I found the closest equivalent research that I think I'm ever going to find - it's a study on changing the airbox parameters such as inlet diameter and length, Helmholtz resonators, etc. with the intention of optimising noise but while also measuring power and torque to find a compromise. They did simulation and real-world and found the sim to be accurate to within 2% of the real engine. The test engine is 1.6L 6,000rpm normally aspirated 16 valve 4-cylinder - that's theoretically only 10% more CFM than an 890cc 10k rpm, - and 4-cylinder instead of 3 only means a proportionate shift in resonant frequencies.
tldr - larger diameter tubes will flow more and produce more power but also more noise, and the resonant effect of the airbox intake snorkel is not that significant in boosting power because they are long in order to suppress noise. It's the inlet tract design that creates the biggest resonant boost.
MohdFaridSaid2019_AcousticStudyofanAirIntakeSystem.pdf (utm.my)
Here's airbox pressures - it's close to atmospheric (1 bar) at idle and drops to 0.985 and 0.98 bar at 6k rpm. The vacuum effect is larger on the engine side of the air filter due to the restriction of the filter.
Longer airbox intake snorkels primarily function to lower noise (and the 890cc airbox has some of the longest intakes I've seen on a larger capacity bike) but they also affect power/torque - longer produces slightly lower top end power and shifts the resonance boost to a lower rpm. This is also confirmed by Vcyclenut's dynos.
Here's David's dyno run of the stock versus cut down tubes and removed top with DNA air filter. Note the loss at 2,800rpm - the original resonance - and the boost at 4,200 - the new resonance of the shorter tubes. Slight increase in top end but the lower resistance of the DNA air filter is helping the modification show gains at the top end.
This is the length of tubes that created the 4,200rpm boost - without the top section, they are all quite similar length.
Finally, here is the effect of larger intake diameter on power and torque. The research paper also shows that a larger intake diameter is also noisier, so the authors of the report recommended the 50mm pipe not the better performing 60mm because noise was significantly better.
So what does this mean for modifying the airbox?
That depends on the project's objectives, and tools available. For me, I'm going to look into enlarging the intake area and leaving myself with two options - 1. standard resonant frequencies but with 50% or more extra flow and 2. also 50% more flow but move the resonant frequencies to about 6,000rpm so the boost enables a broader peak torque plateau. The latter option will require a dyno. Not sure how I can do that since there isn't one here, but I'll look into some way to solve that.
For option 1, the three OEM airbox intake pipes are roughly equivalent to 40mm, 45mm and 50mm diameter pipes.
If DNA’s numbers are correct, then the box needs 50% more flow. This means the 40mm becomes 48mm, the 45mm becomes 55mm and the 50mm becomes 60mm. Other sizes/combos are possible too of course. But let's say it means keeping the largest pipe (but it becomes the smallest), removing the two smallest pipes, and fitting 55mm and 60mm pipes in their place. 55mm is not common, so if the pipes all became 50mm in diameter, the increase in intake area would be only 25%. If one replacement is 50mm and the other is 60mm, the increase is 41%. Using two 60mm replacements and the smallest pipe will result in a 50% increase, .... if the larger sizes can both fit. Then the questions are what length, and can they fit in the box and area around it.
Length is determined by airbox volume, so measuring that is the next step.
