nope, I think you all got it wrong la. I think I now what exactly what you mean. Like this. In old old cars, eg: mitsubishi gallant supertouring 1.6l. or some nissan sunny 160y. You can turn the aircond to max, but then it will make your engine freeze literally. For some weird reason, you can make the aircond cold and yet freeze the engine by adjusting the thermostat. This is normal in old cars. Some come with two thermostats, 1 for AC, 1 for engine temp, yes you can change engine temp but then if you set AC controller to coldest but engine temp medium/ low, you don't get the AC to the coldest. Well its just this sort of thing. If your car is prehistoric where you need to warmup more than 5mins and the radiator does not have that temp stabilizer, then it will have an effect. As for loosing HP over AC, well it used to be alot with old cars, but new cars, its very little and go ahead and turn it to the coldest. In new cars I don't mean a new Kancil as smaller cars are even easier to notice the power drop. I had an old Auto kancil which for some stupid reason the AC would become hot whenever you stall the car in "D" lets say in a traffic jam. If you put to N then its ok But to me its better to have the coldest temperature rather than increasing the fan speed as that requires more power. So overall its not a myth, but then you can hardly notice it. Its not going to transalate into better performance, and its up to you if you want to save fuel open the window or be comfortable. For me the rule is The car IS my slave, not I'm the slave of the car.

Correct me wherever I am wrong.