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Originally Posted by BYprodriver
2.5 engine is factory rated @ 201 HP. I would think Boxster AC compressor load is more like 5HP.
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Thanks for the correction, but that makes the draw even more significant.
I seriously doubt the AC compressor load is only 5 HP.
The concept of compressing freon (or any other gas) is well understood, there is no 'magic' to it. The power required is directly related to the amount of heat (energy) transferred in the cooling process. This is actually much less than the total parasitic draw - somewhere in the neighborhood of 1-4 HP (dependent upon the compressor's capacity).
The biggest factor in the total draw are the inefficiencies involved in transferring the power from the crankshaft to the compressor - a belt and pulley system + the magnetic clutch on the pulley adds additional load by the alternator (small, but measureable). Lots of energy is lost in the transfer as the belt flexes and throws off heat. This is where most of the 10-15HP is lost.
V belt and pulleys systems are pretty inefficient at transferring power, significant power is lost in the transfer. Their advantage is simplicity, cost and adaptability.
The Corvette C6.R does claim a draw of only 1.5 HP using a Sanden compressor. But, this uses a totally different direct-drive system, drawing power from the
driveshaft 'directly' - (also mounted on the rear of the motor). It is more costly, and less reliable in terms of length of service life (as many racing components are) which is why most manufacturers stick with the tried and true pulley/belt system - the non 'R' C6 uses belt/pulley.
Most manufacturers use virtually the same rotary compressor from Sanden - the world leader in automotive AC components. Even those labled as Frigidaire, FrigiMax or Bosch are either re-labled, or manufactured under license from Sanden.
The cases may differ depending upon application, but the rest is virtually identical. In the case of the Boxster, the compressor is a Sanden model #1152. So while I have not personally measured the draw in a lab run, I'm sticking with the industry accepted 10-15 HP average.
Cheers!