Practical Engine Airflow. John Baechtel

increasing elevation, ranging from 29.92 inches Hg at sea level to 23 inches Hg at 7,100 feet. Hale’s description is clear: “Whenever the ‘actual’ ambient pressure equals the corresponding ‘standard atmosphere’ pressure for the ‘actual’ elevation, the ‘corrected’ barometric pressure will always be 29.92 inches Hg.”

Density Altitude

      Density altitude is a great tool for making sure your plane is able to fly in given weather conditions. And it does a great job of helping you jet a carburetor, but it’s not really ideal for motorsports corrections from an engine-tuning standpoint. Hale points out that engine power is not directly related to density altitude. He also states flat out that density altitude is not even necessary for proper engine tuning. Instead he recommends the HP Correction Factor, citing the importance of the type of fuel being burned and whether or not the engine is naturally aspirated.

      According to Hale, engine torque and, thus, power varies with the inverse square root of absolute temperature. He points out that tuners have incorrectly assumed that the VE percentage of a given engine is not affected by changing weather conditions. This assumption suggests that a given inlet path flows the same regardless of pressure, temperature, or water vapor content; and that, of course, relates to your airflow concerns because you select and adjust airflow components based on calculations to achieve maximum VE. The VE percentage increases with rising temperature because the speed of sound increases with the square root of absolute temperature. Hale reminds us that peak mach numbers within the intake runners remain constant for any given RPM. This affects the timing of pressure waves within the inlet tract.

       The Air Density Index based on the MSA equals 100 percent at sea level with 60-degree dry air and 29.92 inches Hg pressure. It also decreases almost linearly (hold a straightedge next to the line), dropping to about 84 percent at 6,000 feet. (Photo Courtesy Patrick Hale)

       Naturally aspirated engines take considerably more finesse to pack the cylinders with air. Each application requires its own optimized package of intake and exhaust components to maximize efficiency.

       Large Roots-style superchargers have always been popular in hot rodding and racing circles. Every hot rodder keenly understands the imposing look of a big blower.

      For a detailed explanation of how this works refer to Hale’s handbook, Motorsports Standard Atmosphere and Weather Correction Methods, available at DragRacingPro.com. It is the most comprehensive description available on weather corrections for motorsports. It is primarily a tuning guide for drag racing, but it discusses how atmospheric conditions affect engine performance. It provides a solid foundation for tuning and makes you aware of the behavior of air from a tuning standpoint.