Correct me if I'm wrong, but the stock airbox tubes are, in essence, rubber velocity stacks which are surrounded by the airbox. In this scenario, you'd get some performance boost from the compression effect of the stacks, because they're drawing less-turbulent, prefiltered air, yes?
So adding plain velocity stacks might actually reduce performance from stock?
The small bell of the stock rubber stack, is unlikely to provide much in the way of air compression, except at extremely high velocities. They do draw air from a plenum of pre-filtered air which is good. The little rubber stack has a bell mouth primarily to keep the inlet flow laminar, or all the air molecules traveling parallel to the runner sides. Lacking this, a sharp corner on the inlet produces turbulence. The airflow then takes on the shape of several curled fingers downstream from the obstruction/disturber, and this then produces alternating high and low pressures on the runner sidewalls. This not only creates air drag, but these fingers then reach into the carb throat where they interfere with the fuel flow entering the carb throat. These "fingers" of turbulent air change length/size as the velocity increases. So, at specific modal velocities these high/low pressure changes reduce fuel flow or increase fuel flow entering the carb, making precise mixture adjustments problematic. For example, you could have a rich mixture at 4K RPM, a perfect mixture at 4.5K RPM, a lean mixture 5K RPM, then perfect again at 5.5 K RPM, rich again at 6K, perfect at 6.5K, etc., as the alternating fingers extend into the intake runner at varying lengths with the velocity changes.
Anyway, those rubber bell shaped stack are there primarily to prevent this, rather than any air compression/oxygen boosting effect. It's what contributes to the these bikes being so "streetable".
This is one of the reasons why I snicker when people tout "pod filters" without even noticing the sharp step they just put at the mouth of the air inlet, adding turbulence. Then to overcome the modal turbulence issues they over jet the carbs to compensate for the fuel flow irregularities. Many are even happier that their engine now uses more fuel than it should, because to them, more fuel must mean "more power". And, what style maven would believe that the stock system works better?
Cheers,
