The rotary and most other engines are a collection of small engines sharing a crank shaft.
So, the rotary is two engines with about the same intake system. On the dyno and on the flow bench, the biggest
influence on performance is the shape of the intake horns. Should look like a trumpet bell. The higher the RPM
the bigger the diameter of the bell. Because: airflow into the bell comes from all directions. A straight tube is the worst. Air flows up the sides of the tube and has to make a 180 to enter the tube. Air has mass and the 180 serves to choke off part of the tube. (As though the tube developed a smaller diameter). So, a big radius allows
for a slower turn in that 180 and flow rate increases. You can find flow going the wrong direction just inside the bell on poor designs. The best design was on old SU carbs where the bell looked like a ring slit into two pieces and mounted on a flat surface. Cheap and the very best design. I will try to find pictures and post them.
Best is a large bell shape and a taper to the port size. The tuned length is for one RPM. The exact length you choose may be great or just short of great. Anything will be better than a stock air filter. Allow at least the diameter of the interior of the bell as a distance to any structure like a tube wall or a flat sided filter box.
A flat sheet with holes the size of the bells is fine. In very few cases will both housings be putting out best power at the very same time.
Round tube on a flat side like a "D" shape works well. Anything flat must be thick and stiff or it will die.
Lynn E. Hanover