Originally Posted by ChrisSlicks

I've used my A/C on track several times to keep the windshield defogged in misty conditions. There was no difference in car feel, acceleration or lap times. An auto A/C compressor isn't quite as efficient as a home unit but the comparison isn't that far off. Power is power as was stated, the difference is in a direct drive compressor you will feel the drag more than the power loss (flywheel effect) and in the low rpm range where the car isn't making much power. At most it pulls 5hp when active and it does seem to disengage under WOT. You'll notice 5hp in a 100hp car, but not so much in a 300hp car.