I have been asked by a fellow forum member to give my take on this thread, and since I hold him in particularly high regard, and although much of it has been said by others, I will do my best not to disappoint him. So, in my hallmark condescending tone of voice, I plunge myself into the flame wars.
First, I'd like to comment on a couple of minor issues early in the thread. I too do not think that it is advisable to restrict the oil flow on a journal bearing turbo.
DanR raised the very valid question of internal vs. external wastegates. An external wastegate is preferable, because internal wastegates tend to make a mess of the flow around the turbine wheel, which may significantly reduce its efficiency (some turbines are worse than others). When we are using the wastegate, we are trying to make a lot of power, so that would be an opportune moment for good efficiency. The argument that the wastegate is there to reduce turbine power anyway is not valid, because with greater turbine efficiency we would be able to open the wastegate more, to get lower exhaust manifold pressure, which is good for engine pump work and cylinder emptying alike (meaning less crankshaft power is expended during the exhaust stroke and more fresh air is sucked in during the intake stroke). Furthermore, an external wastegate lends itself to interesting things. Depending on your preferences and requirements for emissions and sound, you can choose to reintroduce the wastegate flow into the main exhaust flow in a number of ways. Put it in upstream of the cat and you are always emissions and sound compliant. Put it in upstream of the muffler and you are always sound compliant, but the pressure drop over the cat is much reduced. Or just dump the darn gas! You can run fairly high power with a stock cat (and be clean for the vast majority of the time), the pressure drop over the muffler is also reduced, and you get a clear audio feedback on the boost. On my S4, if I ever decide to change the turbo, I want to get a V8 thing-under-the-bumper, and make a wastegate tailpipe to the left. In the unlikely event that I decide it is too loud, a small muffler can be added. Incidentally, this is how aging P911T's work.
The main issue here, however, was turbocharger selection. This is part science, part black magic, and part luck; with skill and experience, informed choices may be made, but the only way to be certain is to experiment, preferably using well designed and executed procedures, so as to ascertain the relevant data and to eliminate the sources of error as much as practically possible. I, too, feel that it will most likely be impossible to declare a clear winner even in a well controlled experiment, as the choice will be highly dependent on personal preference. When speccing a turbo, we always need to make a compromise. We should balance at least the following:
* Reliability (refers unpredictable failure)
* Durability (refers predictable failure or life expectancy, not to be confused with the above)
* Physical size restrictions
* Response at various engine speeds (I prefer that term over lag or spool-up)
* Power or torque at various engine speeds
* Fuel consumption
* Product cost
* Plus whatever I happened to forget
Each person will have different priorities, and different goals leading to a different setup of any number of things around the car, so one person simply declaring one turbo the winner on one car would certainly be interesting to hear about, if nothing more. Some people might favour the cost and reliability of journal bearings, while others favour the response and efficiency of ball bearings; and so on. For the time being, all that seems to be available to us are a few compressor maps on the suppliers websites. We may attempt to compare them (we notice that WC has swapped the axis labels around), and it seems to me that, perhaps, the WC compressor sizes are slightly larger than PUKs at the same stage numbers. But not only are the maps quite different in graphical style, even if they were not, they wouldn't be truly comparable unless they were produced on the same flow bench by the same technician (for example, I would argue that the surge limit is a matter of opinion). There are no turbine maps to compare, but then, turbine maps never did say much to anyone anywhere anyway. There is no telling the response. The data at hand is grossly incomplete. We must experiment, and I too would hope for the suppliers support, since I feel that they are currently asking us to buy expensive hardware of which nothing much is known. Appallingly, the higher spec products don't even come with compressor maps.
Ideally, I feel that the proposed experimental comparison should also include something out of the GT28R range as seen on http://www.turbobygarrett.com/turbobygarre...ts/catalog.html. This would be a bit more involved to do, however, as it is not a direct bolt on fit, but on the other hand, part of the range lends itself well to the external wastegates that I like so much. Probably overkill for this thread, but it would have been interesting all the same.
Personally, I am biased towards preferring good turbocharger response and broad maps. I would tend to be sceptical of clipped turbine wheels and compressors with no maps; and, while flow capacity is nice, I would want a reasonable surge margin. I don't subscribe to the viewpoint that the highest power number at a single rpm number on a dyno run wins, rather, I want the fastest real world car, and that does require good response and a torque curve somewhat wider than the space between gears. For that reason, I think that lap times on suitably fast track should be a prominent feature of the experiment. I think the testing should be done during a period of particularly stable weather to get fairly similar running conditions for each turbo. Meteorological data should be recorded; and tyre pressures and fuel load and all that stuff should be kept the same. When on the dyno, you should, as a bare minimum, record power and boost as a function of engine speed in a tall gear (4th seems like a sensible choice), and also try to record the boost as a function of time when idling at an intermediate engine speed (say 4000 rpm) and slamming on full throttle, to get some sort of comparable turbo response times. If you get all ambitious, you also measure the air mass flow as a function of engine speed (if you can), as this will tell us where we are in the compressor map, which in turn allows us to read theoretical compressor efficiency and surge margin, which tells us a lot about how well matched the turbo is to the engine. If you can find a dyno that can do this, then it is well worth travelling a bit to get there. Also, please try to record the compressor discharge temperature as a function of engine speed at full load (the silicone hose should seal well enough even with two thin wires between the charge cooler and the hose). The higher the temp, the poorer the compressor efficiency. If you get even more ambitious, you also measure pressure and temp before and after the turbine, so we can determine the turbine efficiency (and ultimately the turbo efficiency), but that requires drilling into the exhaust manifold near the turbine, which may not be to everyone's taste, although it affords a perfectly useful pyrometer instrument in the car afterwards (just don't use el cheapo temp probes that break and mess up the turbine wheel). In a professional setting, well over a hundred engine parameters would be recorded, but we also need to stay practical here.
With some experimental data, we might better be able to make informed choices. Now, do I think that would help Bibs in his quest? Well, there is no telling, really. I mean no disrespect to the honourable Bibs whatsoever (or to anyone else for that matter), and I admire his initiative and dedication to our common cause that shows up yet again here, but having read this thread, it seems to me that he may be suffering from PLE syndrome (planless engineering). This is an all too common ailment amongst amateur car modders. "I'll pick up a few goodies and bolt them on, that should make me faster", seems to be the way the majority of these people think. While that theory may well hold true, it is not the recipe for achieving anything resembling an optimum, in which all parts come together in well-matched harmony where no single part is particularly overpriced and overspecced (or underspecced for that matter). What he needs to do is to come up with some concrete realistic goals and a budget to match. This can then be broken down into a coherent plan, and that plan can then be executed. This doesn't have to be something fancy enough to be presented to a board of directors, just scribble something down, but do include the whole car in the process, and make your choices with the greatest of care. Like, for the engine, you might say, I need so-and-so much power, with fair response and torque and whatever characteristics you prioritise. To achieve that we somehow determine that we need, say, a bigger charge cooler, a chip, some injectors, some cams, and a turbo to match. Tally up the cost and compare to the budget. This may be an iterative process, and for something as complex as an engine, professional advice by someone experienced in our particular engine and without further financial stake in the project may be well worth it if you are serious about what you are doing (i.e. don't just ask the guy selling you the parts)(btw I'm experienced enough in the art of engines to realise that I'm not qualified for this). Then, and only then, go out and buy all the stuff you need and bolt it on.
Don't do what the proverbial Bibs is doing. Suppose he chooses his favourite turbo and puts it on his car. Then, a few months down the line, he decides he wants a bigger charge cooler. And so on and so forth. And before he knows it, that expensive turbo of his, that was determined at great pains to be the best available, has become a complete mismatch. Don't embark on your 400 bhp project by first perfectly matching the turbo to 300 bhp. I know this goes against the popular view that tuning comes on in a number of stages, but that is the way I see it. This methodology is called project management, and it is there to get you the most bang for your hard earned bucks (or quid or whatever you use) through minimizing the number of hasty choices and regrettable purchases. While it cannot guarantee that you get the very best available turbo for your finished package, it can certainly increase the likelihood.
Good luck and best regards all around!