In addition to the nuts-and-bolts math of engine design and assembly, it is useful to know a few things about combustion and the effects of atmospheric pressure on engine tuning and performance. In particular, I discuss air/fuel ratios, correction factors for dyno testing, and the effects of altitude on engine performance. There are only a few formulas in this chapter, but they are important to your understanding of how your engine operates and how it should be tuned.
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Atmospheric Effects on Engine Performance
Engines deliver their best performance at sea level because that’s where atmospheric pressure is greatest and air density is highest, notwithstanding atmospheric variations. The common reference for atmospheric conditions is standard temperature and pressure (STP) or the “standard day.” STP is defined as 60 degrees F, 29.92 barometric pressure, and dry air (zero humidity). As the components of STP vary according to weather, air density also changes for better or for worse. By these standards, STP equals 100-percent air density, but zero density altitude. As temperature, barometer (atmospheric pressure), and humidity vary, air density fluctuates accordingly.
Each 5-degree change in temperature (from STP) equals a 1-percent change in air density. Hence a 90-degree day would have an air density of 94 percent if other factors aren’t considered. The change is not entirely linear since it is always accompanied by barometer and humidity changes. Still, it’s close. Temperature typically has a greater effect than humidity; something on the order of nearly 2 to 1. The effect of barometer is linear with density following by percentage. If the barometer drops by 1 percent, air density falls by 1 percent. Humidity is more complicated because of the relationship between temperature and water vapor. Air is capable of holding more water as temperature rises, hence 50-percent humidity at 90 degrees F causes almost double the loss of air density as the same percentage of humidity at 60 degrees F or STP.
These effects are interrelated. If you’re running at a higher altitude and the barometer falls 2 percent to, say, 29.32, you lose 2-percent air density. Then the temperature shoots up to 90 degrees F and you lose another 6 percent. And at 50-percent humidity you lose another 2 percent. Now you’ve encountered a 10-percent drop in air density and, based on the widely accepted power to density ratio of 75 to 80 percent, you’re looking at about an 8-percent loss of power. For your convenience, the NHRA’s published correction factors for elapsed time and speed based on elevation change are listed on page 116. Multiply your ET and speed by the appropriate conversion for your elevation to determine your projected performance at sea level.
Density altitude can be thought of as altitude measured in terms of air density rather than distance. It is basically the pressure altitude adjusted for nonstandard temperature and humidity. When temperature and humidity increase, the density altitude of a given location may be substantially higher than the actual elevation in feet. For example, at Bonneville, you might be racing at 4,200 feet elevation, but the density altitude might be 6,500 feet according to atmospheric conditions. Hence you are racing at 6,500-foot conditions and appropriate power loss can be expected. Here is the standard formula for estimating power loss with altitude:
HPloss = (elevation x 0.03 x HP at sea level) ÷ 1,000
For 800 hp running at 3,500 feet:
(3,500 x 0.03 x 800) ÷ 1,000 = 84-hp loss
This formula yields an estimate that does not account for the displacement of fuel molecules by water vapor in the intake air supply. For greater tuning accuracy, you may want to get a performance weather station to help you account for all contributing factors. It makes jetting and air bleed changes more accurate based on ambient conditions. Optimum conditions for engine performance include high pressure (barometric or ram), low temperature, and minimal water-vapor pressure. When these combine in the right proportions, air density increases and more tightly packed oxygen molecules are available to combine with fuel for combustion. From a tuning perspective, note that excessive vapor in the air also tends to cool the burn and further reduce power. For any water vapor pressure below 40 to 45, percent the engine often responds to slightly richer jetting. Higher percentages displace more oxygen molecules and may require less fuel and more timing to burn the cooler, wetter mixture.
Performance Weather Stations In recent years handheld units have grown in popularity (the PerformAire unit is a favorite). They help you make tuning decisions based on accurate measurements of station pressure (barometric pressure at the track), air temperature, and the water vapor content in the air. Performance weather stations vary in content and to some degree methodology, but they all strive to accomplish the same thing, which is to provide the racer with accurate atmospheric data so he can make appropriate tuning adjustments for race day conditions (see sidebar “Optimum Fuel Mixture Ratios”). Racers have traditionally used air-density gauges to monitor density changes as the air changes during a given day, but air-density gauges do not account for humidity or vapor pressure. They simply indicate the percentage based on ambient temperature and pressure.
If you have ever taken a flight out of the Denver airport, you may have noticed nervously that the takeoff roll seems about 50 percent longer and farther than what you are used to at most airports. There is less air density to support the plane’s wings; hence it requires a long roll and higher speed before the pilot can lift off. In the early days of aviation, piston-engine-aircraft performance diminished as altitude increased. Once supercharging was employed to artificially raise the air density, aircraft performance increased proportionately. The same lack of density takes away performance from your engine on a percentage basis. Any decrease in pressure or increase in temperature or vapor pressure raises the density altitude and reduces performance. (See Appendix B “Handy Conversion Factors.”)
Shame on the Weatherman
Your local weatherman is not a reputable source for racing weather data. He may quote a fabulous day with a pressure reading well above 30, but your uncorrected mercury barometer tells you different—say, 26.10, for example. Why? Well, it’s because the National Weather Service corrects all altitude based conditions to known sea-level conditions for equivalent temperature and pressure. Hence it is critical that you use uncorrected (local) station pressure for your calculations or input to your weather calculator. In a pinch you can get pretty close if there is a nearby airport. Call and ask the tower for an uncorrected station pressure reading. This will be the actual atmospheric pressure at your location without corrections for temperature and water vapor.
It’s also important to use your own weather station equipment when visiting the dyno. Ask the operator to give you the uncorrected barometric pressure that he enters into the dyno software along with the vapor pressure and the specific gravity of the fuel. Make certain that your printouts or the data disc the operator gives you after the test include all observed (uncorrected) figures, which are what the engine actually delivered on that given day. And be certain that you have numbers for the inlet air temperature. There are few unscrupulous dyno operators, but if your operator doesn’t spend at least 20 minutes taking weather measurements and entering them into the dyno software, it would be a good idea to ask why. Observed numbers should match your BSFC numbers and still give you an accurate picture of engine performance even though they don’t seem as impressive as corrected numbers. That’s why there is no point in comparing your numbers to someone else’s engine run under different conditions.
The Motorsports Standard Atmosphere Motorsports engineering authority Patrick Hale has compiled the results of more than 35 years of racing and motorsports success into Motorsports Standard Atmosphere And Weather Correction Methods, a book available through his Web site www.DragRacingPro.com. If you are serious about motorsports tuning and weather corrections, it is pretty much the last word on the subject. In it Hale defines the motorsports standard atmosphere in relation to aerospace and general automotive atmospheric definitions and discusses the intricacies of atmospheric temperature and pressure, measurement techniques, density altitude, water vapor displacement, SAE corrections, corrections for methanol and nitromethane fuels, drag racing ET and MPH corrections, aerodynamics, wind corrections, and more. It’s heady stuff and beyond the scope of this book, but not so deep that you can’t grasp it with a little effort. For those determined to tackle advance tuning issues and the math behind them, I highly recommend it.
Lambda and Air/Fuel Ratios
Lambda and air/fuel ratios are often confused, but the relationship is really quite simple. Air/fuel ratio indicates the mixture ratio of air and fuel. A 12:1 air/fuel ratio indicates 12 parts (or pounds) air to 1 part (or 1 pound) fuel. Because it is a ratio, it can be anything you want. It could be 12 pounds of air to 1 pound of fuel, for example. Calibration engineers prefer Lambda because it gives them an easy-to-read percentage of the air/fuel mixture’s deviation from the ideal ratio for complete combustion. The ideal ratio is called stoichiometric, and it is defined as an air/fuel ratio of 14.68:1 (for gasoline), or Lambda equals 1.00. They both represent the same mixture ratio. The stoichiometric ratio is the control ratio for all modern electronically managed engines and the tuning standard for EFI calibration. This represents the chemically “correct” ratio where emission components are at their lowest and should not be confused with a best power ratio that is closer to 13:1.If lambda represents the stoichiometric mixture, anything greater than 1 represents a leaner mixture and anything less than 1 indicates a richer mixture. In the performance world we know that best power (torque) usually occurs at an air/fuel ratio of about 13.2:1 ± 0.2. The equivalent Lambda is 0.9.
Lambda (λ) = indicated air/fuel ratio ÷ stoichiometric air/fuel ratio
13.2 ÷ 14.68 = 0.899 or 0.9 Lambda
Lambda is defined as an excess air ratio, but it really works both ways. In the preceding example, 0.9 Lambda indicates the engine is using only 90 percent of the air required to achieve stoichiometric combustion. It is running rich to achieve best power (torque). The slightly richer mixture optimizes flame travel and drives the mixture toward a faster reaction rate. When combined with the appropriate quench to produce good mixture turbulence, combustion efficiency and power rise accordingly. With all modern performance engines moving to electronic management and fuel injection, enthusiasts would do well to start thinking in terms of Lambda instead of air/fuel ratio. If you can remember that 13.2:1 is usually best for peak power, it’s just as easy to remember that 0.9 Lambda is the same thing. If you’re going to learn to calibrate EFI systems, it’s good to get comfortable with Lambda. (For an in-depth look at Lambda in the tuning process, see Greg Banish’s excellent SA-Design books, Designing and Tuning High-Performance Fuel Injection Systems and Engine Management: Advanced Tuning, published by CarTech.)
Dynamometer Correction Factors
Common testing procedures can be confusing if you don’t understand where the numbers come from and how the local testing environment affects them. It is important to recognize that a dyno is basically a sophisticated data acquisition system that measures degrees of change (good or bad) caused by various components, modifications, or tuning changes with accommodation to ambient conditions. A dyno provides raw data that is typically corrected to accepted standards for comparison. If you’re testing cylinder heads for example, you don’t necessarily need corrected numbers if your tests are all performed under the same conditions. Observed torque either rises or falls from your baseline, and you are likely to learn more about the head by comparing VE and BMEP. Still, it is important that you understand the correction process and how it can affect your data.
Observed torque is the raw torque value measured by the load cell (strain gauge) on the dyno. It’s basically “what you see is what you get.” Of course the engine’s performance is affected by local atmospheric conditions and that is what you see. Correction factors (CF) are used as benchmarks for comparison purposes. If you’re testing on the same day and conditions remain stable, you can judge performance by observed figures and evaluate efficiency changes by comparing relevant data such as VE. If you’re testing on a different day with new conditions, a correction standard becomes useful. Accordingly it is helpful to know the testing landscape and the terms that define it. Observed horsepower is calculated from observed torque using the formula discussed in Chapter 5:
HP = (observed torque x RPM) ÷ 5,252
Corrected torque and horsepower are the observed figures multiplied by the correction factor. Correction factors are based on observed inlet air temperature, wet bulb temperature, and barometric pressure. Wet bulb temperature is used to calculate the inlet air’s vapor pressure and humidity. A corrected barometric pressure is calculated by subtracting the corrected vapor pressure. Cool air and lower vapor pressure allow for denser (oxygen rich) air and less fuel displacement by water vapor. The dyno measures most of this on the fly and makes the calculations for you. There are two basic correction factors, as described next.
Correction Factor SAE J607
This factor is commonly used by the performance industry, particularly on engine dynos. It corrects observed data to STP, or 60 degrees F at a barometric pressure of 29.92 in Hg and dry air (zero humidity). It subtracts corrected vapor pressure from observed barometric pressure to correct for water vapor in the air.
CF = (29.92 – corrected barometric pressure)1.2 x [(observed inlet temp + 460) ÷ (520)0.6]
The resulting correction factor is multiplied by observed torque and horsepower to obtain corrected figures. Note that temperature in this formula is converted to degrees Rankine, and that this standard yields power numbers approximately 4 percent higher than SAE1349.
Correction Factor SAE 1349 This correction factor is standard to the OEM auto industry and is frequently the standard for chassis dyno work, although most engine and chassis dyno software automatically calculates both. This standard converts to 77 degrees air temperature, 29.31 barometric pressure, and includes a factor for 85-percent mechanical efficiency.
CF = 1.180 x [(990 ÷ Pd) x (Tc + 273 ÷ 298)0.5 ] – 0.18
CF = final correction factor multiplier
Pd = pressure of dry air in hPa (990 hPa = 99kPa)
Tc = air temperature in degrees Celsius
Since most dyno testing is comparative, either standard can serve your needs. For that matter even observed numbers are instructive as long as you maintain consistent comparisons. The important thing is to choose a standard and stick with it for all your testing requirements.
NHRA Altitude Correction Factors
Multiply your ET or speed by the indicated correction factor in the adjacent chart to estimate your performance at sea level. These corrections are based on standard conditions and can be used as a general rule. Actual vehicle performance may vary according to specific local weather conditions.
Fuel quality and fuel mixture ratios are an important component of engine performance. In addition to juggling air/fuel ratios for best performance and optimum fuel economy, tuners are also concerned with fuel heat values, burn rates, octane, and the density or specific gravity of various fuels. For example, fuels with a higher specific gravity affect flow through the jet and alter tuning. The accompanying fuel/mixture ratio and fuel quality specifications are provided for reference. Note in particular the octane differences and the heat values spread between regular and premium gasoline and E85 Ethanol. Also note the difference in stoichiometric ratios between fuels.
Written by John Baechtel and Posted with Permission of CarTechBooks