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So I've been looking around for a 400 CFM AFB to add to the upcoming build of the 261 and I've realized they are kind of hard to find. A friend of mine has an AFB laying around off a Buick but the model number doesn't have a CFM rating. After doing some digging, I came up with this, its a 4331s Buick carb info the info that I can find about the 9400 AFB says the throttle bores and the primary and secondary Venturi are the same size as the 4331s. Would it be a safe assumption that this carb flows at about 400 CFM? Its for a 340 V8 but only 400 CFM seems a little anemic to me for that displacement.

A 340 cubic inch engine at 4,000 RPM operating at 100% volumetric efficiency (impossible) is moving 393 cubic feet of air a minute. A more realistic volumetric efficiency for a street engine without extensive camshaft, valve, and port modifications will be 85%-90%, about 354 CFM max. The 400 CFM carb sounds about right unless you're going for valve-floating speeds.

Edit: The 261 at 4,000 will flow about 270 CFM, so the AFB will be significantly over-carbureting it unless you're planning to run at or near full throttle all the time. A Holley #7448 300 CFM 2-barrel would be a lot more appropriate for a 261 engine. We ran 350 Chevy engines to 6,000-6,200 RPM on the round track with the Holley 2 BBL, but it acted like a restrictor plate at those speeds- - - -manifold vacuum started building up and torque dropped off near the end of the straightaway.
Jerry

Thanks for the quick answer. I'm truly up in the air on how to carb this motor. I know some guys have had luck with the 390 Holley 4 barrel on similar sized sixes but I haven't seen a ton of people on this site using them or the AFB probably because of over carbureting the motor. I like the idea of a 2 or 4 barrel rather than running a multi-carb setup. I was figuring the AFB would run around a little under half its CFM rating before the secondaries start to open due to the smaller primaries, but I might yield to your real world experience. Any other tips or tricks that come to building a warmed up 261 would be much appreciated.