Page 415 of the Bosch Automotive Handbook, 5th edition
http://tinyurl.com/yplr3dhas the two equations you need
to see how horsepower and torque change
when either temperature or pressure change.
For temperature the offical equation
used by Americans (SAE) Europeans (ISO) and Japanese (JIN)
is:
{ Temperature Before + 460/Temperature After +460} raised to exponent 0.6
you need a scientific calculator to do this
but there is one built into every Windows computer
under Start...Accessories
As an example,
where the intake air temperature
drops to 80 degrees
when before the air coming in had been 100 degrees.
{100 +460 divided by 80 + 460} raised to exponent 0.6
={560/540} raised to exponent 0.6
= {1.037} raised to exponent 0.6
= 1.022
You mulitply this number times your horsepower or torque
so if your engine.
If your engine makes 230 hp at peak
1.022 times 230 = 235 horsepower with the 20 degree lower air temperature.
You may have heard the 'Rule of Thumb' that each 10 degree F reduction in air
temperature improves horsepower by 'about' 1% and the equation above is where
that comes from, but is more accurate
For corrections when the pressure changes
the following equation is used:
{absolute pressure after/absolute pressure before} raised to exponent 1.2
As an example
say that the weather is changing where you live
and one day the weatherman says the pressure is
29.5 inches of Mercury as a stormy "Low" passes over
then the next day a clear sky "High Pressure Area" passes over
and the pressure rises to 30 inches of Mercury
{30/29.5} raised to exponent 1.2
{1.0169} raised to exponent 1.2
= 1.0204
"Standard" SAE conditions are 29.6 inches Hg pressure and
77 degrees F (keep in mind this is actually 460 + 77 = 537 Rankine degrees)
