Subtle Variations Can Make a Big Difference in a Racer’s Tune-up

It’s easy to see how dramatic changes in temperature, humidity, and elevation can wildly impact a racing engine tune-up. Racing in a high-altitude desert in the winter will be vastly different from racing at a coastal track in the summer. But how much do less dramatic environmental factors really affect the tune-up if you’re only going a couple hundred miles away (or less)? If you’re just going one town over, do you really need to monitor your fuel system like a hawk for example? Or can you fall back on what you know that worked at your last event? Or last location?

Subtle tuning changes can impact your racing competitiveness. Even going from morning to afternoon in the same location can have a noticeable effect on engine performance. When hundredths of seconds matter, these subtle variables can have big impacts.

Drag racing crew member on the photo left adjusting for full throttle / full power on stack type mechanical fuel injection setup. This is what the ‘atmosphere’ is looking at, so to speak, under full power, high speed, and air density.

Weather Factors and Tuning Changes

The information on the local weather report provides data such as the temperature, humidity, and barometric pressure.  All of these factors in combination affect the air density, or how much oxygen is in the air.  We’ve written about these before and just knowing about these can make a world of difference in getting more power and consistency out of your racing engine and vehicle performance.

Air/fuel Ratio — Weather changes throughout the day affect how much oxygen is in the air and, therefore, the air/fuel ratio in your vehicle.  Since maintaining an appropriate air/fuel ratio for your setup means getting maximum power, tracking weather becomes very important.  Spark plug readings are often the indicator.  Mornings and evenings tend to be cooler which points to higher air density.  However, dew or fog can lower the air density offsetting the increase from cool temperature.  Warmer afternoons are also associated with lower air density from the hotter temperature.  These factors necessitate fuel system changes to compensate.

  • Open loop fuel systems often need adjustment for air density changes.
  • Closed loop EFI fuel systems that are properly setup usually adjust automatically for air density changes. Proper adjustment may include range and response that may need changing for different weather.

As a result, even if you’ve only raced at one track, you probably experienced the issues that come with a changing weather environment.

Atmospheric Pressure

When factoring in weather changes, environmental factors should be taken into account.  For example, higher elevations equate to lower barometric pressure.  A general rule of thumb is

about 1,000 feet of elevation gain

is approximately 1 inHg of barometric pressure loss.

This will equate to lower air density and less power.  For a racing engine, that is about 3% less power per 1 inHG reduction.  This can necessitate fuel system changes to compensate whether they are manual jetting changes or automatic EFI control.

Corrected vs Uncorrected Barometer — The barometer value provided by the local weather report is corrected to provide a standard value at sea level.  If a location at 2,000 feet of elevation is showing a corrected barometer value of 29.4 inHg, the uncorrected value is closer to 27.4 inHg.  In this example, the uncorrected and corrected value would present a dramatic difference in the effect on engine power.  Using the corrected value would provide inaccurate information and lead to potential problems during the run.

Wind Effect

Wind will provide differing effects on a vehicle.  Knowing the track orientation and engine specs can help you plan.

(1) For race vehicles running into a head wind:

  • wind resistance will cause problems slowing the racer
  • added oxygen into a forward-facing air scoop may provide an unexpected boost, speeding up the racer.

(2) A tail wind would provide

  • a little push down the track
  • but may negatively impact a forward-facing air scoop reducing the ram air and engine power.

(3) Cross winds can cause problems with handling.

  • Depending on the aerodynamics of the vehicle, this may be a large or small problem.
  • The forward or rearward component of the cross wind may also affect ram air and engine power.

Wind in Circle Racing — Circle track racers will experience wind problems differently throughout the race in different parts of the track.  These affect engine power, handling, and track surfaces for traction, for example.  The effects are different around the track depending on the wind strength and direction.  To illustrate, a head wind in the back straightaway would be a tail wind in the front straightaway.  Aerodynamics can be significantly affected differently around the track.  Changes can occur from the beginning of a heat to the end of a heat.  An hourly forecast may help.  A forecast may be vital for tuning decisions that lock in the setup at the beginning of the race with provisions for changes at the end of the race.

Humidity & Track Surface — A subtler issue to be aware of in the track is how landmarks and orientation create shade that may maintain dew and humidity on the track’s surface.  This can cause problems with traction long after the rising sun has burned off moisture elsewhere and traction is otherwise improved.

This drag racing setup reveals a Roots blown engine with racing fuel injection. Necessary adjustments include fuel pump size, idle poppet pressure, pump relief pressure, main bypass size, high speed bypass size & pressure, hat-nozzle-to-port-nozzle fuel split, port nozzle fuel distribution, and nitro percentage (when applicable). Having a good baseline for all this makes adjustments for air density easier to track.

Traction and Temperature

Temperature affects the tire traction in land racing motorsports on pavement.  Higher temperatures, up to a point, make more traction.  This affects acceleration, breaking, and turning for circle track racers.

High Temps – Tire traction seems to be the best with racetrack surface temperatures over 70 deg F, up to about 130 deg F. Note that extremely high temperatures above that can flatten out the effect of improved traction.  Tire surfaces can fail from excess temperatures. During the hottest summer months such as over 100 deg F air temperature, pavement can reach temperatures of 150 degrees F with ambient heat and direct sunlight. Spraying water on the surface and letting it evaporate cools the surface down.  A good target is about 130 deg F or below and will increase traction.

Low Temps — Cooler temperatures can reduce pavement traction.  At our local dragstrip, surface temperature below approximately 72 deg F is routinely observed to make it hard for high powered drag cars to hook up.

Direct sunlight can increase the temperature by about 10 degrees over air temperature.  If the ambient temperature is in the 60’s, the pavement may still be in the 70’s.  This will just barely enable drag cars to maintain traction from the surface temperature effect.

Traction and Humidity

Extreme humidity can reduce traction.  The humidity acts like a lubricant on paved surfaces.  A car that is running well and hooking up the tires all day may experience dramatic changes in traction as evening humidity coats the track surface.  However, on racing dirt track surfaces, moisture on the track can maintain traction by keeping the dirt sticky with a harder surface from more texture body.  In this case, humidity can help traction.

Vehicle Specs as Factors

Understanding the specs of your motorsports vehicle helps when dealing with environmental changes.  For example, higher air density makes higher wind resistance.  This can have differing effects on lift for wings or spoilers.  Cross winds will also affect handling differently for different motorsports vehicles.  Hood scoops (when so equipped) will feel the effects of ram air differently depending on how the wind is oriented.

Different Tracks – Different environments

Let’s take a look at two dragstrips in the midwestern United States:

  • Tulsa Raceway Park
    • Elevation: 619 ft
    • Track orientation: runs from west to east
  • Thunder Valley Raceway Park.
    • Elevation: 1,079 ft
    • Track orientation: runs from southeast to northwest

They are only about 145 miles apart, but the different locations can provide different experiences.  Let’s look at weather from both tracks on the same day, April 10, 2022.

Tulsa Raceway Park

The below graphic presents weather at Tulsa Raceway Park on April 10, 2022 courtesy of AirDensityOnline.com.  It is located in Tulsa, Oklahoma and has an elevation of 619 ft.

Tulsa Raceway Park is at an elevation of 619 feet above sea level.  That would subtract an average of about 0.5 in HG from a sea level barometer or about 1.5% in power.  Note that in the image above, the barometer values are uncorrected.  They are not adjusted for the elevation.

For a race day from 8 am in the morning through 11 pm in the late evening, air density varies from 95% at 8 am in the morning, down to 91% at 5 pm in the late afternoon, then back up to 94% at 11 pm in the late evening.  That is 4% down, then 3% back up at night racing.  Power would vary almost that much throughout the day.  More power would occur in the early morning and late evening.

Tulsa Raceway Park Wind Speed & Direction

Now, let us examine the track direction to analyze any wind effect.  This track runs west to east as seen on this page on Air Density Online.

Morning in Tulsa — The wind at 8 am starts out in the morning, from southeast (SE) at 18-mph.  This would be a partial head wind and partial cross wind.  The former would affect a power increase from ram air (if so equipped).  The later would affect an aerodynamic loss from more wind resistance; or traction increase if equipped with a wing or rear spoiler.

Late Afternoon in Tulsa – Starting around 12 noon, wind is 22-mph from the southwest (SW).  This would be again a partial cross wind, but now a partial tail wind reducing power gain from ram air and reducing wind resistance from aerodynamics.  The former reduces performance and the later increases performance.  Wing or spoiler equipped drag race cars may experience a reduction in high end traction from the tail wind as well.

Late Evening in Tulsa – At 11 pm in the late evening, the wind changes to a light wind at 6-mph back from the southeast (SE), approaching the morning wind effect, but not as much.

Thunder Valley Raceway Park

The below graphic presents weather at Thunder Valley Raceway Park on April 10, 2022 courtesy of AirDensityOnline.com.  This track is located in Noble, Oklahoma and has an elevation of 1,079 ft.

Thunder Valley Raceway Park is at an elevation of 1079 feet above sea level.  This would subtract an average of about 1 in HG from a sea level barometer.  That is an extra 0.5 inHg barometer lower than the Tulsa raceway location.  Note that in the image above, the barometer values are also uncorrected.  That is, they are not adjusted for elevation but instead the raw values that affect horsepower and aerodynamics.

For a race day from 8 am through 11 pm, air density varies from 94% in the morning, down to 89% starting at 3 pm in the afternoon, then back up to 92% at 11 pm in the late evening.  That is 5% down, then 3% back up for night racing.  Power would vary almost that much throughout the day.  Again, more power would occur in the early morning and late evening.  That air density spread compared to the Tulsa Raceway at a lower elevation, is in agreement.  A bit of a reduction in air density occurs at this higher racetrack location.  This is a good indication of fairly uniform weather between the two locations.

Thunder Valley Raceway Park Wind Speed and Direction

Now, let us examine this track direction to analyze any wind effect.  This track runs from east to the west, slightly veering north, as seen in this map on Air Density Online.  This is opposite the Tulsa racetrack direction.  For other racing days such as those that may have uniform regional high winds, tuning for wind direction may be quite different between the two locations.

Morning at Thunder Valley — The wind starts out in the morning at 8 am, from the south (S).  This would be a 19-mph cross wind affecting handling.  There would be no head / tail wind, so no effect on ram air (if so equipped) or change to traction from wing or rear spoiler equipped drag racecars.

Tulsa Raceway at this same time has some head wind affecting ram air from a component of an 18-mph wind.  That with a small difference in air density may affect the setup and performance that would otherwise occur at this location.

 

Late morning wind changes from the southwest (SW) affecting performance from ram air and aerodynamics.

Late Afternoon at Thunder Valley – For a short while in the late afternoon around 5 pm, wind is 16-mph, again from the south (S).  This would be only a cross wind affecting handling, again without a head / tail wind affecting baselines from ram air, body wind resistance, or traction from body aerodynamic devices.

In comparison to the Tulsa Raceway at this same time, there is a tail wind affecting ram air.  Tail winds reduce ram air induction and engine power.  This difference with a small change in air density compared to Tulsa, would affect performance compared to what would occur at this location.

Late Evening at Thunder Valley — In the evening starting around 6 pm, wind changes from the southeast (SE).  Around 11 pm, the wind ends up at 13-mph from the southeast (SE).  This would now be a tail wind effect reducing the power addition from ram air but also reducing aerodynamic drag for an offsetting performance effect.

In comparison to the Tulsa Raceway at this same late evening time experiencing a head wind and a small change in air density from the lower altitude, a performance affect may occur compared to this location.

Tracks Comparison Summary

As previously stated, these tracks are 145 miles apart, or about a 2.5-hour drive.  Predicted performance differs from a small change in air density.  That difference appears in line with what would be expected from the difference in altitude.  Differences in wind amount & direction occur between the two locations as well that would influence performance.  The small differences in air density indicate relatively uniform weather across the region.

Elevation with Terrain Differences — Consider different track locations with terrain differences.  Altitude differences are one change with greater influences on the barometer.  Also consider where mountainous terrain may separate track locations.  Differences such as these can result in significant air density changes between two locations as well as wind strength and direction.  Where performance predictions from the weather are important, affects from different locations from terrain may be significant for both air density, wind strength, and wind direction.

It’s Not Just Land Vehicles

Boat racers deal with added effects of wind in the air causing issues with wind strength and direction.  That is the water surface condition from the wind causing chop or waves.

A low-to-moderate wind on a water surface will cause a slight chop on the surface that will help break water surface contact with a highspeed hull against the water.  The race boat may be faster with a wind.

Mirror smooth water from no wind often makes it difficult to get up to top speed.  The boat’s hull stays glued to the water surface.  A bit of chop is best to aerate the contact between the hull and water.  Of course, high wind can cause waves.  Often knowledgeable staff and racing tuners can predict the point where high wind strength make waves too big that may necessitate cancelling a race.

Regionally, as I recall, a wind of around 25 mph was the transition point.  Less wind than that would cause a moderate chop that would allow racing flat out.  More wind than that would cause high waves that would beat the race boat to death.  They could be a handful to race on with continuous throttle modulation to get around the water course.  The wind speed transition point can be different for different race boats.  Larger hulls can tolerate more waves.  Vee bottom or flat bottom boats may be less tolerant.

Conclusion

Predicting motorsports performance can be complicated.  Numerous mechanical and tuning adjustments are often necessary for best performance from a racing vehicle.  In addition, slight differences in weather conditions can add up to unforeseen problems in an otherwise well-planned outing.  While it seems easy to not track changes at an event at your home track or at a neighboring track, those untracked changes can make the difference between a win and a loss from performance differences.  Changes to wind direction or track orientation, weather changes throughout the day, and even geographical features of the track can affect the performance and success of the run.

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