Putting dyno numbers up on a good air day is one thing. Posting up a corrected true 400 whp result is another. Kinda like dyno-jet numbers compared to a heart-breaker mustang dyno results. I became more informed when I swapped a turbo and my dyno results seemed to not change. I said "How can this be?" Then I saw one pull with the old turbo was in the winter. The other pull in the summer. After correcting...glory-be!!... there was a difference between turbos once corrected numbers were used.
If you want to figure it out yourself do this:
1.) Get your uncorrected results.You can ask your dyno facility if they are or are not corrected. Allot of dyno shops disconnect their weather stations because they can give allot of uncertain results and are headaches for tuners. Usually the day of the pull and exact time are on the print out. Find out from a close by weather station (closer the better) what the conditions were like during the pull. Get temperature, dew point temperature ( Used to figure out the humidity), elevation ( usually around 50 to 100 ft. above sea level in the bay rea....hopefully.) Remember temps and conditions will be different in the dyno-room but most likely will be close enough. I like to use this site as it offers weather results for just about anywhere.
http://www.wunderground.com/weatherstation/WXDailyHistory.asp?ID=KCAHAYWA12
Also some of the stations have history going back several years so you can correct your results from a previous day or someone elses too!!!
2.)
Once you have your weather data then plug it in to your SAE correction formula.
http://wahiduddin.net/calc/calc_hp_dp.htm
This will give you a dyno correction factor-at bottom of page. This correction factor is then multiplied to your dyno reading and you now have a corrected reading.
Remember, It's allot easier to achieve good numbers when the atmospherics are good. That's why SAE correction came about. It levels the playing field and takes the variables of temperature,barometric pressure, altitude and humidity out of the equation. This makes you a more informed person when numbers are just given to you. If you make good numbers during warmer months your set-up had to really work to get it. If your dyno readings happened during the colder winter months you probably had a "boost" from mother nature. Thats why our cars run so well in the winter months and cooler summer nights. Have fun and be informed.