What is Density Altitude 'Da' and how does it affect performance?
Here's the short answer: Density Altitude(Da) are the conditions relevant to International Standard Atmosphere(ISA), sea level (0 ft. (0km)) altitude, 59 degrees F (+15c), and pressure altitude of 1013.25 mbar (or barometer of 29.92 inHg). See the links below.
As a rule of thumb from a racing standpoint: For every 1000 ft. the Da goes up, a normally aspirated bike will typically slow down almost a tenth (actually right around eight hundredths or so = .08) in the 1/4 mile. So, for the sake of this discussion, as the Da goes up, it's similar to traveling to a track that is at a higher elevation, and everyone knows that high altitude tracks are slower - right?
Our local track for Brock's Performance, Kil-Kare Dragway in Xenia, OH, has an actual elevation of 803.6 ft. above sea level. When you consider the temperature and barometric pressure (+ various other factors - see links below,) in the springtime, we typically register a Da around 2000-2500 ft. In the summer we reach over 3500 ft. Da (hot/humid, etc...) which means that we slow down at least a tenth in the summer vs. spring. Putting it simply, there is nothing that can be done about this loss.
Some additional tuning, a slightly leaner fuel mixture, and perhaps a bit more ignition timing to help burn the additional water vapor in the humid summer air, can sometimes help the losses from being even WORSE. But the air is simply less dense - or less filled with oxygen + it's filled with more water (that doesn't burn), from the humidity, so normally aspirated bikes must just live with going slower.
Nitrous bikes can add more nitrous, turbo bikes can add more boost - this conversation is related to normally aspirated machines. FYI: a tenth (.10) of a second loss in E.T. in the quarter-mile equates to about an 8-12 RWHP reduction in power for a modern 180-200 HP sportbike. Of course, if the Da goes down, the bike will get quicker, using the 1000' = (.08 -.10) or so formula above. These numbers are general, and they will vary depending upon how quick the bike is in the 1/4 mile. Less powerful bikes tend to slow down more, etc. so do bikes that run standard pump gas vs. oxygenated race fuel.
Your results may vary, but you will slow down as the Da goes up. 1/8 mile racers slow down also, just not as much.
Fun Fact: Both Gainesville Raceway in FL and Valdosta's South Georgia Motorsports Park elevations are around 200 ft (137.8 ft. and 229.7 ft. respectively) above sea level, with their typical Da registering around 1000-1200 ft. so we pick up about a tenth, just by putting the bikes in the trailer and making the drive. Atco Raceway in Atco, New Jersey is also a favorite destination with an actual track elevation of 93 ft. above sea level. Check the link below to view the Kestrel meter we use to monitor Da at the race track.