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


Last edited by Hotrod Lincoln; 11/04/2013 12:06 AM.
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