What's the function of this valve?

[philou320]

As long as the flap can rotate inside the tube, you can cap it, after you do the job, check it moves by actuating it from bellow, it is possible that it allows some air goes in because of the wear and tear.
anyway this flap opens at middle throttle and is closed at iddle.

a better view inside with the flap fully open

[78F350]

Try looking up 'resonance flapper valve' for more info.
I think that any air going in or out of that hole is negligible unless you have excessive wear.

Quoted from: http://press.porsche.com/archive/products/press_kits/press_kits_2001/PDF/Boxster_in_Depth.pdf

"The Boxster inherits its twin-resonance air induction system from the 911
Carrera. The system acts as a “resonance supercharger,” allowing the engine
to draw from higher velocity airflow at certain engine speeds. A crossover pipe
connects the individual air collector/resonance chambers for each cylinder bank.
A flap in the pipe remains closed from idle to about 3,100 rpm. When it opens,
each cylinder bank can draw from airflow “excited” by the resonance created by
alternating induction between all six cylinders. In essence, “dual resonance”
creates two induction paths for each cylinder. Below 3,000 rpm, the cylinders
draw air from a “short” path. From 3,000 rpm to about 5,100 rpm – when the
resonance flap opens – the cylinders draw from a long intake path, which
boosts torque. Above 5,100 rpm, the flap again closes to allow the cylinders
to draw intake air from a shorter intake path to boost horsepower at higher
engine speeds."