When the race is tight and fractions of a second matter, considering every aspect of the environment can help get that little bit of extra speed or faster time. Humidity in the air is one of those environmental factors to consider. It affects both racing engine power and traction.
Humidity in the air can influence not only an engine’s performance but also how the setup for the track is handled. Below are a few ways humidity can impact the engine and the racer setup.
Years ago, many in motorsports overlooked humidity. Popular air density meters from back then would only measure temperature and barometric pressure to determine an air density value for tuning. Now with more recent weather sources such as Kestrel weather meters, CrewChiefPro, and Air Density Online, humidity data is more accurately incorporated into motorsports tuning.
Humidity As A Weather Measurement
Humidity is an indicator of atmospheric moisture. The percentage that is given on the weather report is influenced by many factors and is not always a reliable measure of actual moisture in the air. The percentage is not only a measure of atmospheric moisture but also how temperature and pressure are fluctuating.
Water Grains vs Humidity
Water grains are another measure of moister in the air. Measuring the number of water grains – a small mass of water that can be suspended in the air – is another indication of humidity. There are 7,000 grains of water in a gallon. Even though ambient temperature and humidity may vary throughout a day of racing, water grains tend to remain a more stable value. That is, the number of water grains remain more constant from:
Many tuners rely on water grains to guide their tuning decisions, sometimes instead of humidity percentage. However, air density and density altitude values are both used in racing and are commonly derived from math using humidity percentages. As a result, humidity is a value sustained in the various sources of weather for motorsports.
Humidity in the Air
Humidity expressed as a percentage gives a good idea of how much the atmospheric moisture is displacing atmospheric oxygen and other gasses. As a general rule:
• The higher the humidity percentage, the less oxygen is in the air for combustion and the less horsepower available in a racing engine
• The lower the humidity percentage, the more oxygen is available in the atmosphere for combustion and the more horsepower in an engine.
The actual horsepower effect from humidity changes is quite complex. It is dependent on whether the racing engine is normally aspirated or supercharged. It is also dependent on the type of racing fuel. However, it is illustrated here with an assumed linear relationship with air density. As you can see from the chart above, humidity affects the air density and density altitude values, leading to an effect on air/fuel ratio and engine performance.
Density altitude is another tune-up guide along with air density that is frequently used by motorsports participants. It is remarkable how much density altitude is affected by changes in humidity. Horsepower is more affected by humidity changes at higher temperatures than at lower temperatures. At 80 deg F, a major change in humidity results in a 2% change in power. At the lower temperature, only a 1% change in power occurs from the humidity change.
Humidity’s Effect on Traction
While increased humidity decreases horsepower, it also influences tire traction in different ways depending on the type of track. Increased humidity reduces traction on paved tracks. However, dirt tracks get more sticky with increased humidity – increasing traction.
In circle track racing, more humidity may keep dirt racetracks sticky, resulting in more traction. Less humidity may hasten the dirt tracks drying out. Without moisture, a dry track gets dusty, and traction is reduced. That often needs a touch-up of water spray from racetrack crew to keep the track sticky, especially with a lot of clay in the dirt.
As a result, maximum performance from a motorsports vehicle is complicated by a combination of horsepower management and traction management.
Humidity and Drag Strips
Generally speaking, less humidity results in more traction on paved tracks. Humidity is usually lower in the heat of the day. It is usually higher in the morning and late evening. Traction is more of a problem in the morning and late evening in some cases. At many events, drag strip surfaces are prepared with a combination of spray-on traction compounds and dragging rubber over the track. Recent spray-on traction compounds appear to tolerate more humidity build-up than those from the recent past.
It is reported that ProMod drag cars of this type can frequently black-track (spin the tires) the entire quarter mile race track if power levels are increased above 4,500 horsepower. That is possible with recent blower and fuel system developments. It is characteristic of a front heavy, front engine layout such as this. As a result, much of the racing is reduced to 1/8th-mile events to keep top end speeds not much over 200 MPH in the 1/8th-mile. That seems to be more manageable than higher speeds that would occur in longer ¼-mile racing. Even so, that is a spectacular achievement and a real tuning challenge. Early morning and nighttime races are plagued with considerations of traction loss from humidity with this level of power and performance.
For example, until recently, Sacramento Raceway had good traction in cooler evenings with track surface temperatures down to the low 70s (deg F). That would be about 10 deg F higher than ambient air temperatures that would be in low 60s (deg F). Humidity in the area starts climbing up to around 40%. At main events where high-powered drag cars were run in continuous rounds, traction for the higher-powered cars would hold up even with track surface temperatures continuing to drop into the high 60s (deg F). Ambient temperatures would be in the 50s deg F. Continuous rounds with race cars running back-to-back would keep the track dry with better traction.
However, if the rounds were delayed for more than a few minutes, such as from the occasional racecar breakage/cleanup, traction would be lost when the event resumed. Humidity build-up from the cool evening is a contributor.
Temperature between a spinning tire and the track surface is several hundred deg F. That boils away any surface humidity in the track of rubber that is laid down from tire spinning that increases traction. That would be only if the car is lined up over the tire tracks and remains in that groove on the run.
Humidity and Circle Tracks
Two categories of circle tracks are pavement and dirt. In addition to that, track length is a secondary category. Short ¼-mile circle tracks have tight turns where traction is an all-important contributor to course elapse time. Sprint cars drift in the corners with the back end outside pushing in from engine power. Tire spin can be excessive or moderate depending on the overall circumstances. That would include the sprint car setup as well as the track condition. Dirt tracks are kept damp throughout an event to keep the surface dust-free and stickier. Better traction is a result. Higher humidity contributes to less engine power but a stickier track surface, a peculiar combination.
Dirt track surfaces build up a layer of rubber in the racing groove from cars-after-cars running around the course. On a seasoned surface with a lot of rubber build-up, higher humidity may not be better for that part of the track surface even though it may help the clay/dirt around it. However, from the combination of roughness, grooves, and bumps that form during a race, the difference is probably not noticeable by anyone other than an experienced driver.
Longer tracks such as 1/3-mile and ½-mile are often run with engine throttles wide-open around the track. With a proper bank angle, cars are at full throttle with less drift and less reliance on maximum traction. However, overall good traction would contribute to better maneuverability. Higher humidity is not a welcome condition on pavement tracks but again a peculiar combination on dirt: less power with possibly more traction.
Circle Track Dirt Racing Example at Mohave Valley Raceway
CLS/SWLS, Best of West Round 3, Apr 1, 2023
Location: Mohave Valley Raceway, Mohave Valley, AZ
Event Type: Dirt Racing
Humidity for afternoon qualifying is in the teens (%). It is combined with temperatures in the 80s (deg F) and a low 29.4 in HG barometer. That puts the air density in the 94% range. Evening event racing has increasing humidity and barometer but with cooler temperatures. Air density goes up 4%. That may trigger a fuel system adjustment to chase the better weather in the final racing heat in the evening. Note that humidity goes up 10% from late afternoon into the evening that may affect traction.
Water grains are also displayed. That value shows a gradual drop in the amount of water in the air throughout the day that may affect the chassis and tire setup.
Circle Track Dirt Racing Example at Bakersfield Speedway
CLS with USAC 360 Sprints, Apr 8, 2023
Location: Bakersfield Speedway, Bakersfield, CA
Event Type: Dirt Racing
Looking at last year on this same date, air density is a bit lower than it was for the previous Mohave Valley Raceway event. A proportionate drop in fuel flow may be needed for qualifying. However, air density climbs back up to 94%, similar to the previous Mohave Valley Raceway event. A similar fuel system setup as before may be needed. Humidity for afternoon qualifying started in the 20s (%), dropping into the teens (%), then going up to the 30s (%). That was combined with temperatures peaking in the 90s (deg F) and a low 29.2 in HG barometer. There was a small drop in barometer and a more significant drop in temperature into the evening. Water grains are a lot higher than before. Overall humidity ends up higher than the previous Mohave event. A contribution from humidity change would probably be noticed.
The CLS/WLS event returns to Bakersfield on 5/20/2023. Looking at the historic weather at this time from last year on airdensityonline.com, weather differences added 2% to air density. There was not much change in humidity. Sprint car tune-ups may need further adjustment from this late spring event.
Circle Track Dirt Racing Example at Bubba Raceway Park
Going across country, Bubba Raceway Park in Ocala, Florida is examined at an arbitrary date early in the year.
In this location on this date, humidity is high, especially for an evening final round. Air density is affected by the near sea level altitude and a high barometer for this day. However, looking only at humidity, it is a lot more damp in the air than the locations from the previous western USA examples. Water grains are a lot higher throughout the day than the previous western US events. The track may be sticky where the surface is dirt/clay and slick where the groove is rubber coated.
Summary
All of these track weather examples are dirt tracks. Humidity differences between different track locations may have an effect on engine power. High humidity in Ocala, Florida reduces engine power over that of a lower humidity environment that may be elsewhere. That would be in addition to the effect of temperature and barometer changes between the different locations.
High humidity on dirt tracks would most likely reduce the amount of water spray maintenance on the track during an event. However, where rubber has coated the track surface, high humidity may cause a slippery condition on the rubber surfaces and stickiness on the dirt surfaces. Chassis and tire condition setups are a challenge as well as driving from possible changes in traction.
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