i am wondering whats up with the way holly rates the cfm of their carbs. it has been said that their hp 950 flows about 830 cfm. do their regular double pumpers flow less than stated? like a 4779 750 dp flow maybe 700 cfm? i have run a 750 dp and an 830 annular dp on my 454 and felt no seat of the pants difference. if i replace my stock main body on my 750 dp with the proform piece will it really flow 830 cfm. i thought that demon carbs are rated what they actually flow true ot false? in a recent phr bb build up david vizard states that it is difficult to overcarb a big block chevrolet. there seems to be some misinformation floating around. mike

Well I don't know and might ad more confusion, but i have an old original Holley 3310, its rated at 780 cfm, the newer 3310's are rated at 750 cfm's, the difference I was told was because the original 3310 -780 used downleg boosters and the newer 3310-750 used straight leg boosters, all other things being equal, down leg boosters are supposed to flow more.
Could the type of boosters in the carbs be some of the flow difference?

Carb CFM ratings are dependend on the vacuum used for the test. Holley rates their 4v carbs at 1.5" and their 2v carbs at 3". Since most race engines make much less than this at WOT, most will actually pull less CFM than the rating. One of the reasons to run a larger carb then the calcs would call for.

Boosters play a fairly large part. High signal amplification boosters like straight leg or annulars restrict the amount of air through the carb. Downleg boosters have less signal amplification but are less restrictive. It is why they are better for WOT and high rpm use.

I know this doesn't really answer your question.