Will try it this way.
Can we agree on this:
Lets stick with Holley Quoted cfm numbers first.
Holley says the center carb flows 356 cfm at its rated point 40.8" inches of water test pressure.
Holley says a outboard 6-pack (end) carb flows 500 cfm at its rated point 40.8" inches of water test pressure. This carb has a 1-9/16" venturi and 1.750" throttle blades.
So simple math 355 + 500 + 500 = what?
1355 cfm, right? at 40.8" of water test pressure.
Ok so we have a second Holley carburetor (that is a 2-BBL carb) It is model 4412 and is called a 500 cfm carb by Holley in all of the advertising
https://www.holley.com/products/fuel.../parts/0-4412C
This carb has a 1-3/8" venturi and 1-11/16" throttle blade. It says it clearly right here (2nd page at the very top).
http://documents.holley.com/techlibr...al_listing.pdf
We have a carb #4782 (center 6-Pack carb) that is rated 355 cfm and we have a #4412 (Circle Track 2 bbl) rated 500 cfm.
So looking at the two carbs internally the #4412n Carb has 1-3/8" venturis and a 1-11/16" throttle blade.
The #4782 center carb on the 6-pack has a 1-3/16" venturi and a 1-1/2" throttle blade so you can easily see the difference in cfm
is caused by the 1-3/16" (.1875") vs the 1/3-8" (1.375") #4412 venturis and the 1-1/2" (1.500") vs the 1-11/16" (1.6875"_ #4412
Throttle Blades.
With me so far? A DIFFERENCE in Venturi Area and a difference in Throttle Blade Area.
Tom V.