By on October 7, 2009


TTAC’s David Holzman writes:

One of my good friends asked me a question that I thought you and TTAC’s B&B could answer better than I could. “Hey David, We just returned from the beach yesterday. I tried an experiment with the air conditioner in the car on this trip. If we set the A/C at the coldest setting the car usually gets too cold, we usually adjust the temperature higher to keep us comfortable. However, I think the way the A/C works is that turning the temperature up simply allows some hot water from the radiator to heat up the air conditioned air. So we simply turned the A/C on and off as needed. Admittedly it was not quite as comfy since it would get a bit cold and a bit warm. I have two questions. First: does turning the A/C on and off at ten or fifteen minute intervals damage the A/C (such as putting greater wear on the electric clutch)? Second: does manually cycling the A/C save gas? We measured gas mileage after such a trip and got 30mpg, which was the best mileage we’ve ever gotten on the Camry. But we just had the oil changed and the air filter, which was pretty clogged, changed too. So I don’t know if it was the ac cycling that caused the improvement in mileage or the change of the air filter.”

Sajeev replies:

I’ll answer these questions with my personal experiences: living in Houston and driving older cars with dying A/C compressors lends itself to hearing (old compressors are noisy) and seeing (Mom’s turkey thermometer shoved in the vent registers) your way to the answer.

I seriously doubt the radiator is involved in cold air dilution.  From what (little) I know about HVAC, the compressor cycles less frequently when you turn the hot/cold knob on the dashboard closer to the center.  With less cycling, there is less compressed refrigerant thrown into the system. Best and Brightest: is that fair?

To answer your first question, turning the A/C on and off at ten or fifteen minute intervals will not damage the A/C, but there’s a good chance the A/C works more efficiently when you tweak the blend knob instead.  This is especially true in convertibles or any car with less then Lexus-like door sealing.

To your second question, manually cycling the A/C does save gas. Relative to leaving the A/C at full power, that is. I think there’s more benefit to leaving the A/C on MAX (a.k.a. Recirculation mode) and lowering the compressor’s cycle frequency via the hot/cold blend knob.

Bonus! A Piston Slap Nugget of Wisdom:
Here’s a tip for drag racers: if you have electric fans, check if they come on when running the windshield defogger and turn it to maximum heat. If so, you get the perfect cool-down technique for the staging lanes: the compressor comes on (for quick windscreen defogging in normal situations) which kicks on the fans, by default.
But the compressor rarely compresses, since maximum heat is needed. And the fans subsequently cool down the primary radiator, while the dashboard’s heater core becomes a secondary radiator. Run the fan speed at full tilt, to maximize the heater core’s potential. I’ve seen this work wonders during hot lapping (i.e. legal racing on a drag strip) and I recommend it.

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21 Comments on “Piston Slap: Keeping Cool, Saving Fuel...”

  • avatar

    On most vehicles, the temperature knob operates a blend door between the heater core and A/C evaporator, either directly in the case of manual A/C, or by a computer/servo mechanism in the case of automatic climate control.

    The A/C evaporator is only capable of outputting temperature at a single temperature (very cold). Unless you set the temperature to full cold, what you are receiving is a blend of cold air from the evaporator and hot air from the heater core.

    Manually cycling the A/C compressor may save a little gas (in fact, many Toyotas have an ECON A/C setting that does just this), but really, this seems somewhat labor intensive and distracting, when you should really be focusing on driving the car.

  • avatar

    First: does turning the A/C on and off at ten or fifteen minute intervals damage the A/C (such as putting greater wear on the electric clutch)?
    No, or only in the sense of “the more you use anything, the more wear it gets.” If you were to just leave the system on, it would occasionally disconnect the compressor anyway without your intervention, and chances are good you’d never notice it.

    Second: does manually cycling the A/C save gas?…

    Sho’ nuff. A/C use reduces MPG by roughly 3-5 percent, depending on all the usual variables. So any time you’re not using it, it’s to your favor.
    Manually switching it on and off as needed is a good strategy in many cases, particularly in a car with a relatively small engine.

    Some general notes: With the exception of the most-sophisticated computer controlled systems, generally found on upscale cars, an A/C system is either on or off. The compressor is engaged, or it isn’t.
    Some hi-end systems can actually slow the compressor or let it pump at reduced volume, but they are NOT the norm (yet).
    If you add heat while the A/C is on, you are simply letting warmed air from the heater system blend in with the cold air from the evaporator. In spite of what your wife tells you, you are not “making the A/C warmer.”

  • avatar
    Mirko Reinhardt

    However, I think the way the A/C works is that turning the temperature up simply allows some hot water from the radiator to heat up the air conditioned air.

    That’s true for some systems, the one in Opels for example. Mercedes and others use a variable compressor to only cool as much as necessary.

    Autobild did a very interesting test on A/C efficiency in 2006. They ran some cars through 12 tests, measuring fuel economy with or without A/C. It’s shocking how different the results for the different cars were… At 62 mph, one has a 3.6% fuel consumption disadvantage with the A/C on, the other 12%. At 31 mph, they observed up to 25.6% additional fuel consumption.

    You can read the article here:

    The test results are hidden in the image gallery.

  • avatar

    I seriously doubt the radiator is involved in cold air dilution

    Well, not so much the radiator, but as others have noted, the heater core is involved in temp regulation. That’s how your get warmer, but still dehumidified air into the car!

  • avatar

    I wonder if this is one reason the Prius and other hybrids do well with efficiency?

    They have electric-powered air conditioning compressors which I presume can run at whatever speed is sufficient for demand.

    Likewise electric power steering. Running a hydraulic power steering pump continually when only needing “power steering assist” about 1% of the time when driving, seems like a huge waste.

    Even some non-hybrid cars now have electric power steering and benefit from better efficiency.

  • avatar

    For any of you with a GTI (or a Golf or Jetta I assume) if you switch the display units from MPG to L/100KM when you come to a complete stop the milage display changes from L/100Km to L/Hour. So, at a stop my GTI burns 1.0L/h, turn on the AC and it starts burning 1.5L/h.


  • avatar

    “For any of you with a GTI (or a Golf or Jetta I assume) if you switch the display units from MPG to L/100KM when you come to a complete stop the milage display changes from L/100Km to L/Hour. So, at a stop my GTI burns 1.0L/h, turn on the AC and it starts burning 1.5L/h.”

    Interesting. Which means the A/C burns at most 0.5 L/h, since when the car is moving it gets far cooler with all that air flowing by, esp. the faster it goes. Even 0.5 Lt/h is a pittance, since in one hour you typically do 70 miles or so, and probably get 6-7 lts to do them. And the bigger and heavier the car, the less significant that 0.5 lt max is as a %.

    A/C in the car is far more necessary than A/C at home. Esp. if you have humid weather. I almost never have to use A/C in my home, but I use it in my car all the time (climate control). It is just not worth wasting time to save a pittance. How much is your comfort worth?

  • avatar
    Martin Albright

    I used to avoid cars with air conditioning because of the perceived MPG loss. I actually lived in North Carolina for 6 years with a car with no AC, a car I drove back and forth across the country at least 5 times, often in the summer months (the worst was a trip across Texas in late July, temps over 100 degrees, I was wearing shorts and stripped to the waist with all 4 windows rolled down and driving 70 down I-20. No point in turning on the radio because I couldn’t have heard it anyway. Miserable!)

    Then I realized there was nothing glamorous about suffering and all my vehicles since 1999 have had AC, which I am not shy about using. I use the same technique described (turning AC on and off as needed) and have not had any problems or issues in 93,000 miles on a Ford Ranger; 80,000 miles on a Subaru Outback wagon; and 30,000 miles on a Toyota Tacoma.

    I also did not see a significant difference in MPG whether AC was used or not. I can’t help but wonder if the “AC costs you MPG” assumption is based on older vehicles from the 60’s and 70’s that had less powerful engines and less efficient AC systems?

  • avatar
    Bruce from DC

    In my observation, different a/c systems work differently. For example, in my top line ’08 Honda Pilot, it’s pretty clear that the automatic climate control system cycles the a/c compressor off and on. In fact, you can easily feel the difference in the temperature of the air coming through the outlets. On the other hand, if you switch the system to defrost mode, the circulating fan kicks up to max speed, the compressor comes on, and there appears to be a blend of cooled and heated air coming through the vents.

    OTOH, the bone simple system in my Z3 just has a push button that engages the compressor; and the (manual) temperature control blends heated and cooled air. Only if you run the cabin air fan at the slowest speed and are driving slowly (so that air is not forced into the system from outside at a high rate) in mild temperatures, does the compressor cycle. It appears that there is some sort of low temperature limit on the evaporator coil that switches off the compressor . . . perhaps to avoid icing the coil up.

    On recent trip from DC to east Tennessee in the summer with two passengers and some stuff, the Pilot almost hit 23 mpg on a tank of fuel, cruising at around 70.

    Finally, its not clear to me that running an a/c compressor slowly necessarily saves all that much energy. I would imagine that one problem with engine-driven compressors is that they have to be designed to run at a variety of speeds. By contrast, an electrically-driven compressor can be optimized for operation at a single speed and cycled on and off as needed. I dunno; maybe there’s an a/c expert out there who can correct me.

  • avatar


    I used to avoid cars with air conditioning because of the perceived MPG loss… was wearing shorts and stripped to the waist with all 4 windows rolled down and driving 70 down I-20.

    But, driving on the highway, with the windows down, creates drag that exacts a higher MPG penalty than driving with the AC on.

  • avatar

    The Camry temperature knob just mixes in warm air from the heater, so no energy is saved. Cycling the A/C button manually is the best move. Compressors cycle anyway, so there is little extra stress.

    I use recycle and the lowest fan setting on both Toyotas, but do sometimes cycle the button every 5-10 minutes if it gets too cold for comfort. The defrost setting overrides the button and runs the compressor as if the A/C button is pushed in. The old Fords allowed me to control this myself.

    Our BMW had an automatic climate control that did change the compressor cycling and not just mix in heat.

  • avatar

    maybe there’s an a/c expert out there who can correct me.

    Having worked for a time in Georgia and Texas, I’m surprised Willis Carrier Day isn’t a major holiday down south.*

    * He was born November 26, 1876 and died October 7, 1950 so those dates wouldn’t work. But he submitted the first plans for a modern AC system on July 17, 1902. So, if I were a were a southerner, I would insist on July, 17th being declared an official state holiday.

  • avatar

    JMO beat me to it, he is 100% correct, at highway Speeds (70+ MPH) rolling down one window, let alone … all 4 of them, will KILL any MPG savings from not using the A/C, and then some!

    Killing the A/C will produce any benefits only at low speeds, less than 50 MPH or even less.

    If you are looking around for ways to increase your MPG, AND your safety as well, the No. ONE thing to do is to not only make sure your tires are PROPERLY INFLATED, but actually, whenever you check the tire pressure, put an EXTRA 1-2 PSI anyway, BECAUSE for the vast majority of drivers your tire pressure is BELOW the recommended one most of the time, EVEN if you set it right Once a month (it loses air from there until your next check). And it is NOT safe to run around with way underinflated tires, look at the Exlorer/Firestone debacle.

  • avatar

    David’s right; turning up the temperature merely allows air to blow over both the evaporator and the heater core.

    Cycling the compressor on/off won’t damage it; there is indeed a pressure switch in the system that cycles the compressor off when the evaporator gets too cold as icing the evaporator for long periods of time could cause the compressor to hydro-lock.

    The way to run A/C and still maximize fuel economy is to keep the evaporator as cold as possible. This means using recirculate to cool the already cooler inside air, using low blower speeds, and turning off the compressor when it gets too cold.

    Turning up the blend temp has the effect of the A/C cooling to its maximum capability (too cold for comfort in all but sunny hot days) while warming the car with the heater. You’re wasting gas running the compressor more than you have to.

  • avatar
    William C Montgomery

    I proposed a similar, albeit longer winded, question to those in the know on a Jeep forum. Here’s some of what I learned:

    1) On most cars when you turn the temperature dial or lever on your dashboard does not affect how often the a/c compressor cycles. As noted above, it simply runs some of the cooled air across the heater core to warm it to taste.
    2) Car A/C systems turn themselves off when they get too cold. This means that the A/Cs cycles off more when the outside temperature is cooler, thereby improving your gas mileage as opposed to running the A/C on a hotter day.
    3) Recirculating the air in the cabin when running the A/C causes automobile A/C systems to cycle off more often to prevent it from icing over.

    For the full bloody (and sometimes technical) discussion:

  • avatar

    Chiming in with everybody else, adjusting the temperature knob does nothing to vary the output of the air conditioning system other than blend in some heated air that passes through the heater core. You are essentially doing the same thing when you turn on your defroster, although the blending is pre-set. However, adjusting the temperature for “not so cold” could adversely affect your fuel economy -especially if you are recycling interior air for the AC, because the air temp sensors in the system could keep the compressor engaged continuously as they never sense that the interior air has become sufficiently cooled.

    There will be no harm to the AC clutch from turning the system on and off manually -the AC system cycles it on and off automatically all the time anyhow, just listen to it when you’re idling at a stoplight- tho’ you may eventually wear out your on-off switch prematurely. I also believe that just about all cars are wired so that a wide open throttle disengages the AC clutch until you let up on the gas.

  • avatar

    Maybe putting this into an economic perspective would help. Your AC uses about 0.5L or .1312 gallons of gas per hour, which, at 2.50 per gallon is a whopping 32.8 cents per hour. If you cycle the AC 50% on and 50% of you actually may save 16c over your hour long trip by fiddling with your AC button.

    Alternatively you can just not worry about it, enjoy the cool air and focus on driving.

  • avatar
    Funk Forty Nine

    You mean all those woman in the office I used to work in who turned the thermostat down to 56 so the AC would blow colder were wrong?

  • avatar

    @David Holzman: Could your friend also look into some underdrive replacement pulleys, if the current max output is more than he’d use?

  • avatar

    I’ve got a scangauge in my wrx, and at a stop with no a/c the engine uses 0.3 gallons of fuel per hour, but with the a/c on it reports 0.45 gph.

    Now driving down the freeway at a constant speed, a/c on does use more fuel than a/c off, but the amount is very small. At 55mph I’ll go from something like 33mpg with it off to 32mpg with it on.

    My advice, turn it on after you get up to 30mph, or after you get done accelerating and are up to speed, and always turn it off at any stops.

  • avatar

    Martin Albright

    You should have used approved Texas procedures. To wit, stick a cold one in your crotch to achieve localized core cooling. Bonus – just take a swig when you’re feeling dehydrated.

    least ways, that’s how it was told to me….

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