I've been curious. My heads flow at around 230 CFM @ .500 lift, and I'd say they've got to spend most of their time at half lift, flowing (at least) half that. I have a 650 CFM carb.

If I have 8 ports, doesn't that mean that I'm flowing on average at least 920CFM (115CFM at half lift times 8) for any given minute through the heads? Why, then, wouldn't a 920CFM carb increase my HP substantially?

It would seem that the CARB is the bottleneck for airflow, not the heads. However, conventional wisdom says otherwise... what gives?

Remember, all 8 cylinders are not drawing air at the same time.

Is that where the cubes*RPM/3456 equation comes from?

....and your heads only flow that much at peak lift. Most of the time they flow much less than than the peak flowrate.

Don't think of flow as constant like a fan blowing a stream of air -- think of flow as pulses.

The pulses are being transmitted up through the carb and if the intake runners are all the same length each pulse arrives at the carb at equally spaced intervals. Because of this the carb never sees all 8 cylinders pulling on it at the same time.

For the same reason when you go to multiple carbs (like a tunnel ram or dual quad setup) you need to use more total cfm than you would wityh a single carb. The engine is divided front and back so each carb is only seeing 4 pulses so you can't take and 1/2 size two carbs on a dual quad.

Manifold design has a huge effect on carb requirements for a specific engine.

A single plane manifold with a large plenum can use a smaller carb (relatively) because the entire carb can feed any single runner.

A dual plane manifold will need a somewhat larger carb because the manifold is divided in half and half the carb feeds its respective half of the cylinders.

An independent runner manifold usually requires huge carbs as a single carb barrel feeds a single cylinder with no plenum. For confirmation of this just look at a motorcycle or a hot rod air cooled VW.

There are many other factors that need to be considered in the choice of a carb for a particular engine combination. While a larger carb may give bigger horsepower numbers, there will be a price to be paid in throttle response and low end torque. If you are trailering your car to the drag strip, a larger carb may suit you best. If it's a street car, a smaller carb usually gives the best results.