How many cfm for a 500 or so horsepower small block engine?

I would say at least 850 cfm and a good electric pump.

There is actually a formula, I dont know it by hart but it has to do with cubic inhes and RPM shifting range times those 2 together, then devide one of these carb gurue will know it,, I have aprox 500 HP in a 502 and I use a Holley 833 race carb, works great.

I used a 750dp on a 530hp 400sb for years

RPM X Displacement / 3456

Then multiply by volumetric efficiency (.7--.9 typical)

Then multiply by a correction factor to account for intake manifold restriction (1.1--1.3 for single plane, 1.3--1.5 for dual plane)

SO, for example:

6500 RPM X 350 CID = 2275000

2275000 / 3456 = 658.27

658.27 X .9 = 592.44

592.44 X 1.3 = 770 CFM Carb recommended.

This is a BASELINE recommendation. Your results may vary, particularly if you can't tune a big carb to idle right. If you have more than 1" of vacuum at WFO and max RPM, you need a bigger carb.

If you have more than 1" of vacuum at WFO and max RPM, you need a bigger carb.
NOW all he needs is someone to lay across his cowl with a vacuum guage while punches it until max rpm!:D

Naw, that's why they make 5-foot vacuum hoses...

I vote for a 750

Im with Mike.....750

Ditto.

RPM X Displacement / 3456

Then multiply by volumetric efficiency (.7--.9 typical)

Then multiply by a correction factor to account for intake manifold restriction (1.1--1.3 for single plane, 1.3--1.5 for dual plane)

I did this for the engine I want to build and it reccomends an 830. A 383 @ 6400 rpms, and a single plane intake.