Around 60mm IIRCC.
RSA-5 and FM150 bore diameter at the throttle plate and manifold flange is 2.3125"
RSA-10 is 3.4375"
An FM-200, as used on the 390-119, is 3.0625"
The RSA series has a significant bore reduction at its throat venturi, while the AFP units incorporate a relatively straight bore with a pedestal venturi. The difference is most obvious when comparing an RSA-5 with the FM-150. On AFP's Superflow bench, respective pressure drops at 1560 lbs/hr were 12.75" vs 10".
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For comparison does anyone happen to know the bore diameter of the plenum inlet on the IO-360 and IO-390’s?
The standard horizontal sump should be 2-5/16", like the RSA-5. The "cold air" manifold on the 390C and D is sized for an RSA-10.
That’s interesting about the 390 C/D sump. Unless Lycoming used a different casting than the IO-360A sump, it doesn’t look like there’s room inside the sump to increase the bore 1” or so in diameter: