Practical Engine Airflow. John Baechtel

pressure decreases.)

      • Charles’ Law says that the volume of a gas is directly proportional to its temperature. (The gas expands with temperature.)

      This equation represents the ideal gas law:

      PV = nRT

      Where:

      P = Absolute air pressure

      V = The given volume of air

      n = The actual amount of air measured in moles

      R = The Ideal Gas constant

      T = The absolute air temperature (this is where degrees Rankine fits in: you don’t use degrees F)

      The volume, pressure, and temperature of air are easier to measure than its specific mass. Your primary concern is the behavior of air (a gas) as it undergoes changes in temperature and pressure, which are the parameters that affect engine power output. You want to understand the behavior of air as it moves through the engine negotiating restrictions (area and velocity changes) and reacts to the influence of fuel droplets present in the airstream. The ideal gas laws allow you to predict air movement based on known constants.

Correction Factors

      Correction factors are established to support accurate comparisons of data recorded under different conditions such as location, elevation, time of day, and basic weather data. Although useful in testing, they require careful application and very accurate data to ensure meaningful comparisons. They also invite abuse and the potential for misleading information based on improper input or, in rare cases, attempts to fool the sensors. Although most dyno testing is comparative, both observed and corrected data can serve your needs.

      For that matter, observed numbers are often more instructive as long as you maintain consistent comparisons. Observed numbers tell you what is really going on, right then and there. The important thing is to choose a standard and stick with it during all your testing. There are two basic dyno correction factors: the SAE (Society of Automotive Engineers) and the STP (Standard Temperature and Pressure). SAE numbers are used by OEMs for all their testing and they are generally the standard for most chassis dynos as well. These are automotive standards.

      Most dyno shops and “magazine” testers stress STP numbers because they are roughly 4 percent higher than SAE numbers. STP numbers are recognized as the “motorsports” standard and are described as such by motorsports authority and author Patrick Hale in his book, Motorsports Standard Atmosphere and Weather Correction Methods.

      Hale is the founder of Racing Systems Analysis (RSA) and the author of the popular Quarter and Quarter Jr. dragstrip computer simulation software and the Engine Pro engine simulation program. He eventually sold off the RSA software business and now operates Drag Racing Pro (DRPro), a motorsports consulting firm. Hale’s book defined the Motorsports Standard Atmosphere and how it applies to weather for tuning purposes.

      Engine dynos collect only raw uncorrected data. Whatever correction factor is applied is merely a percentage calculation based on contributing factors input by the operator or recorded by sensors. Your dyno printout can be set to list observed (raw or as-recorded), SAE, and STP numbers plus the actual numerical multiplier so you can determine the “validity” of the correction factor for yourself. Serious tuners use uncorrected numbers and raw recorded data and recognize that the larger the correction factor, the more likely the numbers are to skew incorrectly.

      Correction Factor SAE J607

      This factor is common to the performance industry, particularly for engine dyno testing. It corrects observed data to standard temperature and pressure (STP), or 60 degrees F at a barometric pressure of 29.92 inches Hg and dry air (zero humidity). It also subtracts corrected vapor pressure from observed barometric pressure to correct for water vapor in the air. Numbers according to this correction factor used to be referred to as STP corrections, but are now commonly referred to as Hale’s Motorsports Standard Atmosphere (MSA) corrections.

      SAE J607 = (29.92 – corrected barometric pressure) 1.2 × [(observed inlet Fahrenheit temperature + 460) ÷ (520)0.6]

      Where:

      29.92 = standard barometric pressure

      460 = Rankine conversion

      The resulting factor is multiplied by observed torque and horsepower to obtain figures corrected to the common standard. This most often results in a higher reading than the observed numbers. Note that temperature in this formula is converted to degrees Rankine (by adding 460 to the Fahrenheit number), and that it yields power numbers approximately 4 percent higher than SAE 1349.

      Correction Factor SAE 1349

      This is the auto industry standard and is the normal correction for chassis dyno work, although most engine and chassis dyno software automatically calculates both. The software converts raw data to 77 degrees F air temperature, 29.31 barometric pressure (990 millibars), dry air, and includes a factor for an agreed standard of 84.7-percent mechanical efficiency. Numbers according to this correction factor are commonly referred to as SAE corrections.

      SAE 1349 = 1.18 × [(29.31 ÷ Pd) × {(Tc + 460) ÷ 537)0.5} – 0.18

      Where:

      29.31 = barometric pressure

      Pd = pressure of dry air in hPa (990 hPa = 99kPa)

      Tc = air temperature in degrees Celsius

      460 = Rankine conversion

      Why Use Corrected Numbers?

      Uncorrected test figures are your benchmark; they permit accurate analysis of fuel and air usage, and BSFC numbers. If you’re seeking or selling dyno numbers, you need to know the correction factor. If you’re an engine builder validating your combination, uncorrected numbers tell you everything you need to know. The chances that your engine will ever run with the same conditions as the correction factor are slim to none, so what’s the point?

      Magazine testers use corrected numbers because they are higher and editorially expedient. Engine builders glean most of what they need to know from the raw data, particularly with regard to BSFC, VE, air consumption, fuel flow, air/fuel ratios, airflow, brake mean effective pressure (BMEP), and the shape of the torque and power curves.

      If you think about it, the engine makes what it makes and the data recorded tells how and why it performed the way it did. The numbers change wherever you go. Correcting them to some pie-in-the sky number serves only your ego and can lead to considerable confusion.

      For example, if you were in San Diego the engine would make more power. Well, yeah! If the air were colder it would make more power and on and on. If you’re testing and tuning in Denver, the numbers are different than if you go to the beach. But it doesn’t matter if you only race in Denver.

      Bonneville racers routinely see big numbers on the dyno and then go to the salt flats and experience density altitude numbers anywhere from 4,000 to 8,000 feet and the corresponding loss of power they have come to expect. Either way the tune-up is different.

      The correction factor’s real value is not in predicting power based on an arbitrary correction; rather, it provides the direction for tuning adjustment to compensate for actual weather conditions. Either way, your engine is still going to make less power in Denver, but standardized corrections can help you optimize its performance for existing conditions. That doesn’t impact actual engine airflow from a physical standpoint to any huge degree. But it does influence the weight and content of air, which has some effect on air movement through the inlet tract.

The MSA and the Air Density Index

      The motorsports industry has universally adopted the MSA as described by Patrick Hale. That reference is for 60 degreees F, 29.92 inches Hg (sea level), and dry air (the absence of water vapor). These conditions constitute an Air Density Index of 100 percent. Any elevation above sea level or temperature above 60 degrees F has an Air Density Index