Reviewing this
"classic" article (i.e., going back to the basics), I believe that better air flow is causing better atomization of the fuel, reducing particulate matter/soot emissions. Since soot seems to primarily be a problem in the cold start to runup range, the air flow is improved AT LEAST through the part throttle range (since most people won't go WOT until engine is warmed up).
...As the fuel enters the air stream, it is pulled apart and broken into even smaller particles. This process can be likened to spraying an aerosol can into the atmosphere. The liquid in the can is pushed from the high pressure in the vessel to the lower pressure in the atmosphere, and is broken into what the consumer would identify as a fine spray. The proper term is atomized...
...The smaller the fuel droplets are, the greater the surface area for a given amount of discharged fuel (in pounds/hour) and, thus, there is more exposure to heat, so the phase change to a vapor becomes much more efficient...
...As the engine speed increases, the velocity through the intake manifold also goes higher, and more of the fuel is atomized than would be at low speeds...Possibly better injectors with finer atomization patterns would be a solution, but unlikely to be an inexpensive one on a custom basis.
Not GDI related, but this Hot Rod
article provides insight into fuel/air interaction.
