So I was driving around yesterday with the AC on, and was reminded of dumb ricers claiming they lost the race due to their AC being on. Maybe I'm just a fool, but i don't notice any significant loss in power when my AC is on.

So whats the deal? How much HP does the AC compressor really rob? Has anyone dun dyno tests on this?

 

Depends on the car/weather/conditions/etc.

More often than not it's probably only noticable in the low end, and probably only on a butt-dyno. It steals enough to lose by a carlength or two in a ¼mile, but in day-to-day driving, even if you've got a leadfoot, it's an insignificant amount.

 

With a brand new AC system, after 2000 we'll say, its not going to use THAT much power to turn. Plus you have to remember that the compresser(the part that spins) only turns on every now and then. It gets your car cold then it turns off, the car warms up a little bit, it turns back on. So the compresser isn't using any hp unless its on and even then its only about 5hp.

So if someone starts bit**ing about losing because of the AC being on, they would have lost anyway.

 

Also depends on the car.

AC on and off makes a HUGE difference in day to day driving in my 1990 civic. I mean, the car only puts like 45hp to the ground anyway, so that 5 or whatever the A/C steals is a big difference.

 

i forgot to turn off my ac once at the track and ran a 15.3 my slowest run ever. i usualy run high 14's

 

In the past decade most car companies have put in switches that turn the A/C off at wide-open-throttle so you don't lose power when it is obvious that you need it.

That was happening in the 1990's and is pretty much standard these days for most cars.

There's no real way to calculate how much power you lose unless you throw your car on a dyno and do a part-throttle pull with the A/C on and then off. Even then it'd be hard to replicate two runs at the same throttle percentage.

 

The compressor takes about 20 HP to run, but it shuts off at throttle positions above 70% or so, depending on actual load and RPM.
The alternator pulls another 5 - 15 under heavy load, which can occur when the fans kick on full due to the running compressor.

 

Oh, I have a question related to this matter. How much more gasoline is consumed when the AC is on?

 

As MM stated, parasitic loss is quite significant without a disengaged compressor clutch. And as Tessai stated, the effect is emphasized with a lower output motor.

 

heh - this could be why so many people say the underdrive pulley makes a difference for to the mid range - under partial throttle .

 

Well, lets say I get 17mpg for the tank with no AC (I never drive with my AC on anyways) What would be the difference in MPG if I were to always use the AC?

 

This is probably the simplest approximation formula:
20 (hp needed to run AC compressor) / 232 (crank hp) = 8.6% more gas consumption.

This formula is an oversimplification, so take it with a grain of salt.

 

How much power does the AC take? Well, let's just say my 89 Escort had optional AC:

Option 1 - run the AC
Option 2 - drive the car

That thing was dangerously slow with the AC running (and it was the "GT" model to boot)!

 

Well, calculation smake sense since 20 is 8.6% of 232... so I guess that'll work

So instead of 17mpg with no AC, it'll become 15.5mpg with AC... it is an approximation though so don't count on it.

 

its not even an approximation because the compressor is only on for less than 50% of the time anyway .

 

Haha, I think many people are familiar with this option. I know I had it on my high performance 100 hp Saturn. If I had the AC on and needed some extra power I would exclaim "hit the turbo boost, KITT" and press the AC button to turn it off. Haha, good times.

 

AFAIK, car AC compressors are not thermostat controlled like room ACs. I think the only time it's off is when the compressor clutch is disengaged (a la WOT) or it's really off.

Therefore, if someone has the AC on and is put-putting around town, I'm pretty sure the compressor is on all the time.

 

Good times !

 

my first mazda pickup truck didn't even have an AC, and my friend's 240sx s13 dont have it either.......sad sad
yea, now adays it seems trivial for me to even turn on the AC since its nice and cool anyways

 

Unless your AC is set to MAX AC at all times, the compressor disengages. I could hear my AC in my old accord kicking on and off as i drove around. In my 8 I can feel it turn off as the air suddenly goes from cold to lukewarm. The first day i had it, i thought I had a bum AC until i turned the thermostat all the way to cold, and promptly froze the hairs on my arms. If i turn it back towards the center I can feel it turn on and off. You can hear the engine noise change when the compressor kicks on at idle too.

 

Who cares about partial-throttle power? Ridiculous. The mechanical advantage of the longer lever-arm generated by the UD pulley cannot be detected by your butt at part throttle. You may notice that additional pedal travel is needed, but your power and acceleration forces are imperceptibly changed.

The A/C compressor is never "continuously" on. It is regulated by a temp sensor on the condenser and a pressure sensor. It usually runs at a 50% - 70% duty cycle, depending on measured performance.
The gas consumption formula above is completely spurious.

 

And I stand corrected. Thanks all.

 

MM that avatar me no like

 

This has already been answered a few times, but I think I can make it ultra clear:

This is a trick question.

The answer is zero.

When your car is wide-open-throtle (WOT), the AC is switched off automatically, so you lose nothing. When you're crawling around town, the AC is on, but it's using HP you don't need and aren't really using anyway... so no loss that you'd notice.



On a dyno, you will mash the throttle, so the AC would switch off - thus dyno results would show no change from AC on or off.

 

DAVID?