Jerry mentioned CFM; if one is not familiar with "CFM" this link to my website may help:

CFM

And one more, although specific for 4-barrels, more information about CFM and Jerry's comment about over carburetion:

More on CFM

And this is at the end of the second link, for those of you who do not like links:

"Finally, there is an old story here in the Missouri Ozarks which, unfortunately, is ignored by many enthusiasts when choosing the correct carburetor size. Seems like a young hillbilly reached his 16th birthday having never owned a real pair of shoes. His uncle told him to go into town, and come up with a new pair of shoes, and the uncle would buy. So, much later in the day, the youngster comes walking back from town, with the shine of new boots visible through the dust of the dirt road. His uncle asked what size he bought, and the youngster replied "wal, the salesman thot I oughter wear a 9, but 10's felt so good, I got 11's!".

If acquiring a carburetor for a street engine, give the above some thought!"

Generally, on a street engine, over-carburetion is worse, sometimes MUCH worse than under-carburetion.

And Jerry, I must disagree on the 90 percent efficiency figure; most street engines won't make 90 percent.

Jon

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop