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Old 08-09-2012, 09:31 PM   #1 (permalink)
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A/C? (instantaneous mpg drops between 4 and 6 mpg with AC on)

Hey everyone!
I just got back from an 800+km trip.
I have the PLX Kiwi monitoring my mpg.
On the last leg of my journey, I decided to play around with my A/C.
I noticed that on a consistent flat road, when I turned on my a/c, my instantaneous mpg went down between 4 and 6 mpg. (depending on the road)
I am totally a newb. At tech school we were taught that a/c didn't take away power or mpg. This blew my mind!
Has anyone had a similar experience?
Also, I challenge you to find the flattest road you can and set your cruise and test it. A/C on and off. See what your mpg does.
Thanks for being patient if this is a total newb question!

- Buzz

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Old 08-09-2012, 09:58 PM   #2 (permalink)
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i did this test sometimes ago and i get a minimum 6 mpg drop
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Old 08-09-2012, 09:59 PM   #3 (permalink)
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It varies from vehicle to vehicle. In my 02 Insight it was a 10 MPG hit, even when you set it on recirc, with low fan speed and just enough temp to be barely comfortable. On my Fiesta its much less, probably barely over 1 MPG. The manufacturers have done a lot with compressor technology as well as orfices instead of expansion valves to reduce the power requirements dramatically. The Fiesta is tolerable on low fan, recirculate, with the temp knob about 25% above max cool. I actually watched the instant MPG reading climb to 46.7 MPG running the AC the last time we rode in the Fiesta. In my Insight it would beat that MPG by about 7 MPG, not a huge difference.

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Old 08-10-2012, 10:14 AM   #4 (permalink)
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With the Cobalt, I've only recorded one trip with AC on, it was 42(why record bad trips) vs same trip without AC averages about 48, trip average speed is about 50 mph. Comes to about 12% loss.

At idle gph goes from 0.3 to 0.4 no air vs air. So just for comparison. Changing the average speed to 48 mph to make the math easier, without air I use a gallon per hour. If with air I use 1.1 gallons to go the same 48 miles I get 43.6 mpg. (9% loss)

AC setting pretty much same as Old Mech, I'm usually ok until temps in 90's, and I get to park in the drive thru at work so car isn't really hot when I leave.
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Old 08-10-2012, 10:24 AM   #5 (permalink)
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Quote:
Originally Posted by buzz_42 View Post
At tech school we were taught that a/c didn't take away power or mpg. This blew my mind!
Sorry, but your tech school is 100% wrong. Energy to do anything has to come from somewhere. That place on a car is your engine. To do anything requires power from it.

Frank Lee posted up some good info on the A/C penalty a while back.

http://ecomodder.com/forum/showthrea...ighlight=a%2Fc
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Old 08-10-2012, 02:29 PM   #6 (permalink)
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I wonder if the Tech Teacher really meant AC doesn't hurt mpg anymore than the aero penalty for having windows rolled down?
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Old 08-10-2012, 02:50 PM   #7 (permalink)
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I kill the compressor when I am idling and let the fan blow over the cool evaporator. My car also stays in my garage so it is always cool when we start off, except when we park somewhere in the sunshine. The Fiesta made one 50 mile trip this week, while the motorcycle went 215 miles at 90.84 MPG. I don't intentionally ride the bike in the rain, but I have become good at watching the weather radar and dodging the storms we have almost daily this time of year.

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Old 08-10-2012, 03:12 PM   #8 (permalink)
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On my 2002 Elantra I know it take a LOT of power from the engine, never checked the MPG but when I turn the A/C on you feel the car jerk "back" right away (it slows down quite a bit). This is a know problem on this generation (2001 to 2006) but it was fixed later (2007 and up).

My best FE with the A/C on was 6.4l/100km (almost 37 MPG) on a highway trip, so I'm guessing I could've easilly gotten above 40 MPG if I had turned the A/C off!
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Old 08-10-2012, 03:50 PM   #9 (permalink)
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On my matrix its a 4-6 MPG drop with AC on. windows down is no drop up to 60 MPH. since I am almost never over that speed when its hot out my ac stays off.
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Old 08-12-2012, 04:55 PM   #10 (permalink)
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Quote:
Originally Posted by buzz_42 View Post
Hey everyone!
I just got back from an 800+km trip.
I have the PLX Kiwi monitoring my mpg.
On the last leg of my journey, I decided to play around with my A/C.
I noticed that on a consistent flat road, when I turned on my a/c, my instantaneous mpg went down between 4 and 6 mpg. (depending on the road)
I am totally a newb. At tech school we were taught that a/c didn't take away power or mpg. This blew my mind!
Has anyone had a similar experience?
Also, I challenge you to find the flattest road you can and set your cruise and test it. A/C on and off. See what your mpg does.
Thanks for being patient if this is a total newb question!

- Buzz
I tried your challenge last Summer on a very slightly downhill section of road with my Mustang. Cruise control was set at 35 MPH. Scangauge indicated 80 MPG with A/C off. It dropped to 60 MPG when I turned on the A/C. 20 MPG/25% difference! Under those low load circumstances. I'll have to check the difference in GPH when I get a chance.

Living in the mountainous part of Arizona, I rarely use A/C except on road trips. When I need to use A/C, I set it on recirculating and "pulse & glide" the compressor.

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