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Discussion Starter #1
A question..

Why does adding a mbc (of any type)increase the low down grunt and increase spool up...?

the wategate actuator is a diaphragm and a spring
boost pressure acts on the diaphragm and when it gets over a pressure (base boost ) the pressure on the spring opens and moves the rod which opens the wastegate and so limits boost

mbc's interferes with this and leaks some air away so fools the actuator by applying a lower pressure to the diaphragm (normal - leakage )
so attaining higher boost pressures
all well and good but in theory it should have no effect when the boost is low
But we all know that it does
Now take into account the apc valve
this is an electronically controlled valve which manages the pressure on the actuator ie an electronic boost controller ..which can leak pressure away from the actuator for more boost but if the ECU detects knock or conditions which could lead to overboost it can switch the pressure on to the actuator,opening the wastegate and so limit boost
So no effect in the low boost area then...
So why does an mbc have such an effect on the low down boost response ?
Supposition....
Does the APC valve allow the wastegate to open at low boost until it senses WOT and shuts it an allows the pressure and the turbo to spool up
if the mbc is interfering with this process then I can understand it but how can the apc valve perform this function ?
The spring in the actuator has still to be overcome and it will keep it closed until its threshold is exceeded

Any Ideas ?
Have I missed something ?
 

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I am no expert in MBC, but I'll have a go...:

As I understand it, the stock ECU does not use boost pressure as feedback. Instead it blindly cycles the wastegate control solenoid open and closed at the mapped rate in a manner similar to the fuel map in order to vent pressure out of the wastegate actuator. I would therefore deduct that the MBC uses this particular mapping feature of the ECU in order to increase the boost since the ECU is operating from its map, rather than the boost pressure increase. But its just a theory
 

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Originally posted by StanleyB:
[qb]I am no expert in MBC, but I'll have a go...:

As I understand it, the stock ECU does not use boost pressure as feedback. Instead it blindly cycles the wastegate control solenoid open and closed at the mapped rate in a manner similar to the fuel map in order to vent pressure out of the wastegate actuator. I would therefore  deduct that the MBC uses this particular mapping feature of the ECU in order to increase the boost since the ECU is operating from its map, rather than the boost pressure increase. But its just a theory  
 [/qb][/b]
I'm not sure how it works on T5, but on T7 the boost climb rate is highly variable. I played with an MBC and did not get any increase in boost climb rate over cool driving conditions.

Warmer weather is another matter. Boost may reach the stock 15 psi in some warm weather, but the boost climb rate is significantly reduced because the ECU doesn't "trust" the engine not to knock. In these conditions you might see more power with MBC, but you'd also threaten knock on the engine, or it would just pull the ignition timing way back because the boost is rising quicker than it's supposed to.

To summarize my thoughts ...

The MBC is commited to whatever boost setting you adjust it for. If it detects knock on the way to that boost setting, TOO BAD.

The variable adjustor however adjusts at a rate where the Trionic ECU can "see knock coming" ... as unlike most ECU's Trionic can see small amounts of knock before it becomes serious. But it can only react quickly enough if the boost climb rate is slow enough.

In theory a BPCV should be just as quick as MBC ... but it's probably better that in practice it generally isn't.

Dubbya~
 

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Ylee,

I think the theory is, when for example base boost is set at 6psi, when boost is 5psi, the wastegate is only 1 psi away from opening... but.... there is also a pressure diferential accross the valve in the turbo itself which may (will) be over 1psi (more so if you have a feree flow downpipe such as the JT one. the sping also needs to overcome thsi pressure differential.

the MBC would stop any of the pressure getting to the WGA until the MBC's set point had been reached, this would mean the the wastegate valve is being held shut with the full spring pressure.

Andrew
 

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Discussion Starter #5
so on acceleration the wastegate is pushing against a constantly reducing pressure (std mode)
and could be opening or tending to open so reducing the exhaust push on the turbine
In mbc mode the wastegate is pushing against the full spring pressure
So the mbc works because even at less than base boost pressures the wastegate could be being pushed open
the mbc makes sure it is shut.. hence low speed grunt and quicker spool up
and more pressure on the turbine wheel (as it is always full-on at low pressures )

Now with the apc cycling away thinking it is on the map the mbc has taken it off the map
hence overboost spikes

have I missed anything ?
 

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Yes,

Except with the MBC, the boost is rising much quicker below base boost as the waste gate is being held shut under it full (6psi in this case) weight, the ecu might not be quick enough to catch the boost as it flies past 6psi and clips the over boost. This is especially true of fast sppoling turbo's such as small T25's and bigger ball bearing turbo's.

I think that the pressure over the physical wastegate valve can be quite high so that the wastegate can be leaking gases from even quite low boost pressures.

I think that some older turbo's were limited to 2:1 diferential between compressor and exhaust sides (i.e 1psi boost, 20psi exhaust pressure).

It would only take in this example a 6 psi differential to open the wastegate weven with am MBC.


A
 

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I tried an MBC on mine prior to Stg3, and with it fully open it would just work, with barely any more of a turn it would over boost at 2750 in 4th and 5th once but every acceleration, bust base boost was still set correctly.

I've not tried it with the stg 3 with it's higher (23/24 psi?) boost limit

a
 

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Discussion Starter #8
And we get down to the nub of it ...
before I got my stage 6 my car was ferocious above 2k with the mbc..
With the stage 6 it does not get going till 3k
I miss the ferocious take off And I find it dificult to wait till june till I get it re-programmed
so....I guess it is worth a try even set at a low figure it should help the low speed response even with the larger turbo..
my fear is over boost
..It would blow the top off the engine
 

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ylee you don't WANT any boost below 3,000 rpm with a GTBB30 turbo ...

With a typical Saab engine a 500+ hp turbo shouldn't be spooled up until after 3-4K rpm ... if it spools up before this it is well outside the appropriate compressor map, and subject to both compressor stall, and surge loading! Not to mention sky high intake temps as the efficiency in that region is horribly low.

A better, albeit more expensive, solution is to change the gearing so that the 4,000+ rpm band can be used more readily. The only problem with that is you then have too much accelleration for your tyres to use and all goes up in smoke.
Oh well ... you could always put a smaller turbo on it. The GTBB32 is right about perfect at 38 lbs/min an a 420 hp rating. It should spool considerably faster, but then what would you do with the GT30R?


Dubbya~
 

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Discussion Starter #10
I do not understand
the Turbo is independant to the engine as far as spooling up
as long as the gas flow is there to turn the turbine and the compressor wheel it does not matter what revs the engine is at
it matters what the turbo is "revving " at ...
so if you can get it moving at low engine revs so that it supplies puff at these revs all you have to deal with is the torque !!

or are you saying that you need the air flow through the engine (revs) or the compressor does not flow efficiently
To determine this you would need to know the flow characteristics of the head and the camshaft duration and overlap added to the flow characteristics of the plumbing and i/c

as well as the compressor map combined with the
exhaust speed /turbine speed characteristics ...

Interesting

If I remember right my son (motorsport engineering degree ) had a program which you could dial in all these characteristics and would predict output ...

I must break into his computor and have a look !!
 

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Or you can go here and plot flow and boost maps onto existing compressor maps:

http://www.turbofast.com.au/turbomap.html

Compressor maps for the GTBB series turbos may be found here:

http://www.atpturbo.com/root/index.htm

The engine speed matters greatly. Engine speed makes a HUGE difference on where things are on the compressor map. And if you operate outside the compressor map on the left hand side you will "surge load" ... very very bad for both the engine and turbocharger. Play around with that turbo map tool for a while. It will help you see why big turbos on small engines have a narrow operating band, and have to use high revs.

Dubbya~
 

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Discussion Starter #12
Narrow ?
well narrower than my old set up
but I can do 40 -100 in about 8 secs
and at least 2 of these is turbo lag !!!
I must say Maptun have done an incredible job in giving me a perfectly tractable car which,driven normally ,feels completely benign ...drop a gear or two and watch these wheels light up !!
 

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Quote Adrian_W:
 The operating band of a high boost application is naturally narrower than a corresponding low boost application.  [/b]
To that you should add: unless the ECU has been re-mapped to even out things a little. If you go for optimum re-mapping over a selected RPM range as opposed to maximum re-mapping (like many of the ECU tuners do) you can get a far better result IMHO.
Take for instance the map of my ECU; it is still producing loads of boost between 5000 to 6000RPM, whilst the regular re-mapped Abbott ECU more or less runs out of steam after 4000RPM (on a couple of 2.0L engine that I have tried anyhow).
But there are of course the cases where the tuners moved the whole power map to a higher RPM range. Take for instance the (previous?) map of /John's ECU. I noticed that it came in a around 3500RPM. I never did find out where it ends by the way
. I treat other people's car with a lot more respect than I treat my own
.

In the case of engines where no ECU is present, things are of course as per Adrian's description I would say.
 

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Err ... what I meant is the turbo map will be narrower.


Irrespective of ECU mods, a high boost map does not have the "ability" to be as broad as a low boost map. That's just due to the nature of all centrifugal compressors. Various turbo trim levels and a/r ratios can alter things, but that's what it eventually boils down to.

Sometimes ECU's are tuned more conservatively than the turbo's compressor map. In which case they can later be broadened, which is what you're talking about.

Dubbya~
 

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Discussion Starter #16
Sometimes ECU's are tuned more conservatively than the turbo's compressor map. In which case they can later be broadened,  [/b]
and that is exactly my position
I know my head breaths well but the tuner has to be conservative so I reckon we can broaden it out nicely
 

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 Sometimes ECU's are tuned more conservatively than the turbo's compressor map. In which case they can later be broadened, which is what you're talking about.  [/b]
The ECU can be re-mapped for both height and width.i.e: more/less power at a given RPM, and more/less power at either end of the RPM range. Most tuners go for the height. Just super-impose the standard SAAB power curve over that of the majority of the SAAB tuners to see what I mean.
To get both width and height, you are likelyto have to go for a customised re-map like I had to do.
 

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This is true ... the ECU can be re-mapped up down and sideways, but the compressor on the turbo still has a limited useable map.

For instance a 600 hp turbo will never ever ever ever be fully spooled to 2 bar on a 2.3L engine by 1,500 rpm. Even if you used a special injector to force the turbine side to spin up it simply won't do it.

I think my point may be getting lost so I'm going to try to explain why this is the case.

A centrifugal compressor works on inertia. The air towards the outer edge of the compressor is spinning quickly, and there is no centripital force to prevent it from being flung outwards. Because of this it "falls" out of the edge of the compressor and into your engine. As it falls away from the compressor it causes a vacuum where it used to be. This draws in more air and the cycle continues.

The amount of force with which this compressor can move the gas depends primarily on the compressor wheel's speed. When moving quickly it can push lots of air. Of course a larger wheel can flow more air at the same speed as well.

There are other things to consider like "trim" on the compressor. Low trim makes for a large ratio between inner and outer diameters. This increases the inertial differential between the two when the compressor is spinning, and while not generating any more flow, it allows it to generate more pressure. At any given turbo speed the maximum pressure difference is equal to the inertial difference between the air in the center of the compressor, and the air at the edge. But now we're running into a problem ...

A turbo of a given size MUST flow a certain amount of air in order to keep from freewheeling. Imagine this:

You have some device which spins a turbo really quickly, but you've closed off the turbo outlet with a plug of some sort. The turbo pressurizes it's high pressure side, but then the air has nowhere to go. Because the pressure difference across the compressor relies on the flow of the air moving out of it (remember it generates the vacuum to pull more air in) suddenly despite spinning very quickly the air goes the WRONG way through the turbo charger.

Once enough air from the outlet has been evacuated the air will flow the right way again until the pressure is once again too high for any flow to take place, then it will go the wrong way again. This process is nearly instantaneous and called "surge loading". When you hear a car like a Supra going "WHEW WHew whew whew whew whew" they are surge loading. It's the exact same sound you'll hear if you plug your BPV or BOV. (You're welcome to try it if you like, though I don't reccomend it.)

Now this would be a non-issue if the air flow had to be completely stopped for it to occur. Unfortunately this is not the case.

The point at which the flow in a turbocharger is no longer sufficient to generate the appropriate pressure differential is proportional to its speed and the pressure difference across the compressor. Because of the larger area in the vanes of a larger turbo, it requires more flow to maintain the appropriate pressure difference. This is why larger turbos cannot spool up at lower RPM.

To the left side of this compressor map (TD04HL-18t) there is an area known as the "surge zone" ...



At low RPM an engine simply is not taking in enough air to avoid surge loading the compressor. So why not turn up the boost? Well if you raise the boost you raise the pressure differential, and the higher the differential the more flow is required. Raising the boost pressure generally creates a much larger pressure difference with very little increase in flow.

Operating in the surge zone doesn't mean immediate surging however. As long as you are only baaarely inside it you may get away with it most of the time, but it's not a good place to be. Very low efficiency combined with risk of turbo damage. Most turbo companies tune their exhaust side to not allow the turbo to be spun into the surge zone. Or at least not easily. Hybrids sometimes upset this ballance, but very rarely. Hybrids just require more careful tuning to avoid it.

At any rate, this is the primary reason above all else, which prevents a large 550 hp GT30BB turbo from being usefull below 3,000 RPM on a Saab. To find more exact points of reference the afformentioned Ray Hall Turbocharging site can provide you with near exact flow points from which to work with. Hopefully it will be helpfull.

Dubbya~
 

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Discussion Starter #20
interesting as always..
however Ray Hall is makeing huge assumptions about such things as volumetric efficiency etc and while this is good for ball park figures when it comes to fine tuning ecu maps you have to be able to measure flow through the engine.. as I have said before this will determine where you draw the line as far as where the boost starts.(or you do it on a dyno empirically )
My inference is the ..."better the flow the lower the blow "
has a certain ring to it doesn't it
 
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