Air Density Tuning Elite 16 ProMods Drag Racing @ Texas Motorplex Nov. 1-3, 2018

Screen snapshot from Air Density Online for Texas Motorplex showing an example of air density, density altitude, and grains of water for October 25, 2018.  This weather data is used by professional racers for critical race engine tuning.

Via Air Density Online

ProMods and Top Sportsman drag racing cars are the wildest motorsports door slammers around!  Midwest ProMod Racing Series (MWPMS) host a crazy event featuring these wild drag racers at the Texas Motorplex, Nov. 1-3, 2018.  Top drag racing ProMod racers running power levels well in excess of 2,000 HP, gamble for a $50K purse with air density tuning that is all-important for max performance.  In addition, the very entertaining Top Sportsman (TS) racecars also compete for a healthy purse.  Power levels are a bit less in many of the TS competitors where predictability is more important than making max power since these cars run within an ET bracket.  For some of these participants, ET dial-in adjustments are done from air density changes.  For others, engine, launch RPM, or shift RPM changes are made for air density changes to reproduce an elapse time target.  In most competition cases, air density knowledge is important! is filled with racecars and parts servicing this category of racing.

Bit of Air Density Science

That altitude puts a theoretical nominal barometer at about 29.4 inches HG or about 98% air density for STP (standard temperature pressure) weather conditions.  That would be for an ideal sea level baseline although those conditions seldom exist in racing.

Back to the science.  The air density affects the power output of a racing engine:

  • less air density >> less oxygen in the air
  • less oxygen in the air >> less power.

Air Density Affects Racing Engine Tuning

The following are examples of different weather conditions at Texas Motorplex and how they might affect engine tuning.  Track altitude is 515 feet above sea level.


Screen snapshot from Air Density Online for Texas Motorplex showing an example of air density, density altitude, and grains of water for October 25, 2018.  This weather data is used by professional racers for critical race engine tuning.

Air Density the Year Before

One year before this 2018 event, on Nov. 1-3, 2017, Texas Motorplex air density was about 97% in the morning and dropped to around 91% in the heat of late afternoon from warm temperatures well over 80 deg F.  Humidity was high that also affects the amount of oxygen in the air:

  • more humidity >> less oxygen in the air
  • less oxygen in the air >> less power.

Fuel system tuning for these kinds of air density changes is more dramatic.

Air Density Forecast for Theoretical Racing Time October 18-20

Air density information was re-examined a couple weeks before the event for this year.  That is shown in the next screen snapshot from Air Density Online for the Texas Motorplex.    It looked like much cooler temperatures and higher overall air densities than last year if the race was run at this time.

Screen snapshot of Air density forecast chart from race track weather for Texas Motorplex showing 3 day forecast of air density, density altitude, and grains of water for October 18-20.  The forecast shows vital tuning data used by serious racing tuners.

Air Density Variation Effects

This particular weather shows good air density that only varies from a high of 99% to a low of 98%.  If racing was done during these days:

  1. The first tuning task is to adjust the fuel system for this local air density. Essentially the fuel system should be adjusted a comparable amount to the air density change from the racer’s baseline.  That often depends on where the racer came from, which would determine a prior high or low air density baseline standard for that racer’s tune-up.
  2. Once that is done, tuning would be minimal, in this case, because of this narrow air density range throughout this theoretical event time. Spark plug readings, as tuning indicators, should remain consistent.  The air density simply does not change very much, and fuel system demands would not change very much.

Air Density Forecast for Theoretical Racing Time October 25-27 Showing Rain

Air density during the week before the upcoming event was further examined.

It looks like rain for a few days preceding cooler weather.

Air density is 98% in the morning going down to about 94% in the warmest afternoon with a temperature of around 60 deg F.  That is lower than last year around this same calendar period.  This is shown in the next screen snapshot for the Texas Motorplex for October 25-27.

Screen snapshot from Air Density Online for US Tracks, specifically the Texas Motorplex showing 3 day forecast of air density,  density altitude, and grains of water for race tuning on theoretical event time of October 25-27

More Air Density Variation >> More Oxygen Variation, >> More Power Variation

Air density varies from a high of 98% to a low of 94%.  If the ProMod racing event was done during these days:

  1. The racecar fuel system should again be adjusted from the racer’s baseline for a comparable amount of fuel volume change for this increased amount of air density change at the start of racing.
  2. The fuel system should be adjusted for the changing air density at it occurs throughout race day. Changes in spark plug reading become more dramatic as tuning indicators.

Humidity Effects on Drag Racecars

Note 1: The best racetrack surface is usually right after a strong rain storm where the pavement surface is washed clean from the rain.  On the chart, rain is likely with a forecast of 100% humidity.  If rain clears on race day, that is then followed by a good coating of traction compound and rubber dragging deposit before the race track.  That is key before it gets dirty from dust and oil/exhaust byproducts from subsequent racecars.  Without a prior rain storm, national event preparation often includes manual washing the racetrack surface as clean as possible prior to traction compound application and dragging.  Racetrack preparers say when it does not rain ahead of an event, washing is way more work than applying the subsequent traction compound & dragging.  Prior pavement cleaning is more critical to maximum traction for a subsequent event.

Note 2: Note the humidity change data.  Typically the higher the humidity during an event, the more sensitive the racetrack surface is to loss of traction.  Traction is very critical to high powered ProMods and Top Sportman racecars.  As the temperature goes down in the evening, the racetrack temperature drops, and humidity goes up.  In many events, as long as the event proceeds on a continuous basis, one racecar-after-another keeps ‘the traction groove’.  If the event is interrupted from a crash or oil down, sometimes ‘the groove’ is lost.  Subsequent traction is difficult to achieve, especially in front heavy, high powered ProMods and TS racecars.   They tend to unload the rear tires when the front end comes down after a wheels-up launch when there is any loss of racetrack traction from humidity or cooler temperature.  More free info is in reference 1.

Different Racers from Higher Altitude Baselines >> Different Tune-ups

Competitors from different parts of the country attend this event.  Some competitors developed tuning baselines from higher altitude, lower air density such as mountainous regions.  Here air densities are often well below 90%.  Lean spark plug readings would be common, and the racer must tune-up to the Texas event.

Different Racers from Lower Altitude Baselines >> Different Tune-downs

Some competitors developed tuning baselines from lower altitude, higher air densities such as coastal regions with higher air densities than elevated central regions.  Rich spark plug readings would be common when the air density is lower as you go to the higher elevation racetracks.  The racer who traveled from lower elevation regions must tune-down to warmer afternoon competition when it occurs at this Texas Motorplex location during warm weather.

Air Density Effects on Different Engine Combinations

Different racing engine setups are common to ProMod and Top Sportsman.  As temperature, humidity, and barometer vary from STP weather conditions, different air densities result that affect engine power levels especially in both the popular carbureted gas mountain motors and the blown alcohol competitors.  With the right fuel adjustments, power goes up or down with air density in either setup.  In both setups, fuel amounts should be adjusted in proportion to air density changes from the racer’s baseline.  The proper amount is weight-of-air to weight-of-fuel or air/fuel ratio.

For nitrous oxide (NOS) assisted engines, baseline engine setups “on-the-motor” w/o nitrous are affected equally as much by air density changes as normally aspirated mountain motors.  However many competitors simply adjust the NOS for the air density differences.  Adjustments for nitrous oxide amount, fuel amount, and bottle temperature are the ones to compensate for air density effects on these very high power racing engines, with many that are well over 2,000 horsepower.

EFI and centrifugal superchargers are a combination also affected by air density.  With closed loop control using oxygen sensors, the fuel is automatically regulated by the fuel map to maintain air/fuel ratios for air density changes.   However, power goes up or down with air density.

Carburetors and centrifugal superchargers are another combination affected by air density.  Carburetor re-jetting or other changes such as power valves or float bowl pressure are necessary to chase the air as air density goes up or down.  Because of high boost, carburetor tuning may be more vital than in normally aspirated engines with carburetors.  At high boost, great power or bad problems may be amplified without proper fuel system adjustment.

EFI and turbochargers are also one of the ProMod combinations.  They are less affected from air density changes since the exhaust waste gate(s) regulate boost to a target value, regardless of air density changes.  These setups, commonly running closed loop with oxygen sensors, automatically adjust the fuel as well.  That is again done according to the electronic controller fuel map properly matched to a constant boost that is regulated by the waste gate(s).

That logic would apply to a carburetor and turbocharger setup as well.  Waste gates tend to keep everything the same, and tuning for air density changes from a good baseline may be minimal.

Most Engine Combos and Air Density Tuning

For any of the racing engine combinations that do not run NOS, the oxygen from the air supports the combustion process.  The amount of oxygen in the air determines the amount of fuel.  The proper air/fuel ratio determines the amount of power.  Higher or lower air density affects the amount of oxygen in the air and subsequently the amount of power.

Note: Top Sportsman setups that use a blower can change the blower overdrive to compensate for a change in oxygen in the air from air density changes.

Our ProCalc fuel injection jetting calculator is a tool to determine the amount of overdrive change to compensate for air density changes to reproduce a power level for racing mechanical fuel injection.

ProMod engine tuners who their engines at the blower overdrive limits specified by the event rules cannot make this change.  Fuel system adjustments are needed instead.  The effect on air density from changes in temperature, humidity, or barometer is outlined in reference 2 & 3 and illustrated with math in these free references.  ProCalc is again a tool to determine fuel system changes for air density changes in racing mechanical fuel injection setups.

Air Density Effect on Horsepower Corrections

ProMod competitors strive for max power.  Other than the turbocharged racecars with boost controlled exhaust waste gates with relatively constant fuel systems, air density affects the fuel delivery needs (tuning) in the other engine combinations.

Air density changes also affect the horsepower

but in a different amount than the fuel delivery needs.

The effects are quite complex.  Changes to temperature, humidity, and air pressure (barometer) each have different effects on horsepower.  This is illustrated in the next example.

Example: Assume a dramatic temperature change by itself that causes a 5% change in air density.  That would necessitate a 5% change in fuel delivery needs from a baseline.  Now assume a different racing location where a dramatic barometer change by itself causes a 5% change in air density.  That would necessitate the same 5% change in fuel delivery needs from the same baseline.  Even though the air density change would be the same in both cases and the fuel system change would be the same, the horsepower correction would be different.

  1. Temperature changes have one effect on horsepower.
  2. Barometric pressure changes have a different effect on horsepower, even though the engine fuel system may be the same in both cases.

More Information about Horsepower Corrections

Reference 4 highlights the science behind horsepower corrections.  This source illustrates an example where an air density of 98% was achieved with a specific combination of the three weather values.  An engine with 2,200 horsepower was reduced to 2,165 horsepower from this weather combination.   The illustration shows that different weather combinations with the same air density could have different horsepower corrections.

Humidity vs. Water Grains

The previous charts show both humidity and water grains.  Many racers pay more attention to water grains.  That is because the relative humidity % changes with temperature, and water grains do not.  They stay relatively constant regardless of a temperature and relative humidity % changes that may occur throughout a racing day.  They may change from a weather front that may be moving in, but not from regional effects, especially where the wind speed is low.

Humidity Effects on Horsepower

  • Years ago, I was told to ignore humidity in tuning because its effects were minimal.
  • I was also told that more humidity made more power because of detonation retarding.
  • I was also told that it made more power from cooling of the air.

That is really not the case.  More humidity displaces oxygen in the air making less power.  Reference 5 illustrates an example where 50% relative humidity by itself reduces the air density (and oxygen content in the air) by over 2%.  That would be a reduction in fuel system delivery by over 2% to maintain an optimum air/fuel ratio, just from low vs muggy humidity.  That would be a comparable reduction in power.

ProMod at MWPMS and HP Correction

The performance change of the ProMod in changing air densities does not exactly follow the air density percentage line event though the fuel system correction may.  These racecars can overpower the tires, at will, almost anywhere on the racetrack.  Horsepower corrections from the weather value combinations become a vital part of the tuning data base to try to avoid tire spin.

Top Sportsman at MWPMS and HP Correction

The performance change from different weather value combinations in Top Sportsman may be more critical.  Tuning with air density changes to control a target ET dial-in really depends on the specific combination of weather values.  One weather value combination affects horsepower correction and the ET.  Another weather value combination even with the same air density affects horsepower corrections and the ET in a different manner.

In Summary

  • Air density forecasts are available through Air Density Online for racetracks all over the US.
  • Current air density for international tracks all around the world are listed as well, and metric units for weather conversions are provided for these.
  • Air density is key to fuel system tuning in most high performance racing engines.
  • Fuel system tuning goes up and down with air density.
  • Engine horsepower corrections go up and down as well but at a more complex rate.
  • Several air density and horsepower corrections results were provided from several references.
  • Air density tuning is a vital part of motorsports setups bought or sold through com.

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