We hope everyone is staying safe and healthy during this unusual time. If you’ve got some time at home, this is a good opportunity to brush up on some engine tuning skills. The following article describes the basics of elevation and weather in engine tuning commonly practiced by professional motorsports competitors. Further reading can be found at Air Density Online.
Motorsports performance depends on air density: The more oxygen in the atmosphere, the greater the potential engine power and performance. Numerous factors affect how much oxygen is in the air, including weather and elevation. Higher elevations have thinner air, so lower air density. Other atmospheric factors not only affect air density but also a racing vehicle’s overall performance. For instance, tire traction is affected by humidity.
Racers with AFR meters or lambda meters often ignore air density, relying on their meter for info or correction. Racing engines that are run rich can produce misleading results from these fuel mixture meters. More or less after-burning in the exhaust for rich or lean mixtures from air density changes can add error to meter readings. Attention to air density changes, in addition, can reveal more effective tuning trends and adjustments.
Racing engines with custom exhaust, lengthy pipes and heavy mass can leak. This will introduce outside air into an exhaust, causing mixture meter errors. Add to that changes from air density and mixture meter tuning – an AFR meter by itself can be a shot in the dark.
Weather Factors Influence a Tune-up
Elements in the atmosphere can affect the whole car in various ways, but if the engine isn’t giving its best performance, the rest might not matter. In a racing engine, the right combination of air and fuel produces the best combustion and, therefore, the most power. Keeping an engine running in top shape means maintaining that air/fuel ratio when the atmosphere is changing.
In the simplest terms, temperature indicates how much atmospheric molecules are moving in the air: The faster they are moving, the hotter it is. In a cubic foot of atmosphere at ground level, there is more room for other molecules like moisture and pollution when it is hotter and atmospheric molecules are moving faster.
Atmospheric pressure also determines how much oxygen is in the air. Less pressure means fewer molecules in a cubic foot of air and more room between oxygen molecules in that air. More pressure means everything is more compressed together, so there are more molecules in a cubic foot of air and less room between those atmospheric molecules. In terms of elevation, higher elevations have less pressure, which equates to less air in the atmosphere and less oxygen in the air.
Corrected and Uncorrected Barometer Values
When checking the local weather report, the barometer value that is commonly given is a corrected value in order to provide a standard reading adjusted to sea level. The actual, uncorrected barometer value gets lower as elevations increase and atmospheric pressure decreases. This becomes important in racing engine tuning because an environment with a standard 29.92 inHg barometer reading can be thrown off by the elevation.
A corrected barometer reading in mountainous Colorado could be 29.82 inHg. Even though that looks like a good pressure value, the uncorrected barometer value would be approximately 5 inHg lower. That means the air density is much lower at 85%.
Effect of Humidity on Air Density
Races that are won or lost by a fender length or less can be from correct or incorrect tuning for humidity changes alone. Humidity droplets occupy air space, reducing the amount of oxygen in the air. The effect of humidity, by itself, on air density and engine power is illustrated in the screen shots below.
Humidity not only takes up space in the atmosphere, it also affects the road surface that vehicles run on. When racing on pavement, a layer of rubber builds up from race car traffic. Humidity can cause declamation of the rubber coating, leading to chunks of rubber breaking loose from the surface as race cars drive over it.
Race tracks often use a spray-on surface treatment to increase traction. Treatment can compensate for some humidity buildup but not very much.
Let’s compare two race tracks to see how their atmospheres might affect an engine tune-up. At a glance, Tucson Dragway and Palm Beach International Raceway are obviously in very different climates. Tucson is in the desert and at a high elevation of 3,000 feet. Palm Beach is in a tropical climate at sea level, or approximately 29 feet above sea level.
That is an approximate 10% difference in the weight of air. With all else the same for a race car in the two locations, Tucson would require 10% less fuel by weight. That should also mean less engine power; however, depending on the fuel, expected power level differences can change.
Methanol would not experience as much of a power reduction as gasoline because methanol contains 50% oxygen in the fuel by weight. It is not enough to sustain monopropellant combustion, but enough to make the fuel a bit less sensitive to air density changes than gasoline.
Nitromethane would not experience as much of a power reduction as methanol because it has more oxygen in the fuel. In fact, it is enough to sustain monopropellant combustion. In many nitro racing classes, tuners can simply adjust the volume of fuel to some extent to compensate for changes in air density.
Engine factors make a difference as well. A roots or screw blown engine would not have as much power reduction as a naturally aspirated engine. The blower at a fixed overdrive pumps a lot more air through the engine than the same size engine that is naturally aspirated. Because of that, complex thermodynamic changes occur between the two engines that make boosted engines less sensitive to air density changes.
In racing classes where the overdrive of blown engines is not specified, simply changing the blower overdrive an appropriate amount can compensate for different air densities.
A turbo-charged engine with boost-controlled waste gate may not experience any power reduction with less drastic air density changes. The boost would remain constant with the waste gate control. Power should be constant.
In some cases with more dramatic changes in altitude or a significant change in air pressure, the turbocharger components need to be changed to reach boost. This is a common problem in turbocharged land speed record vehicles dyno’ed at sea level. The turbocharger components can be sized for the boost in that location. That same boost may not be attainable with the same turbocharger components at a high elevation location such as Bonneville in Utah. Air density may be more than 30% lower. Different turbocharger components would be necessary to reach the same boost to maintain the horsepower.
Comparing Weather at Both Tracks
Going back to Palm Beach and Tucson, both experience extreme heat, but Tucson is a very dry climate while Palm Beach is very moist. While Palm Beach is at sea level, the moisture in the atmosphere would affect engine performance in numerous ways.
As illustrated before, moisture displaces oxygen from the air. The moisture effect depends on temperature since warmer temperatures leave more space in the atmosphere. See the examples below.
After comparing all the factors that come into play, neither track is necessarily going to provide a greater advantage when it comes time to race if proper compensation for air density is not done. Tucson has a disadvantage for being at a much higher elevation while Palm Beach has a disadvantage for having much higher humidity. Proper adjustments based on good air density data in both situations can ensure a successful run.
Engine tuning and the atmosphere
As always, the fuel system should be tweaked for the altitude and barometer value. Depending on the type of engine, adjustments may be subtle or drastic.
Mechanical Fuel Injection: Nozzle changes for maintaining air/fuel ratio
For racing mechanical fuel injection, a well-tuned engine may need the main bypass nozzle changed to accommodate changes in air density. In a new or unfamiliar setup, changing engine nozzles might also be necessary to control fuel pressure.
The first step is having a good understanding of what the best air/fuel ratio is for your engine. From there, main bypass and nozzle changes can be determined.
Roots or screw blown engines: blower overdrive
Roots or screw blown engines can maintain their fuel system in different environments by changing the blower overdrive. This method maintains engine horsepower without needing any other fuel system adjustments.
For example, a local racer with a match racing funny car dragster ran a 6.8 second ET at a sea level race track on an 80 deg F evening. He also ran a 6.8 second ET at a track with a 5,500 foot elevation on a 105 deg F afternoon. He did this by increasing the blower overdrive by 27% to compensate. The amount of overdrive change was computed based on the air density change.
Blower overdrive adjustments can be done for other air density parameters, such as temperature. Several years ago, our dragster ran in a 7.9 second bracket. We adjusted the overdrive in our 8-71 blower for qualifying in a hot afternoon temperature of approximately 95 deg F. As the day progressed and the weather cooled almost 30 degrees, the air density increased. We reduced the blower overdrive by 10% and maintained 7.9 second ETs through the final run late in the evening. We ran full throttle all day and into the evening without lifting the throttle. The racecar was repeatable due to the appropriate blower overdrive adjustment for the air density change.
Forward air scoop
In engines with a forward facing air scoop, the barometer change from altitude can be a mixed blessing. For higher altitude race tracks with a lower barometer, less air density means lighter air is going into the air scoop – creating less increase in power from ram air. Lower altitude race tracks with higher barometer values have heavier air going into the air scoop and tend to make more power increase from ram air.
In an offset difference, higher altitude race tracks with thin air have less aerodynamic resistance to racing vehicle bodies boosting performance. Where a larger air scoop is used for more ram air boost, less wind resistance occurs from pushing that larger scoop through the thin air.
Lower altitude race tracks with thick air have more aerodynamic resistance to racing vehicle bodies, detracting from performance. Where a larger air scoop is used for more ram air boost, more wind resistance occurs from pushing that larger scoop through the thick air. Power may be increased causing better ET, but the trap speed increase may not be as dramatic.
Considerations in different racing environments
Mechanical fuel injection engines: adequate spare jets and nozzles and adequate fuel pump size for high air density locations such as from sea level locations or cooler weather.
Carbureted engines: adequate spare jets and other carburetor components to lean or richen the carb as needed for the air density environment that will be entered.
Humid track locations: new or near new tires for best traction under high humidity conditions.
Turbocharged engines: confirmation of compressor map range for turbo considering the air density of the environment. For example (as stated earlier), many turbo racers with an optimum low altitude setup could not make boost at high altitude tracks such as Bonneville without appropriate turbocharger component changes.
Air density changes due to weather and altitude are a major consideration for tuners when going to a new track. While altitude primarily affects the baseline barometer, track location may affect the baseline humidity. Coastal tracks would likely require power considerations due to higher humidity and lower elevation. Other locations would have atmospheric and elevation factors to consider that may be very different. With proper planning and air density information, a good tune-up can be better maintained at different track locations and altitudes.