Air Density Effects on Great Motorsport Races

Succesful races have a lot more to do with just how fast your car is, or even the track conditions for that matter, performance can have a lot to do with the environment. A well-tuned race vehicle should be calibrated for atmospheric and environmental factors but sometimes the run does not go as expected.

What is really going on when a racer suddenly sets a personal best run or even a world record? Is it luck or science? In some circumstances, the best-tuned racing engines might run poorly and in others, they might exceed expectations. Outside of the engine, what might be going on in the environment to contribute to an outstanding run?

World Record Setting Runs at Las Vegas Motor Speedway

On November 1, 2019, Brittany Force ran a new Top Fuel drag racing record of 338.17 mph at the NHRA Nationals at Las Vegas Motor Speedway. In fact, she did it in 1,000 feet of race track length instead of the traditional 1,320 feet (one-quarter mile) used in slower classes.

Top Fuel racer, Billy Torrence, ran a really fast 332.38 MPH during that same round of racing. In addition, several others ran well over 320 mph. Even though Brittany Force stole the show with that speed, others were running really fast as well.

What were some of the environmental factors that might have contributed?

Historical air density data from record running NHRA national event Top Fuel drag race.
  • Track elevation about 2,000 feet
  • Uncorrected barometer was about 28.14 inches HG which is typical of that elevation
  • Temperature was about 68 deg F which was on the cool side for drag racing, bumping up the air density and the oxygen in the air for power making.
  • Humidity at 6% indicating less oxygen displacement in the air. In other words, more oxygen in the air for combustion as well as more fuel vaporization from unsaturated air.
  • Wind was from the South West at about 3 mph, making a small tail wind for the mostly South-to-North race track direction. This indicates a slight reduction in ram air at high speed that could require a little bit less fuel in the last half second of the run; tuning for these 3 second races is done in fractions of a second time intervals.
  • Clear through the day indicating sunshine on the racetrack surface keeping it warm and indicating better traction. The sun on the race track will typically add 10 to 15 deg. F of surface temperature for a track temperature in the 80s (deg. F) for this run time. That is really good for traction at a national event drag race.

That is a lot of tuning information for that event and this run that is vital to motorsports tuners.

NHRA Top Fuel dragsters have to conform to an extensive set of rules to limit their speed. The overall combination in this class establishes quite a complex and involved tuning challenge that brings hit-or-miss results. Top Fuel tuning can make a record running amount of power, an excessive amount of power melting the engine, or a reduction in power from dropping cylinders from just a little bit too much fuel or ignition timing. Any of the three outcomes can occur at anywhere in the run. The crew is challenged by making many adjustments to the setup to try to target the best outcome.

Air density is one of the critical values in the tune-up. Changes in air temperature, humidity, and barometer all make up one air density value. However, each of these have different effects on horsepower and different effects on aerodynamic drag. For two different racing events or locations with the same air density but different combinations of temperature, humidity, and barometer, the horsepower and the aerodynamic drag could be different at those different locations. This is a crew chief challenge in a professional team such as Top Fuel.

This particular run is unique since it was done at a 2,000 foot altitude. That was with about a 7% reduction in air density from a sea level baseline, just from the barometer, with a corresponding reduction of oxygen in the air for power making combustion. For a Top Fuel drag racecar, Don Jackson, DJE, says that oxygen can be brought in with engine adjustments such as more blower overdrive. In this case, the power and revving range was somehow preserved in the tune-up. The reduction in air pressure from the reduced barometer however reduced the aerodynamic drag from fixed racecar features such as the cockpit, engine protrusions, and tires. Assuming rear and maybe front wind adjustments to restore the down drag for traction, the overall combination was well adjusted for the air density conditions. The ridiculous high speed is the final indicator.

Weather variables

Temperature, humidity, and atmospheric pressure all affect the air density, or how much oxygen is in the air. Generally, lower temperatures, low humidity, and higher atmospheric pressure result in higher oxygen content for more combustion power potential. Outside of oxygen content, the weather can affect a run in varying ways. Websites or handheld weather stations such as Kestrels display many weather variables besides air density and noting those changes can be enlightening.

The air density percentage can be the same at 2 different locations, but the weather factors that contributed to that number might be different. A race in Las Vegas might have lower humidity but higher temperatures while a run in Pennsylvania might have higher humidity but lower temperatures. Both combinations result in a similar air density value.

Here are some factors that might contribute to a good run under different weather conditions.

A comparison of screenshots from Air Density Online showing how different weather variables can still produce the same air density value. Daytona is near sea level while Las Vegas has about a 2,000 foot elevation. Engine tune-ups would most likely be different at the different locations.

 

 

Fuel dissociation

Fuel dissociates into intermediate chemicals during combustion. It is part of the normal combustion process but can be very complex. Most fuels including gasoline, alcohol, and nitromethane dissociate but they will each vary in dissociation steps. Air density affects dissociation which adds to the complexity.

For racers running methanol as their fuel of choice, some of the fuel dissociates into hydrogen prior to the ignition process. Dissociation occurs during compression for most engines as well as during boost for supercharged engines.

In common internal combustion engine designs, hydrogen is very prone to backfire. When the air density is high, more air goes into the engine causing more boost and more compression pressure under certain tuning scenarios. That results in more hydrogen dissociation. At a certain point in air density increase, a record running combination may backfire due to the increase in dissociated hydrogen.

Without this awareness, a tuner will richen a backfiring engine or reduce ignition advance. That usually works by cooling the engine, however, power may be reduced. Cooling the engine in some other way such as increasing water cooling or from fuel redistribution could work without a power reduction.

For example, supercharged engines often run fuel injection nozzles before the blower, and a second set after the blower. By redistributing more fuel above the blower by changing nozzle sizes, the intake manifold will be cooled. This changes dissociation, reducing hydrogen formation when air density is high. Note that the overall amount of fuel into the engine would not change with this method, just the distribution.

Air Saturation

Relative humidity is significant to a motorsports tune-up in more ways than often perceived. Higher humidity displaces oxygen which reduces air density and the power potential as a result. High humidity also saturates the air while low humidity leaves the air dry.

Racing fuels will readily vaporize in dry air. More fuel vapor has many advantages to power making. In contrast, fuel that is injected or carburetted into humid, saturated air is less prone to vaporize. The same amount is still there for the same air/fuel ratio. It is just not in a highly vaporized state. The fuel may be atomized by the fuel system, however it is not carried very well by the air through bends and distortions in the intake pathways. Less fuel vapor has many disadvantages to power making.

Comparing similar air density percentages with different humidity values can yield different run results. A higher humidity environment may produce less power, not because of the humidity displacing the oxygen in the air but because the saturation makes fuel vaporization more difficult.

Ram Air

In engines using a forward facing air scoop or air inlet, ram air increases the air to the engine as speeds increase. This can provide an increase in oxygen to the engine and more power if the appropriate amount of fuel is added to maintain the air/fuel ratio.

Depending on the wind direction, the effects of ram air can be dramatic. A head wind will increase the air into the engine while a tail wind will decrease the air into an engine at a given speed. For example, assume a track runs west to east. Also assume that there is a 20 mph head wind coming from the east. A vehicle running 200 mph will see 220 mph air going into the air scoop. That head wind for drag racecars running into it will affect the air/fuel ratio as the vehicle reaches higher speeds.

Aerodynamics

Wind running against the car can also have an aerodynamic effect. Higher air densities and lower elevations mean there is the potential for more drag on the body. Higher elevations and lower air densities can be beneficial in some circumstances due to the lower aerodynamic drag.

Naturally aspirated engines may be hindered by higher altitudes due to the loss in horse power. With lower air density, a naturally aspirated engine cannot make more power when there is no supercharger to compensate. Aerodynamic drag reductions may not overcome the power loss. Highly streamlined race car bodies can see minimal improvements in aerodynamic drag reduction from high altitude. Larger, less streamline bodies such as sedans and especially stock bodied nostalgia racecars from the 40s and 30s with protruding fenders, flat radiator grill shells, and wind shields may see great improvements in aerodynamic drag reductions.

A supercharged sedan with blower overdrive correction may run a lot faster in higher altitudes with thinner air. Where rules permit, the blower overdrive can be increased to compensate for the thinner air density and restore the power. The thinner air density will reduce the drag on the car body at speed making a boost in performance.

For cars with a rear wing, such as in NHRA Top Fuel, angle-of-attack adjustments can be made for more slope. This increases the down force to maintain rear tire loading for traction. The result may be no reduction in aerodynamic drag from the wing as it is adjusted with a greater angle of attack for thinner air or higher altitudes.

Tuning for successful (or unsuccessful) runs

Paul Polito was the NHRA Division 7 Champion in Pro ET in 2019. He and his crew chief, Mike O’Donnell, use air density tuning for Paul’s 572 cubic inch Ford powered Mustang. Mr. Polito repeated four elapse times all in a row. He ran exactly 9.280 seconds for the quarter mile performance by adjusting for weather as well as other track variables. Paul’s Ford engine is powered by a Rons Terminator fuel injection system running on methanol. This team is always a contender at the events they enter even when traveling to different tracks. We are the first to admit that while air density tuning was a major contributor to Mr. Polito’s success, he and his team exercise a lot of talent in drag racing bracket setup, tuning, and driving.

A Cautionary Tale!

Several years ago, we watched three top running drag racers at our local track. They were all in a popular nostalgia drag racing category with superchargers.

The temperature was in the low 50’s deg F, and humidity was high from a lingering weather front. Air density was over 101%. Density altitude was -370 feet. That is negative 370 feet indicating dense thick air. Most nostalgia events in this area are run in mid 90’s percentages for air density and density altitudes over 1,500 feet.

All three race cars were running close to 50% blower overdrive. We ran tuning math calculations to simulate the performance in a baseline for common air density around 96%. The math indicated the blower would have to be backed off to less than 40% overdrive to compensate for the increased air density at 101%. Prior to the runs, all three decided they were not going to adjust the blower for the cooler weather since they had raced in similar weather before without a problem.

On that day, all three race cars backfired and blew the burst panels out of their engine blower manifolds. The saturated air from the higher humidity may have inhibited fuel vaporization and fuel distribution causing an excessively lean cylinder. Higher air density may have caused more hydrogen dissociation with more sensitivity to backfire. It was peculiar that all three teams with similar engines backfired but these factors might have contributed.

Air Density Online has several tools to help you get the right tune for your setup including track weather information, a ProCalc jetting calculator, and even custom Pro Tune setups.

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