CFM ratings are one of those really grey areas. The test is done at a WOT [wide open throttle] at a given suction value, or vacuum figure, however, there is no industry standard for this.

If a manufacturer wants to make his carb look better, then just strap it onto the flow bench & jack up the suction.
From memory, a commonly used figure in the aftermarket industry is 3" of water suction. That is, enough suction to lift a column of water 3"
Thats not a lot of vacuum, but thats the lowest figure that most four barrel carbs will need to pull fuel through the nozzles. That also means that a lot of air is flowing, but pull it up to 4" of water & thats a 33% boost in suction. It makes a diference to the CFM rating.

CFM ratings are a bit like factory performance claims,... more of a guide than an absolute.

I dont know if iam 100% right but some one told be 45mm webers in pairs can flow over 1000 cfm but i could be wrong.
A rally car i navigate for with a 1900 l series & twin 45s gets about 2 km per litre. In a rally.

I have a quick question about carbs I am used to 4 barrel downdraft carbs found on american muscle cars and used to their ratings in cfm as well. What I want o know is what are the cubic feet per minute ratings on 1.5" SU's and the 40mm sidedraft webers and why they don't say the cfm rating on them? I'm desperate so ne thing will help thnx