How much power a set of heads can support, based on their intake port flow. Useful when comparing heads, as long as you remember it's a ceiling, not a promise.
This widely used rule, popularized by engine builder David Vizard, reflects how much air a two-valve naturally aspirated engine needs per horsepower. It gives a realistic upper limit for a well-developed combination.
Flow benches are sometimes run at 10 or 25 inches of water instead of 28. Airflow rises with the square root of pressure, so the calculator corrects to 28 in first. Compare heads using figures from the same bench whenever you can.