Step #1 in stock air box modification: rip out the internal snorkel.
Step #2: Use an angle grinder with cutting wheel to hack out the internal divider wall.
My thought in removing the internal snorkel and divider wall was to free up air flow paths to more of the surface area of the air filter to reduce the pressure drop I was seeing.
Gone are the internal snorkel and divider wall.
To test out the modifications, I went out to my super-secret testing location where I could do 3rd gear pulls on a nice and flat surface. Unfortunately, I couldn’t do a back-to-back testing as it took time to do the modifications, but the trends are still the same. This is to say, the modifications did nothing readily apparent to improve manifold air pressure. The intake does have a slight hum to it now though.
The blue line is the stock air box from previous testing done at a higher sample rate (hence more data points) and higher ambient pressure (so the line is higher). The red line is the hacked air box. Notice the trend of the intake manifold air pressure dropping with engine speed is still the same. I put in linear trend lines to characterize the trends and the slopes are pretty much identical. Air is pretty good at finding the path of least resistance, so it appears that gap between the internal snorkel and air filter is enough for the air to distribute around the air filter to minimize pressure drop. It appears the intake air flow restriction is located elsewhere in the intake system. One potential culprit is the intake tube being of the OEM ribbed design and another suspect being the intake manifold itself. On a side note, the stock intake system seems to have a resonance around 2100-2200rpm. Looking at the data logs all the way down to 2000rpm, there’s a big spike in intake manifold air pressure that range along with above average MAP readings at ~4200-4400rpm and the spikes again at ~6300-6600rpm. The next spike would be in the ~8400-8800rpm range which works pretty damn well on the AP1 S2000 with the F20c engine revving to 9000rpm. I told ya those Honda engineers optimized everything.