Great tuning starts with knowing the weather at your events. Understanding how your race vehicle handles weather in your location is a great advantage. The slightest edge can mean the difference between beating the competition or finishing behind. With all the effort put into maximizing an engine and a racing vehicle body’s aerodynamic performance, missing out because of a weather change can be a low blow.
Combustion in an engine is determined by the oxygen in the atmosphere. As temperature, humidity, or barometric pressure fluctuate throughout the day, they affect how much oxygen is in the air, affecting good combustion. In addition, these weather values affect the amount of fuel vaporization in a racing engine. More vaporization = more power. The amount of humidity (often expressed as water grains) has an effect on vaporization. This vaporization depends on the temperature at the time as well as the barometric pressure.
Some motorsports racing engines, such as popular bracket drag racing, are tuned to a targeted horsepower goal. Other motorsports participants tune to the maximum power output. However, ultimately, either goal – target horsepower or maximum power output – is determined by how far your engine can be pushed depending on the weather at the time of your run.
Air density values
The combination of temperature, humidity, and barometric pressure – along with local elevation – create an air density value that measures how much oxygen is in the air. Generally, higher air density (or higher oxygen levels) indicates good air and good power out of an engine. By comparison, low air density (or lower oxygen levels) indicates thin air and can indicate less power from an engine.
As air density fluctuates, different types of racing are affected in different ways. According to Andrew Ensminger at Kestrel Instruments:
“Along with grains of moisture, density altitude can play a role in car performance. Generally speaking, fewer air molecules [oxygen] will mean lower engine performance, so engine performance should drop at higher altitudes where the air is less dense. Density altitude measures the altitude at which you’d find your local air density, assuming standard atmospheric conditions (ISA) and is expressed as an altitude above sea level. As the temperature rises or pressure drops, your density altitude will increase. As the temperature drops or the pressure goes up, your density altitude will decrease.
For example, let’s start at sea level with an initial temperature of 85 F, pressure of 29.00 in Hg, and relative humidity of 50%. Since we’re at sea level, your altitude is 0 feet, but with those weather conditions, your density altitude is 3000 feet. If everything but the temperature were to stay the same, and the temperature dropped to 48 F, your density altitude would then be 400 feet. Conversely, if everything but the pressure stayed the same, and your pressure dropped to 27.3 in Hg, your density altitude would be 5000 feet. You would then want to not only tune for moisture in the air, but also your local density altitude, rather than your standard altitude.”
Dramatic changes like this could occur when traveling to different locations. They illustrate the effect on density altitude.
Different types of racing will use air density in different ways. The example above uses standard air density calculations to determine density altitude as one way to inform the tuning of your engine. Either is used to maintain the best air/fuel ratio for your setup to ensure best power. Knowing the air density enables accurate fuel adjustments for the air/density changes.
To further complicate things, air density also effects the aerodynamics of the racing vehicle in a way that opposes the effect on power. High air density indicates more air drag on the racing vehicle body (slowing you down), while low air density indicates less air drag (speeding you up). Depending on the atmospheric conditions, adjustments may be needed such as ride height or wing angle on those so equipped.
Weather variables in racing
The specific variables that affect air density are temperature, humidity, and barometric pressure. In addition, wind speed and direction can play a role in a car’s performance. We’ll investigate those using a typical summer day.
The temperature can fluctuate over 20 degrees throughout the day. Cooler temperatures generally mean higher air density while warmer temperatures mean lower air density. From just that change, engine fuel systems can need up to a 4% adjustment. Horsepower can drop more than 4% from that amount of temperature increase and get restored from that amount of temperature drop.
Closed loop engine EFI fuel systems are designed to compensate automatically while those that are open loop must be manually adjusted. Mechanical fuel injection and carbureted fuel systems must be manually adjusted to maintain an optimum air/fuel ratio for that temperature change.
Humidity can fluctuate by 50%. Humidity indicates how much space the moisture is taking up in the air. That amount of humidity increase can reduce horsepower almost 2% by itself. It can be counterintuitive, however, because higher humidity values do not necessarily indicate more moisture in the air or that more moisture is filling more space.
For example, if the temperature is rising, the air is getting thinner and there is more space for humidity. The current amount of moisture in the air might not change but the humidity percentage value will change. In that case, the horsepower will not change from the humidity percentile change, although it will change from the temperature difference. Tuning for humidity can be complex.
Many tuners use water grains as a measure of how much moisture is in the air. Water grains in the air are less affected by the temperature. Tuning with water grains is often based on the tuner’s database generated through trial-and-error.
During a racing event, barometer values can change by a half inch on the mercury scale. That amount of change, by itself, can affect horsepower by almost 2% in some racing engines. Barometer indicates the weight of the air. Lower elevations indicate higher barometer values where air is more dense. Higher elevations indicate lower barometer values where air is less dense. Changing barometer values at a specific location can also indicate an incoming storm system where rain and wind are possible, indicating a temperature change or a humidity change.
Not only does higher elevation indicate thinner air, it also affects the barometer values as mentioned above. Barometer taken from a local weather report is adjusted to sea level to provide a standard measurement, referred to as a corrected barometer. Higher elevations will have a much lower unadjusted barometer value. For example, a corrected barometer at Denver can be typically 29.9 inches of mercury. A typical uncorrected barometer in Denver, Colorado would be 24.5 inches of mercury. It is this unadjusted value that is most useful in engine tuning since this represents the actual localized air pressure data. Note that there is approximately one inch of barometer reduction on the uncorrected scale for every 1000 foot elevation increase.
Wind does not necessarily affect the air density measured at a location, but it can have a huge effect on the aerodynamics of the racing vehicle performance. Racing into a head wind puts more air drag from wind resistance on the racing vehicle body. However, it makes more power with racers with forward facing air scoops. The increased speed of air going into the engine increases the volume and oxygen from the air. Racing into a tail wind can reduce drag on the car body but also provide less power for engines with forward facing air scoops.
Effects of weather on a racing vehicle
Altitude of a racing facility has a dramatic effect on the barometric pressure in the air. Higher pressure from lower altitude makes more power with the proper tune-up. Measurement of the weather values become vital, especially for racing teams that travel to different locations. Changing weather and wind affect the aerodynamic drag on a racing vehicle body. Racing vehicles with ram air induction are affected by the alignment between the wind heading and the racing vehicle heading.
On the racetrack, heat and moisture affect race car tire traction. For drag racing, the launch depends on that traction. More traction, a quicker launch. At the circle track, turning and exploding out of the turn both depend on that traction.
Wind can affect waves and chop for race boats. Popular tunnel hull and hydro racing boats rely on a short chop from a mild wind speed to skim over the water at the fastest speed. The chop aerates the water against the sponsons, reducing hull surface water drag. The amount of wind affects that water surface characteristic. Off shore motorsports depend on wave characteristics. Wind speed together with current direction affects wave height and frequency. Excessive wind with the wrong current can make large waves with hull damaging outcomes from attempts to keep the speed up.
For bracket racers, air density changes indicate an amount to adjust the engine in order to reproduce a target ET. For class racing, compensating for air density changes can affect the maximum power. Without maintaining an optimal air/fuel ratio, engine performance can be unpredictable.
In engines with closed loop electronic control that are common in modern EFI, some level of compensation is automatic. However, racing engines are usually run rich with after-burning in the exhaust that can affect the closed loop control sensors. Air density monitoring helps to get indications for tuning in these setups as well. Tuning may be necessary from closed loop errors that can occur with exhaust after-burning from fuel rich mixtures. For nitro racing classes, air density and water grains units of humidity information are used to help select nitro percentages and volumes, especially in high horsepower classes. These are in addition to other engine adjustments such as compression ratio, blower overdrive, and the ignition advance curve indicated by a professional tuner’s data base.
The chassis is tuned for the track from humidity and temperature weather data for the best traction that is available. In addition, when it occurs, excessive cross wind on the racetrack can limit the handling of the car. Most drag racing takes place on a quarter mile racetrack, however, excessive cross winds can limit competition to shorter eighth mile racing for safety, or even termination of an event.
Top Speed Events
Noting elevation changes at event such as Bonneville are necessary. Bonneville is over 4,000 feet above sea level, creating thinner air and lower air density. Other locations closer to sea level will have much higher air density. Bonneville racers often travel from dramatically different locations around the country, most often from lower altitudes with higher air densities. Racing setups built in lower altitudes are difficult to convert to the high elevation with the thin air. For example, it can be difficult to pull boost in turbocharged engines originally set up for low altitude, local locations. Turbo housings and other components are not necessarily functional for much of a difference in altitudes. Knowing the effect of elevation changes on the engine power, such as turbocharged engines as well as most others, and body aerodynamics are key.
While drag racing and land speed racing take place at a specific point in time, sprint car racers will run an event over a longer time interval. The race cars have to be tuned for different scenarios that occur from the beginning of that event to the end. Temperature and humidity fluctuate as the day progresses. A hot and less humid afternoon might give way to a cool damp evening final event. Correct tuning is needed to handle both extremes.
Wind speed and direction are key to circle track handling around an oval course. Course direction into a head wind will increase down force in winged vehicles while a tail wind will reduce down-force in winged vehicles. Awareness of wind direction can assist in chassis setup in preparation for the event and driving technique throughout the event. Increases in humidity going into an evening can change the traction in a dirt or clay course. Some dirt courses get dramatically bumpier as a race progresses. High humidity may affect that deterioration, making position often difficult to hold onto. On paved courses, pavement traction may be affected by temperature changes as well as humidity buildup.
Using the environment to your advantage
Over time, and after trial-and-error , we developed a better understanding of how the weather effected our engine’s performance. Now, having an air density weather source such as Air Density Online and a portable weather station provides an accurate picture of the current weather conditions at our location. We can ensure the engine performance we know we can achieve. Knowing the air in the changing environment measured by air density is key to the precise adjustment of the fuel to get the most performance.
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