etc
Click to expand...
Ah yes, should have mentioned that!

It's a custom scale to match the layout of the AEM wideband next to it :

tmp2005_0323_080744AAcrop.jpg
 
Graham, Very neat setup, I really like the bank of switches without the S/R switch too

I am planning to derestrict the snorkel on my intake somehow, needs to stay OE look, but that should finally clear the airflow from intake to back bumper. It may increase the crossover pressure?! The 968turboS had this mod, I realise the 3.0 capacity made this essential, but interesting how the factory did it via the bonnet.

keep up the good work
George
944T

the Subaru forum will snap up that dump valve, £500 buy it now[;)]
 
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