For someone trying to run 3 X 2 Rochester 2G: there are 2 different SAE bolt patterns and throttle plate sizes, sometimes referred to as small (1-7/16") = Chevy V8, large (1-11/16") = Oldsmobile.
In the larger 2G there are 4 venturi sizes 1-3/16" to 1-3/8" rated 352 to 435 CFM @ 3.0" Hg.
Rough approximation for a cylinder pair with 1 X 2G:
CFM = engine size ÷ 3 (78.5" for a 235 etc.) X VE (use 90% or less) X RPM (5,000 or less) X 3.3 ÷ 3,456
78.5 X .90 X 4,500 X 3.3 ÷ 3,456 = 304 CFM @ 1.5" Hg (430 CFM @ 3.0" Hg)
Hot 261: 87 X .95 X 5,000 X 3.3 ÷ 3,456 = 395 CFM @ 1.5" Hg (558 CFM @ 3.0" Hg)
