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Discussion Starter #1
I would like to pull outside air through the vents on pleasant days (SF has many 75 degree days). But I'm getting warm air, not hot, just not outside temp.

I hate to run the AC when its 70 degrees outside.
 

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Unfortunately other than setting the temp down to 60, it will still pull in warm air. The air comes in from outside through vents at the base of the windshield and passes behind the dashboard before coming out the vents. The path it takes means it will never be as cold as outside air. A 70 degree day in SF! When I lived there I remember 65 was considered temperature to turn on the AC!
 

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Like Eric says, turn the temp to the minimum setting to get air as close to outside temp as possible, but I find on really sunny days the hood heats up the air too much and it's actually much nicer to turn off the vent and open the windows... if I'm driving slow of course.
 

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Tyler_Canada said:
...to turn off the vent and open the windows... if I'm driving slow of course.
I remember reading, I think on this site, about a comparision of driving with windows down.... botton line driving was that driving with the windows down didn't effect gas milleage.
 

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Gonzo said:


I remember reading, I think on this site, about a comparision of driving with windows down.... botton line driving was that driving with the windows down didn't effect gas milleage.
It depends how fast you are going. Bottom line, going slow, open windows. Going fast, use AC.
 

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Well I think that is just it... even going 75MPH it didn't have much effect... these are cars not jet airplanes. You raise the gear on a small single engine at 150 MPH you only gain about 1-2 MPH.....
 

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Gonzo said:
Well I think that is just it... even going 75MPH it didn't have much effect... these are cars not jet airplanes. You raise the gear on a small single engine at 150 MPH you only gain about 1-2 MPH.....
Since using the AC has a "fixed cost" in terms of load on the engine (the compressor is either on or its off, no in between) there is probably a speed where its more efficient to use the AC than keep the windows open. Mythbusters on Discovery channel revisited the segment they did on this "myth" and concluded in an informal study that the speed where this crossover occurs is around 45mph. Not completely accurate, but good enough for me.

I like it quiet when I drive so I can hear my music very clearly, so I tend to use the AC all the time anyways. Also prevents annoying bugs from flying into the car and buzzing around.
 

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Eric L. said:


Since using the AC has a "fixed cost" in terms of load on the engine (the compressor is either on or its off, no in between) there is probably a speed where its more efficient to use the AC than keep the windows open. Mythbusters on Discovery channel revisited the segment they did on this "myth" and concluded in an informal study that the speed where this crossover occurs is around 45mph. Not completely accurate, but good enough for me.

I like it quiet when I drive so I can hear my music very clearly, so I tend to use the AC all the time anyways. Also prevents annoying bugs from flying into the car and buzzing around.
Yeah, Mythbusters did it very badly the first time. Quite simply, A/C uses a fixed amount of horsepower, while the drag from having the windows down requires an exponentially increasing amount of horsepower as speed increases. At some speed the power required to overcome the wind resistance will be more than the power required for the A/C, and it's probably between 45mph and 60mph.

I believe the first Mythbusters only tested 45mph and 55mph the first time, and the results were therefore inconclusive.
 

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Here is a simple and not very accurate test... but what the heck.....

Opps never mind... the MO doesn't have instant/current fuel economy readings.


I **WAS** going to suggest drive at a constant RPM/Constant Speed/Constant pedal position on a flat road and take a reading at 45, 55, 65, 75 MPH.... with windows up and repeat with windows down. Compare gas readings.

Would this have worked for a simple experiment?
 

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Gonzo said:
Here is a simple and not very accurate test... but what the heck.....

Opps never mind... the MO doesn't have instant/current fuel economy readings.


I **WAS** going to suggest drive at a constant RPM/Constant Speed/Constant pedal position on a flat road and take a reading at 45, 55, 65, 75 MPH.... with windows up and repeat with windows down. Compare gas readings.

Would this have worked for a simple experiment?
Maybe if you drove the exact same stretch of road, at the same temperatures, using the cruise control. That would indeed be a crude but demonstrative test. You can google for hours and hours on this topic though, and read about it.
 

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MPG Loss Due to AC

A slightly different but related topic.
It seems that the my gas mileage averaged for a tank of gas does not change noticeably whether I use AC or not. I have not used AC for the last two weeks because of the cooler weather in NE but the gas mileage for the last tank was the same as when I was using AC, about 21 MPG. Could this be because the cooler temperature reduces the efficiency of the engine?
 

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Discussion Starter #12
What have I started?

I just wondered if anyone else gets hot air. I want my wife to stop complaining everytime she "JUST WANTS A LITTLE AIR".

I will have the dealer check if the plimun (spelling?) is moving fully from side to side.

Like these forums often go, it turns into a physics lesson about the aerodynamics of planes and cars, not to mention someone even had the nerve to question the accruacy of the Myth-Busters. (My kids favorite show).

This sounds like the thread about mileage .... tailgates up or down.... Over at the Ford Truck Web Site.

I have my first service soon, I'll have them check it.

Ken D.
 

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Not much to check that is just how it is.... the dash is getting warm from baking in the sun. I find a sun shield helped out a bit. Try parking in the shade that makes a huge difference too.
 

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Re: What have I started?

kdesvern said:
Like these forums often go, it turns into a physics lesson about the aerodynamics of planes and cars, not to mention someone even had the nerve to question the accruacy of the Myth-Busters. (My kids favorite show).
They did a poorly designed test, and the result was inconclusive. The fact that they revisited the topic shows that they eventually figured that out.
 

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I've been having the warm air issue in my '06 SL.

I think I may have solved it by making sure the climate control is turned off. I then pressed the "outside air in" button until it turned on and stated blowing. The A/C did not turn on and the air was considerably cooler than before. The air tem was app. 75.
 
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