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  #1  
Old 09-26-2002, 07:25 PM
Red SVX 92 Red SVX 92 is offline
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Best highway option: A/C, or rolling down windows?

I was just reading an engineering textbook, and a sidebar text section mentions tips for minimizing gas consumption. I'd quote the exact statement, but I'm lazy right now, so I'll paraphrase. Basically, the book said that at highway speeds, turning on the air conditioner to achieve a cooling effect was more fuel-efficient than rolling down the windows and sunroof, and noted that this was the case especially in newer, more aerodynamic vehicles. Now, I know that this is just a generalization, and may not apply to the SVX, but I'm curious: would anyone have any substantiated evidence or knowledge which proves or disproves this statement? I usually drive on the highway with the A/C off and the windows and sunroof down, because I believed that I was saving gas. I like the feel of the wind, but I'd rather have A/C, a quiet passenger cabin, and no funky cow manure smells if it saves gas.
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Old 09-26-2002, 08:02 PM
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<<and no funky cow manure smells if it saves gas.>>

Maybee stay away from the buffet bar at lunch

I usually roll up the windows and turn on the AC on the freeway. I don't think it will amount to more than a few dimes each tankfull that you'd save if at all.

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  #3  
Old 09-26-2002, 08:38 PM
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<<and no funky cow manure smells if it saves gas.>>

Try living in Amish country......it's a daily occurence here...this time of year it's everywhere...the liquified stuff....stinks to high heaven....spraying it on their feilds....stench would gag a maggot.......
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Old 09-26-2002, 08:53 PM
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Another consideration is the compressor being of variable displacement. As more cooling is needed it increases its effective displacement and vice-versa. So once the interior is cooled down the load is decreased considerably.

The one detrimental item I can recall from my first SVX was the effect of having a bra on the car - it was good for a 2mpg hit on a road trip.
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Old 09-26-2002, 11:43 PM
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By golly that's it!! I have about that much difference between my two cars when it comes to mpg on a road trip. I have a bra on one and not on the other. It is about 2 mpg difference. Didn't even think about that. Thanks
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  #6  
Old 09-27-2002, 05:35 AM
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Also if you run "economy" or "vent" with the windows closed it will both save gas and aerodynamics

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  #7  
Old 09-27-2002, 05:36 AM
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Re: Best highway option: A/C, or rolling down windows?

Quote:
Originally posted by Red SVX 92
I was just reading an engineering textbook, , turning on the air conditioner to achieve a cooling effect was more fuel-efficient than rolling down the windows and sunroof,
I have not tried it both ways cause of the noise. I always have the windows up unless the traffic is slow and it's a beautiful day. The buffeting air noise in the SVX is very strong. I can't stand the beating.
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Old 09-27-2002, 10:58 AM
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Some anecdotal evidence

Years back my techno gaget loving uncle bought a Merkur and was told that the car gets better mileage with the windows UP and the AC on rather than the other.

He drives a Legacy GT sedan now! Purty car!

On a side note. I discovered the natural resonant frequency of my cabin with the windows shut but sunroof open occurs at about 27mph. The beating can be quite disturbing. Something like 3-5Hz I'd say. If I changed speed it would go away.

Just like a VERY low frequency whistle, as air passes over the opening it setup a resonance in the 'chamber'.

It makes your chest pound and your hair (what little there is left) bounce.

Besides who can STAND driving THAT slow for too long!
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Old 09-27-2002, 12:01 PM
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It would make sense that, with windows down, at highway speeds, the drag on the car would be increased. Windows up would increase the efficiency of the .029 coefiicient of drag, I would think. That, and the variable displacement, pointed out by Beav, could possibly equate to better fuel efficiency, but, no, I have not substantiated evidence.

My father used to make us swelter in the summer because he didn't want to waste gas running the air conditioner. Of course, that was in the good ol' days, when "heavy metal" was used to refer to cars instead of music, plus, aerodynamics took a rumble seat to styling.
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  #10  
Old 09-27-2002, 12:02 PM
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Back in the early 70's, Car & Driver did some testing on this. There was a definite negative impact on gas mileage when driving with the windows open vs leaving them closed. I don't remember if they tried it with AC on and off.

They also tested a bunch of other mods, such as front air dams and reducing the opening in the grill, and found some of them really worked.

Their test vehicle was a Pinto. Today's more aerodynamic cars might not get such favorable results. It would be an interesting project for them to redo.

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  #11  
Old 09-27-2002, 06:09 PM
Red SVX 92 Red SVX 92 is offline
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Re: Re: Best highway option: A/C, or rolling down windows?

Quote:
Originally posted by svx_commuter


I have not tried it both ways cause of the noise. I always have the windows up unless the traffic is slow and it's a beautiful day. The buffeting air noise in the SVX is very strong. I can't stand the beating.
Personally, I love having the windows down on the SVX. The wind and noise is a fraction of what it is in the other cars I've driven. In my previous car, I constantly had problems of stuff flying out of the rear windows, while I never worry about that in the SVX. If I open the sunroof, it does get a lil loud and harsh, but still not overly bad. That, and I don't like the laggy feeling I get when accelerating with the A/C on.
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Old 09-27-2002, 06:27 PM
Red SVX 92 Red SVX 92 is offline
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Quote:
Originally posted by Aredubjay
It would make sense that, with windows down, at highway speeds, the drag on the car would be increased. Windows up would increase the efficiency of the .029 coefiicient of drag, I would think. That, and the variable displacement, pointed out by Beav, could possibly equate to better fuel efficiency, but, no, I have not substantiated evidence.

My father used to make us swelter in the summer because he didn't want to waste gas running the air conditioner. Of course, that was in the good ol' days, when "heavy metal" was used to refer to cars instead of music, plus, aerodynamics took a rumble seat to styling.
Don't worry Aredub, they still do that in modern times. My parents put me through the same exact thng, and I can't even legally drink yet. Grrr... just a few more months... *gulp*

Another thing I just remembered: internal combustion engines generally get higher thermal efficiencies at full throttle, as opposed to partial throttle, because of turbulence in the um... air throttle mechanism(?). Not sure if that only applies to carbureted engines, or to FI engines as well, but if it does, would it seem logical that the car would be running more efficiently by turning on the A/C (I don't mean using less gas by turning on the A/C, but that adding the A/C load to the engine would increase efficiency and thus require less of an increase in gas added to the engine)?

Wow, looking back at what I just said, it seems very convoluted, but I can't think of any other way to explain without diagrams and a 40-minute discussion on the topic. Any takers?
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  #13  
Old 09-27-2002, 08:12 PM
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Don't bother - I know what you're trying to say and you don't want to be known as the person that said it.
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