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Discussion Starter #1
This one confused me no end at the weekend... and I still can't come up with an explanation, so I'll throw it open to wider debate...

I've recently shelled out for a very, very sexy roller bearing turbo, and am now starting to tune it up (the reduction in spool up time is phenomenal
).

My previous hybrid had developed a minor bearing weep so I deal a deal on the upgrade. I was running a 12PSI (0.8bar) actuator and MBC valve, very happily.

As fitted, the new unit had a base boost of 0.5bar. I had plugged this up to the regular APC solenoid, in order not to start flogging it too much whilst new. It was happily peaking at about 1.2 bar and then dropping back as normal.

After a bit of fiddling underneath, I shortened the actuator rod as much as I could without needing to get into re-threading (there wasn’t much scope for adjustment ) and got a base boost of 0.6bar. I connected it up to the MBC again and went off to adjust it. Except I couldn't get any more than about 0.8bar boost, no matter how far I opened the MBC valve. Previously it would have hit overboost cutoff after about 3/4 turn, but here I was 2 turns open .

So I reconnected the APC solenoid and everything was fine- peak boost a bit above 1.3 bar and holding pressure slightly higher as revs increased- exactly as expected.

So why, oh why, oh why can I not get the MBC valve to give me enough boost?

I've checked it- as you blow through it and undo the screw, you get progressively more air coming out of the bottom port.

Longer term I'm talking to Maptun about a custom mapping and new injectors blah, blah, blah (300hp? that's nothing ) but I just want some fun NOW!

Has anyone got any ideas?
 

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Personally, I think you're being very greedy running all those mods anyway. Theres only a finite amount of horsepower on earth you know, put any more into Bubbles and the rest of us will need to scour the galaxy to find other horsepower resources which have not yet been tapped.

So why, oh why, oh why can I not get the MBC valve to give me enough boost?    [/b]
Sounds like your either losing pressure due to a leak at the exhaust manifold or that theres some thermodynamic stalling. i.e. turbulent flow of exhaust gasses over the turbine wheel.

I read that last bit in a book BTW.
 

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I think it's that roller bearing turbo. Saab's were not designed for then or else they would have been fitted as standard.

Forward it to me and I will see that it is destroyed in the correct manner....
 

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Discussion Starter #5
Hmm... specs, piccies you say?

I can't remember the full specs off the top of my head- they're on a piece of paper carefully filed somewhere


But basically from memory it's a T3/T28 roller bearing hybrid (T28 turbine, T3 compressor) with a Garrett -31 actuator, good for up to 18psi base boost
. And it's chuffin big (see pic). The roller bearings basically dramatically reduce friction and spool up time, thereby giving you all the top end performance benefits of a big turbo without the lag. The A/R's have been modified from standard to cope with higher boost (I'm looking for a broad range up to about 1.6 bar) It's rated "comfortably" for 350hp. And it does feel very quick.

Now has anyone got any ideas...

 

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Discussion Starter #6
Originally posted by Jason (Mr Torque-Steer):
[qb]Sounds like your either losing pressure due to a leak at the exhaust manifold or that theres some thermodynamic stalling. i.e. turbulent flow of exhaust gasses over the turbine wheel.

I read that last bit in a book BTW.  
 [/qb][/b]
But then it wouldn't be working properly with the APC solenoid then, either...
 

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Ah well, I only half knew what I was waffling about anyway. Perhaps the reason why boost isn't building to such a high pressure is due to the balance of the system, i.e. the big low friction turbo will have less thermodynamic resistance (I forget the proper term) and therefore the rate of gas flow through the system is higher. If you get my drift ...

I reckon the only way to up the boost further is to improve the fuelling to match, which you've already mentioned. I reckon you'll be buying your tyres by the dozen shortly Mark.

Edited because it made no sense at all previously.
 

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That's a very strange thing going on.
I guess you're using the same three vacuum hoses for both the selenoid valve as for the MBC.
Are you sure there are no very sharp bends when one of the two devices is fitted? It could influence pressure.

BTW, nice turbo
 

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Discussion Starter #9
1. Jason, not having a pop, just clarifying for other readers...
Previous turbo with same MBC setup peaked 1.5 bar, held 1 bar
This turbo peaks fine when using APC solenoid, not with MBC- which as Maarten says, has the same 3 pipes connected so there can be no difference.

2. Maarten, I did wonder that, but the only way this can be true is if the R pipe is being blocked, but as this is the one that doesn't bend at all when fitted to the MBC I find it hard to believe. I guessI'll just have to try gain...

PS sorry it's a bit spotty, I did put high temp paint on but then hit an unexpected patch of wet whilst I was out "baking" it...

[edited for too many "a"s in Maarten
]
 

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Mark,

Have you tried running it with just a piece of pipe between the turbo and the pressure source, ie remove the MBC from the circuit.

If this gave the same response as with the MBC then I would suspect the boost controller.
 

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Whats the cost of a roller bearing turbo then. I want to put it in my budget for next year, I planning to spend 2K each year for 2003 and 2004 so I'm hoping the turbo alone won't wipe out that budget.
 

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Discussion Starter #13
Roller bearing turbo? That'll be about £1100 to you, sir, plus the fitting cost etc etc...

You're welcome to try the lack of spool up for such a big turbo at the SSc if you make it sober

Oh, and sorry, Alex, I missed your response but I'm not sure what you mean. Are you thinking I'm running the MBC between the APC valve and the actuator? I'm actually trying to run it in place of the APC valve. For a more detailed expansion on the possibilities, see this link

HTH
 

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You're welcome to try the lack of spool up for such a big turbo at the SSc if you make it sober  [/b]
Well, it turns out I'd cocked up my diary entries; I'm boozing it up on Saturday night. So I will be sober on Saturday, I'll just need to leave Chichester about 4ish to get to Mansfield for 7-30.

£1100, well my initial budget for a turbo upgrade was £750, but what the hell! Thats now gone into 2004's budget.
 

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Mark,

I assume that you are running the pipe from the compressor outlet nipple, through the MBC then straight into the wastegate actuator. If you were to replace the this with a piece of pipe running straight from the compressor nipple to the wastegate actuator and the behaviour was the same as with the MBC then this would tend to suggest that the MBC was at fault.
 

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Mark,

I have a bleed valve and a Dawes Device. They both give different power characteristics. A Dawes gives more low down torque:)

Having read about your problem it would seem to me that not enough air is being bled off from the bleed valve (mbc), operating the wastegate and giving base boost. There must be a route for the wastegate signal air pressure to be released or the wastegate stays open.

A Dawes Device holds back the signal air from the turbo until the preset boost pressure is reached, then releases it to operate the wastegate. A Dawes prevents pressure spikes as the turbo spools up wheras a bleed valve lets the surge of pressure through to the wastegate because of its design and if more signal air is coming through than can escape this could be your problem.

Has the bleed valve reached it's air flow volume limit used with your new super dooper turbo?

By the way I'm not a salesman for Dawes Devices, but they are more fun

You can always tell me to shut up if I'm teaching grannies to suck eggs
Hope this helps

Keith
 

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Discussion Starter #17
The comment about flow limit via the MBC would be true if there was a leak in it- but there isn't. it is, in effect, a constant pressure chamber in as much as the pressure applied by the air is equally opposed by the restoring force from the actuator spring.

Keith, I must admit I don't follow the logic of your statement:

"A Dawes prevents pressure spikes as the turbo spools up"

Whilst a turbo is spooling up, by definition it isn't generating full pressure, so the actuator wouldn't be moving, and pressure "spikes" as you call them are generally considered to be what happens when you get overshoot ie the pressure goes too high.

I too have a Dawes devicebut having bought it, and considerd howe itr worked, I decided I wasn't too keen on it's ability to cause damage- especially as I run high boost pressures. At least with an MBC you have a more gentle, controllable boost characteristic.

I also don't see why a Dawes should be any different on low down torque as again the turbo won't be at maximum speed/pressure- unless of course you are running a low base boost pressure.
 

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I too have a Dawes devicebut having bought it, and considerd howe itr worked, I decided I wasn't too keen on it's ability to cause damage- especially as I run high boost pressures. At least with an MBC you have a more gentle, controllable boost characteristic.[/b]
I was under the distinct impression that the Dawes devise is an MBC .

As far as the spiking is concerned: that can be prevented by following the set-up instructions to the T.
 

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Discussion Starter #19
Yes, technically the Dawes Device is an MBC or Manual Boost Controller. However in this context the MBC referred to takes the place of the 3 port APC solenoid. For more info see my link above...
 
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