What is Density Altitude and how does it affect performance?
Density Altitude is a measure used in aviation to assess the density of the air. It is not a true altitude but rather a calculated altitude that reflects the density of the air in terms of its effect on aircraft performance.
Density altitude is primarily affected by changes in temperature, humidity, and altitude above sea level. As the temperature increases, the air becomes less dense, resulting in a higher density altitude. Conversely, lower temperatures increase air density, resulting in a lower density altitude.
Density Altitude in Drag Racing
In the realm of racing, a general rule is that with every 1,000 feet increase in Density Altitude (DA), a normally aspirated motorcycle tends to decelerate by nearly a tenth of a second in the quarter-mile (approximately eight hundredths, specifically 0.08). To illustrate, an elevation in DA is akin to racing on a track situated at a higher altitude. Conventionally, tracks at higher elevations are known for slower performance, paralleling the effect seen as DA climbs.
Our local racetrack, Kil-Kare Dragway in Xenia, OH, stands at a physical elevation of 803.6 feet above sea level. However, when factoring in temperature, barometric pressure, and several other variables (detailed in the links below), during the springtime, our Density Altitude (DA) typically ranges between 2000-2500 feet. As summer arrives with its hotter, more humid conditions, our DA often climbs to over 3500 feet. This elevation in DA means that we experience a slowdown of at least a tenth of a second compared to our performance in the spring. In essence, this loss in speed during the summer is an unavoidable challenge that we encounter.
By implementing additional tuning adjustments, such as slightly leaning the fuel mixture and potentially tweaking ignition timing to aid in combusting the increased water vapor found in the humid summer air, it's possible to mitigate the extent of these losses. However, it's crucial to recognize that the air, during these conditions, becomes less dense with reduced oxygen content, coupled with higher humidity, resulting in an air mixture less conducive to combustion. As a consequence, normally aspirated bikes inevitably contend with the reality of reduced speed under these circumstances.
Nitrous-fed bikes have the option of introducing more nitrous, while turbocharged bikes can increase their boost levels. However, this discussion specifically pertains to normally aspirated machines. It's worth noting that a tenth (.10) of a second lost in the quarter-mile translates to an approximate 8-12 rear-wheel horsepower (RWHP) reduction for a modern 180-200 HP sportbike. Naturally, when Density Altitude (Da) decreases, the bike's performance improves, employing the earlier formula of 1000 feet resulting in a reduction of about .08 to .10 seconds. These figures are general and can vary based on the bike's quarter-mile speed. Typically, less powerful bikes and those running standard pump gas experience more pronounced slowdowns compared to bikes using oxygenated race fuel.
Your results may vary, but you will slow down as the DA goes up. Eighth-mile racers will also slow down, just not as much.
Fun Fact: Gainesville Raceway in Florida and Valdosta's South Georgia Motorsports Park both sit at similar elevations, approximately around 200 feet (with specific elevations of 137.8 feet and 229.7 feet, respectively) above sea level. Typically, their Density Altitude (Da) hovers around 1000-1200 feet. Consequently, just by loading the bikes onto the trailer and embarking on the journey, we automatically gain about a tenth in performance. Another favored destination, Atco Raceway in Atco, New Jersey, boasts an actual track elevation of 93 feet above sea level.
For monitoring the DA at the race track, we use the Kestrel 4250 Weather Meter.