A/C Usage and Impact on FE

[bowtieguy]

now that summer is here, and 3 digit temps(heat index) with it, it is suprising how much impact my car's a/c has on FE.

let me first break down a/c on vs a/c off on the scangauge. this is at standing idle only since variables are eliminated...

IGN on: 14, off: 12
LOD on: 20, off: 15
GPH on: .4, off: .2

i cannot comprehend that gal/hr is doubled at idle w/ the air on. obviously w/ throttle and driving situations the % is no where near that but...

according to SGII, there can be as much as 4mpg difference in the same trip, EXACT conditions. i ran the air on the way to work the past 2 days yielding ~36 and 37 mpg. typically i'll get ~40-42 air off.

i am accounting for a bit a variables, so i dare not claim a 6mpg difference. remember, everything has been nearly exact and the same--route, weather, breeze(none), etc. typically, this time of year, my only variables are traffic lights. i time them very well, and usually make them all but 1 or 2.

so, at 2mpg, the loss is ~5%, and 4mpg it is ~10%. maybe closer to 4 and 9% respectively. still, that is significant. am i figuring this correctly?

[theholycow]

Originally Posted by bowtieguy

i cannot comprehend that gal/hr is doubled at idle w/ the air on. obviously w/ throttle and driving situations the % is no where near that but...

At idle it only needs to make enough power to overcome engine friction, which isn't much at idle RPM and closed throttle. I can see A/C being as much as that.

[dkjones96]

MPG impact is directly related to engine power.

Larger engines don't get affected as much by ac. I would venture to say that the MPG impact is about as noticable as the performance hit. That's what my experience has been anyways.

[GasSavers_maximilian]

Are there any cars that kick off the A/C when they need more power? I used to do it manually when climbing the steep hills in Providence in my 87 Sentra.

[bowtieguy]

Originally Posted by dkjones96

MPG impact is directly related to engine power.

Larger engines don't get affected as much by ac. I would venture to say that the MPG impact is about as noticable as the performance hit. That's what my experience has been anyways.

i think you're spot on there. i don't remeber my V6 olds losing that much. besides if the % was the same, obviously the loss in larger engines and lesser FE would not be as noticable.

[theholycow]

Originally Posted by maximilian

Are there any cars that kick off the A/C when they need more power? I used to do it manually when climbing the steep hills in Providence in my 87 Sentra.

AFAIK, most modern cars turn off A/C at WOT.

[Jay2TheRescue]

I think larger engines don't suffer as much... I tested this a year or 2 ago. Drove my truck on its daily drive, with the A/C on max for a week, then drove it with the a/c off and windows up for a week and noticed no difference in my mileage.

-Jay

[Jay2TheRescue]

Chryslers turned the a/c off under heavy loads in the 80's. I hated it. Hit a good sized hill and the a/c would shut off. I'd hate to have to drive it up a mountain in the summer...

[GasSavers_maximilian]

I hadn't considered that. Pretty ironic given where I live!

[GasSavers_GasUser]

Yea, but you are in Florida too.

I have actually only needed to use mine a couple of times this year and mine makes no difference on the highway where 95% of my driving is.