S4 Airbox Mod.

[Howie]

That box is the resonator.  AFIAK, it is there to reduce intake noise, not enhance performance.  Your googling seems to substantiate this.  To know for sure is difficult, a little like "does that big ass wing on the back of my car really help?"  It seems most people get a slight gain with proper fueling, but since there is a gain from proper fueling anyway, I think whether it is worth it or not is both subjective and dependent on the person doing the dyno tune.  I would think the gain is minimal since people get away with an open box and just a CO trim, which really has a diminishing effect as speed/load increase.  Brad Black may have enough info to make you happy and save you many hours and dollars on dyno time.    http://www.bikeboy.org/performance.html



Do keep in mind the butt dyno is important too.  It may not be a real measure of performance, but aren't we doing these things for the smile factor?  If I had the money and another bike, my bike would get high compression and flat slides.  A lot of work and money to still have a slow bike, but...

[Langanobob]

That box is the resonator. 

Interesting and you're probably right that it's not there for any performance gain.  I think the rest of the box is supposed to resonate also, although at a much different frequency.

Do keep in mind the butt dyno is important too.  It may not be a real measure of performance, but aren't we doing these things for the smile factor?  If I had the money and another bike, my bike would get high compression and flat slides.  A lot of work and money to still have a slow bike, but...

You're absolutely right.  I'm interested in this airbox subject mainly as a wannabe scientist, not to eke out another few HP from an M620.