Originally Posted by mixja
Go to http://www.csgnetwork.com/relhumhpcalc.html
Plug in standard condition values (i.e. the values that give "Calculated Relative Horsepower To Rated" value of 100%):
Air Temperature - 77F
Ambient Pressure - 29.235 inHg
Relative Humidity - 0%
Elevation - 0 feet
Now keeping all other parameters constant, change the elevation. You'll see that the "Calculated Relative Horsepower To Rated" drops to 82.5% - i.e. indicating a loss of 17.5% due to elevation.
this works well, but it over compensates slightly.
If I use the constants and only put my altitude of 5300 ft, it gives me a calculated air pressure of 24,03 in Hg. But at my location now the actual air pressure is 25 in Hg. It also shows a 22% loss of 78% of sea level power, which is over by 3-4%
You need to get the corrected to sea level pressure from your local air port or weather station for the day and plug that in. You also must input the humidity. When you do this it will make the percentage loss slightly less, and is more accurate to real wold conditions
The easiest and most accurate method is to divide the sea level air pressure by the actual air pressure at your altitude to get your % ratio
otherwise your correction formula will cause an inflated figure for sea level.
In actual environment, I have a 6psi pulley on my charger but at 5300 it read 4,78psi, roughly 82%