I know in the old days A/C took quite a few pony's when it was running. Then at WOT the compressor would turn off in later years to prevent this on some cars. I can't notice a difference really with this car with the A/C on or off. Has anyone dyno'd with it on and off or know the answer?

 

Dyno wouldn't show anything because the compressor does kick off at WOT.

 

I ran a 12.75 with the AC on, so it can't be taking that much power...

 

The compressor does kick off under WOT...in the mean time, you're adding a lot of heat to the engine bay and taxing the cooling system a bit more...and if the motor is hotter, you'll make less power...
...presumably the ECU will dial back the timing, etc.

 

It would need to be really really hot out for the AC to put enough extra heat in to force the ECU to cut back timing. Like, 130° hot.

 

Lol . Right on!

 

Umm...Engine bay temp has little to do with IAT's (used to determine timing) when the airbox is fed by outside air... The only power loss with AC on is the power it takes to spin the compressor (minimal on a 400+ hp motor)..

 

Probably true but any heat added to the cooling system will mean a hotter engine with hotter heads and a less dense charge in the combustion chamber.

It would definitely make a knock more likely in a car that was border line (like being run hard or one that was operating with a custom tune)...

Basically, it probably makes little difference but if you're flogging your car it's not a bad idea to just roll down the windows and give your motor a break.

 

Basically, if your engine is so close to knocking that the A/C causes it to go into knock, you got bigger problems than your engine knocking.