How much air your engine actually breathes, and the carb size that matches. Displacement, redline, and how hard the engine works, nothing more.
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The 3456 converts cubic inches per minute into cubic feet and accounts for a four-stroke engine breathing once every two revolutions. Every carb maker's sizing chart is this formula with a VE assumption baked in.
Four-barrel carbs are flow-rated at 1.5 inches of mercury of vacuum; two-barrels at 3.0. A "600" from one test bench is not identical to a "600" from another, and dry-flow ratings differ from wet. Treat the output as a size class, not a lab number.
An engine only pulls its rated CFM at full throttle near redline. The rest of the time a too-big carb weakens the signal at the venturi, which dulls response and mileage. When between sizes on a street car, choose the smaller one.
A mild 350 that spins to 5,500 RPM needs about 475 CFM by the formula and runs best on a 600 CFM carb. A built 350 turning 6,500 needs roughly 590 CFM and wears a 650 or 750 depending on cam and intake. Run your exact redline above.
Yes. An oversized carb weakens the vacuum signal at the booster, which dulls throttle response and low-speed metering. The engine only pulls rated CFM at full throttle near redline; everywhere else, smaller meters better. This is why 750s on mild small-blocks feel soggy.
For draw-through and blow-through carb setups, boost raises effective VE well past 100 percent, so treat the all-out setting as a floor and size up with your tuner. For modern EFI engines, the injector sizing calculator is the right tool.
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