An interesting point Jacek, but with no mods to the intake and the replacement of the stock filter with a free flowing one would surely, by your own logic, not alter the resonance and/or shockwave patterns nor would the overall fluid dynamics have been changed, but it "would" be able to flow more air. Given a K&N usually gets added after the cars have been increased in power from stock 210/200 to 250/260 this extra flow of air would benefit the increased airflow request from the Turbo, or perhaps not...
With regard to the point on trumpet shaped volume flows, do you have examples of the shapes etc? If you look at the design of the initial intake pipe from the radiator area to the airbox it is very narrow so as to squeeze past the rads. This, along with the entry point into the airbox itself has been modified by at least 2 forum members who have significantly tuned their cars (clogz & Coxy) Clogz did quite a lot of data logging on the subject before making the change to the first stage intake, but for the reasons you stated, kept the airbox itself.
Although I would like to keep the airbox, the intercooler that will be fitted might not make it viable to get a cold air feed to the airbox without cutting a hole in the bumper, so switching to a more direct feed system abale to scavenge air may be the only solution. If I do end up switching I do not intend to change the MAF or resonator out but instead just replace the airbox that connects to the MAF with a direct Maxogen Cone. This way I keep the shockwave absorption designed by the engineers and gain a scavenge air feed.
The resonator box has two isolated chambers, one before and one after the MAF, not interconnected so the shockwave absorption turbo side would not be effected by removing the airbox itself.
As a matter of fact, turbo is capable of delivering 2.8Bar (or more, but i've not tried it myself) with stock intake.
Less restriction in the airbox might benefit it by improving turbo lag - but this would have to be tested.
However i would rather not use K&N filter but a bigger paper filter as K&N filters are allowing a lot more dirt through (which will not cause the engine to explode immediately, but will increase wear over time).
It would be good idea to measure turbo inlet pressure to see if there is any significant vacuum there - if you are getting atmospheric pressure (or very close to) at that point - then you can't get it any better and modifications to the intake system will not improve it.
As for the trumped shaped inlets, wikipedia is probably the best place to start Velocity stack - Wikipedia, the free encyclopedia
- intake system on 156 used this type of inlet in few places (i've never had a chance to take apart the one on 159 so can't say anything about it), for example MAF was attached to a trumpet which then was attached to the airbox.
Cold air feeds on turbocharged intercooled engines are another thing that is bit of a waste of effort - in the end what matters is intercooler outlet temperature and you will gain more by changing the intercooler (or even cleaning it - especially after it's all gunked up with oil and all sorts of other stuff inside and dead bugs on the outside) than by messing with cold air feeds. So you can suck very hot air in, but as long as your intercooler is cold, air going into the engine will be cold - and with standard intercooler, it does get pretty warm so there is a lot more to be gained there.
Also, there is a huge difference in how you design whole intake and exhaust system for naturally aspirated engine and forced induction engine.
Exhaust system is a lot simpler for a forced induction system as you can just stick a big thick pipe and you don't have to care about flow velocity, instead you want lowest possible pressure at the turbocharger end, where in naturally aspirated engine exhaust plays a big role in cylinder scavenging and you want nicely tuned exhaust system that has highest possible flow velocity.
For the intake, all you care about is for temperature in the intake manifold to be lowest and pressure to be highest, this is true for both types but it's achieved differently, and with nicely tuned N/A system you can even get 1.2bar or so in the manifold (think about VTEC yo! or any other kind of variable manifold system) - with forced induction, you can get more air by just increasing boost and don't bother with tuning it.