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LOL.
there is a simple formula to work it out.
CFM = (CID x RPM x CF) / (2 x 1728)
CFM = cubic feet per minute
CID = displacmente
RPM = well if you don't know this one you may as well go to a different site
CF = correction factor. this is based on the VE% (volumetric efficiency) of the engine. i have a nice chart here for that in relation to cam duration but for stock heads and cams in the 240-250 duration @ .050 use .9
these numbers are all based on the standard 1.5" pressure drop across the carby. this is the part where it gets interesting because it doesn't take a lot of effort to get a decent intake to have less then 1.5" drop which changes the CFM rating of the carb.
there is however a formula for that too.
CFM / square root of( old pressure drop/ new pressure drop)
or 800cfm / sqaure root of 1.5/.5
= 800/1.732
= 462 CFM so a 800cfm carb (measured at 1.5" drop) only flows 462CFM @ .5" drop
i'd also go for a vac secondary carb for street use, this way you can also get a slightly larger carb without the problems associated with large DP style carbs i.e. it won't suck gas as much
this post has now been saved to my desktop