once AC is on, fan settings don't matter to gas mileage?

skim · 05-13-2009, 11:39 AM

gas mileage is not what i'm concerned about, but just curious if what my friend told me is true
once the AC is on, it doesn't matter if the fan setting is at the lowest or full blast in terms of it using the engine power?
what my friend told me makes some sense, but i just wanted to make sure
this guy doesn't turn on AC in his car unless it's sweating hot, but he likes turning it on once in my car.. cheap ass friend..

Georgeair · 05-13-2009, 01:16 PM

Old school, only minimal difference from fan speed due to very slight variance in alternator load, but when the a/c compressor was engaged it was sucking energy regardless of fan setting.

For this car I'm interested in seeing the answer. I almost think the compressor clutch disengages at something approaching WOT based on the temp and "smell" of the a/c. That would be pretty cool.

Someone who actually knows something will be along shortly I'm sure!

valeram · 05-13-2009, 01:32 PM

First law of thermodynamics, energy in = energy out. The higher the fan speed, the more volume of air to transfer the heat (or cold) for that matter. Therefore the compressor will run more frequently when you blast your fan away until the interior temp has reached the thermostat settings. But using low fan speed or high fan speed will only dictate the time to cool down the interior. The total energy to cool down the interior is the same for both approach.

05-13-2009, 11:39 AM	#1
skim Captain 142 Rep 661 Posts Drives: 07 Z4MC Join Date: Nov 2007 Location: SoCal iTrader: (0)	once AC is on, fan settings don't matter to gas mileage? gas mileage is not what i'm concerned about, but just curious if what my friend told me is true once the AC is on, it doesn't matter if the fan setting is at the lowest or full blast in terms of it using the engine power? what my friend told me makes some sense, but i just wanted to make sure this guy doesn't turn on AC in his car unless it's sweating hot, but he likes turning it on once in my car.. cheap ass friend.. __________________ Future mod consideration: exhaust, pulley otherwise, very happy with how the m3 was born
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05-13-2009, 01:16 PM	#2
Georgeair Captain 80 Rep 803 Posts Drives: '08 Melbourne E92 M3 Join Date: Apr 2009 Location: Madison, MS iTrader: (2)	Old school, only minimal difference from fan speed due to very slight variance in alternator load, but when the a/c compressor was engaged it was sucking energy regardless of fan setting. For this car I'm interested in seeing the answer. I almost think the compressor clutch disengages at something approaching WOT based on the temp and "smell" of the a/c. That would be pretty cool. Someone who actually knows something will be along shortly I'm sure!
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05-13-2009, 01:32 PM	#3
valeram Major 37 Rep 1,413 Posts Drives: 992 TTS, 991.2 GT3 T, F80 M3 C Join Date: Oct 2006 Location: Dublin, OH iTrader: (0)	First law of thermodynamics, energy in = energy out. The higher the fan speed, the more volume of air to transfer the heat (or cold) for that matter. Therefore the compressor will run more frequently when you blast your fan away until the interior temp has reached the thermostat settings. But using low fan speed or high fan speed will only dictate the time to cool down the interior. The total energy to cool down the interior is the same for both approach.
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