I guess it needs putting on here now, so i have Copied and pasted it from elsewhere as i certainly don't know it...
This standard assumes intake air temp is the same as ambient.

The SAE J1349 (Jun 90) power correction formula is:
Metric units millibar / deg C CF = 1.180 [ ( 990/Pd) x (( Tc + 273)/(298 )) ^ 0.5 ] - 0.18
in hg / deg F units CF = 1.180 [ ( 29.23/Pd) x (( Tc + 460)/(505)) ^ 0.5 ] - 0.18

Pd = dry air pressure ( ie absolute air pressure minus the contribution of the partial pressure of the water vapor in the air)
Tc = Ambient air temp ( ie temp outside the car, not the intake air temp)

The formula is supposed to give you the hp the car would make if it was at an absolute air pressure of 990 mb (29.92 in hg) 0% humidity and 25 deg Centigrade (77 deg F) outside air temp.