L/D, Coefficients & Other Jargon

Before cutting seats and grinding valves, experienced head specialists often whip out a calculator or spreadsheet to figure out what might work best for a given combination. Critical elements like bore, stroke, rpm range, cam lift, and duration are needed along with a few formulas. The basis for valve selection is the curtain area. That is defined as the circumference of the valve times the valve lift, and it represents the entire area that the air must pass through on its way into or out of the chamber. As the piston moves down the cylinder and draws air in, there comes a point at which the velocity of the air becomes so great, called the sonic choke, which is at the speed of sound, that no more air can physically move through the given area. At high lift or high rpm this is usually limited by some restriction further up the port such as a pushrod pinch. At lower lift or lower rpm, it is limited by the curtain area, Ac, as seen in the following formula:

Ac = π × valve diameter × lift

There are more complicated formulas for curtain area, which take into account the actual valve angles. Rick Ferbert wrote a small program to determine actual curtain area for their valve jobs. “Having steeper seats is going to close up the curtain area, especially down at low lift. It has a significant effect on it. That reduced amount of flow acts like cutting the duration down on your cam.”

Determining a set of Lift-to-Diameter (or L/D) ratios can be used to generate proper flowbench lift settings. The aftermarket has grown accustomed to using flowbench lift numbers at every .050 inch or .100 inch, and we have done that in this test for comparison purposes. The flaw with that is that it does not correlate directly with anything that can be used to quantify and compare efficiency. In other words, comparing the flow efficiency at .400-inch lift for a head using a 2-inch valve and one with a 2.30-inch valve is not an apples-to-apples comparison. They must be compared at the same L/D ratio, say .2 for example, which would be .400-inch lift for the 2-inch valve, but would be .460-inch lift for the 2.30-inch valve.

With a known curtain area, a theoretical or “potential” maximum valve flow number can be determined by either plugging the curtain area into a complex third-order polynomial, or just comparing it to a chart available from the flow bench masters at SuperFlow. We chose the latter. Our 1.94-inch test valve had a curtain area of 3.18 square inches at max lift (.522 inch) and a max L/D of .27. Looking at the SuperFlow chart, our wedge head valves had the potential to flow right at 100 cfm/square inches. In a perfect world then, the 1.94-inch valve would be able to flow 318 cfm. Right. We flow tested the heads at a nearby .550-inch lift, corresponding to about 102 cfm/square inches on the chart, and giving a max potential of 324 cfm, that was what we used for comparison. Our baseline airflow was measured as 229 cfm at .550-inch lift. With all that, actual valve flow efficiency (Cv) can be calculated. Cv is a direct result of the valve job, and is represented by the following:

Cv = Actual flow volume ÷ Theoretical flow volume

For the baseline then, it would be 229/324 = .70 or 70 percent efficient. Not spectacular, but it is a number that can be used to compare against the different valve jobs on this head, or against the valve job on any other head, regardless of valve size or configuration.