Normal A/C vs Max A/C

Discussion in 'Fuel Economy' started by Unleaded, May 13, 2013.

  1. Unleaded

    Unleaded Well-Known Member

    I have been wondering for a while if there are efficiency differences between using Normal or Max A/C (a/c using outside air vs a/c using recirulated air). Now, I'm not asking about which should be used under certain conditions but rather which causes the most fuel consumption.

    Hope you guys understand what I'm getting at here.

    Here is my theory. When selecting normal a/c the recirculation door opens to let outside air into the cabin. This would seem to increase the drag on the car, making it consume more fuel. Now when Max a/c is selected, the recirculation door closes blocking ouside air from coming into the cabin, thus reducing the drag. In this case it would seem that Max a/c would be more fuel efficient than normal a/c.

    Does this sound right to anyone? Yes, I do realize that no a/c is more fuel efficient but I am talking about when the a/c is in use.

    So in technical terms, is my assumption correct?
  2. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    Most cars draw in air from the hood-windshield corner which is fairly stagnant air. I doubt you'd be able to measure that difference even in a windtunnel.

    What you could measure is the effect of the systems intake air temperature. When it's 100F outside and 120+ in the car, outside air will clearly be the best. Once you've cooled it below 100, inside air will be better. I usually override any "Automatic" air system that defaults to Max AC when it's really hot, until things cool down a bit.
  3. Prozac

    Prozac Well-Known Member

    ^ This.

    Once the vehicle has cooled down some, max air is more benficial because you will require less compressor time cooling down recirculated cool air than hot outside air.
  4. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    Which is exactly backwards from most Auto climate controls. They go MAX until it cools down, then switch to normal.
  5. rfruth

    rfruth Well-Known Member

    Here in Houston (read humid) more compressor time is good (no max A/C if possible) ((with the A/C on you get more dry air, increase the fan speed and you feel cool) ((with the car set to 78 degrees with higher fan speed you feel about the same as at 75 with a lower fan setting) but max runs everything at full tilt boogie, you have no control so max A/C is not terribly efficient (but all cars are different)
  6. 2011accent

    2011accent Well-Known Member

    Drag from in-taking air from the engine bay probably doesn't exist in any car. It might even be beneficial to take air from the engine bay into the A/C system because it would probably increase good aerodynamics by lowering the engine bay air pressure.
  7. Unleaded

    Unleaded Well-Known Member

    Didn't think of it that way. It does make a lot of sense, thanks! And I agree about auto climate control systems. Mine will start off with Max, stay there for a while, then switch to normal.
  8. xcel

    xcel PZEV, there's nothing like it :) Staff Member

    Hi Unleaded:

    There would be no measurable drag effect between Recirc and outside air.

    Using A/C smartly however is far more important.

    Here are few suggestions that may help?

    To begin, try not to ramp up your A/C when you leave in a steaming hot closed up vehicle. Park in the shade if possible, use a sun shade or leave your windows cracked during the day. Before turning the key, open up all the windows and allow the interior to come close external ambient. When it gets close, get in and take off allowing the natural speed generated breeze to air the car out further. Once to the external and hopefully you are closer to your higher speed section of the commute, go immediately to recirc but do not use MAX. If your A/C is smart, it will cycle the A/C compressor in a reasonably efficient manner. Keep the temp as high as possible for lower humidity and lower temps but just so. If it is not a late model unit, you can cycle the A/C yourself. Any accel, shut it down, Any fuel cut inducing drag, ramp it up to maximum. Cycle as necessary during your DWL and stop light to stop light daily grind in this manner.

    If you can do without, driver side front and passenger side rear cracked open for cross flow and begin ramping up fan speeds with only your vents open. Once at max, drop the the windows some more. And when/if it is to hot, button up and being the A/C cycle routine in most non-electric or smart compressor equipped vehicles.

    I hope that helps?

    Last edited: May 14, 2013
  9. Unleaded

    Unleaded Well-Known Member

    Hi Wayne,

    Originally I was simply curious to know weather there would be any measurable drag between recirculated and outside air. Having said that, I appreaciate the great info on how to use the a/c smartly.

    I am glad to say that I do practice many of the tips you have provided. Such as a sunshade (I ALWAYS use one, some people do not realize what kind of difference it makes). My windows are tinted at 35% (legal in AZ) which also makes a big difference in temps. Others like, cross ventalation from the windows and using outside air without a/c and engaging the a/c when in DFCO I learned from this great site. :)

    My a/c system is smart enough to cycle the compressor once the cabin is at my desired temp. Hows that for 17 year old technology :p

    One thing I wanted to point out is that the setting MAX a/c in my car is the same as recirculate when it comes to a/c. I do not have a seperate recirculate button on my climate control. Mine consists of this, MAX a/c, Normal a/c, VENT, FLOOR, Defrost/Floor, DEFROST and AUTOMATIC.

    I know in other vehicles like BMW, that the MAX setting simply sets the temp to 60, turns the fan to high, and closes the recirculation door.

    Anyway, I'm rambling. It's good to be reminded on how to use the climate control wisely, and I do appreciate all the helpful info that is given on here.
  10. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    I do it pretty much like Wayne says , but I open the passenger front window an inch and do the same on the driver side rear window. I find it makes for a bit less wind noise at 45 MPH.

    So....... with less wind noise , I can really enjoy the famous road noise a Civic at 45 psi makes !

    Also , the first thing I do when I get in the car that has been baking all day ; fan on high , floor and dash vents. I like to think this will cool the ducts a bit before I put the A/C on. That's IF I put the A/C on , I very rarely do it unless I have a passenger.
  11. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    I used to do both front and rear driver's side windows in my old Civic. Why? Hand crank windows. Now with power windows, I'll do all 4 about an inch down, or sometimes close my window for lower noise.

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