On my L-235 with the O-290-D2 engine, I used an analysis similar to what Chris Z. just posted. I figured out the velocity of the air going through the carburetor and compared that to the velocity of the airplane and came up with an expanding ram air inlet that would slow the air down from 170 KTS to whatever the carb needed (can't remember). I'll have to measure my intake opening, but I believe it is ~ 2-3/4" in diameter and expands to 4" in diameter over 11" for a 7° taper on each wall (total included angle of 14 °). The ram air inlet is connected to the K&N filter and airbox with a short piece of 4" scat duct. I hope it works (grin).

Note that I made the "cod piece" removable for easy access to check the lower part of the engine and change the oil.