Quote:
|
Originally Posted by insite
topless -
your calculations aren't right. you're just assuming a 3% loss for each 1000'. the pressure drop is NOT linear.
the SAE correction equation is:
cf = 1.176 * ((990/Pd)*(Tk/298)^.5)-.176
one over this cf yields the percent power available at that pressure. Pd is the pressure of dry air at altitude, and Tk is the temperature in degrees kelvin.
i'll use 15 degrees C since that's what my altitude pressure tables are calculated for. at 5,500', the pressure of dry air is about 820mb.
if i plug all this into the equation, i get a cf of 1.22 or an HP yield of 82%.
that means that if your assumption of 260RWHP at sea level were used, the HP available at 5,500' asl is 213.
FYI, Raby says he usually sees about 240RWHP from the 3.4L. this would yield 196RWHP at that altitude.
|
Of course you are right that the alt./air pressure relationship is not linear. Your pressure figures are good. I have seen several 996 3.4L dynoed in San Diego between 255-260 rwhp so I think those are good numbers for a healthy motor near sea level. Jake's shop is at 1500' or so, which might explain his numbers. Any way you slice it the OP should have some hidden juice somewhere in that motor even at 5500'.