Enquiring Minds need to know: Mythbusters and 100 Series aero-dynamics

Submitted: Monday, Feb 13, 2006 at 21:13
ThreadID: 30750 Views:2929 Replies:13 FollowUps:17
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[another TV program driven question from enquiring minds!]

Background:

Dad drives the 100 Series normally aspirated manual diesel at about 90 k's.

When The Chancellor of the Exchequer is in the passenger seat Dad likes to show how frugal he can be (with the diesel that is).

So he turns off the AC and winds down the windows and tells the kids to send their complaints to the bureaucrats at the Chancellory - of course, says he, it is more efficient to drive with the AC off.

Then, kids watch Jamie & crew of Mythbusters fame on SBS, and tell dad he doesn't know what he is talking about, and send a stinging letter to the Chancellory. Windows up and AC on is more efficient, they say.

Surprise, surprise, Mythbusters revisit the question with some more real-world testing (yeah sure), and to everyone's surprise, prove that this holds only (for their test vehicle) at speeds above 50 mph.

The situation:

Now - Dad's cruiser has an added burden and higher coefficient of drag than your everyday San Franscisco runabout rental car that Mythbusters use. Especially with the Sahara bar on the front, the tyres on the back, the roof rack and everyone's luggage on the roof, the whip antennae, etc.

It also has different power/weight ratio, power curve and air-con compressor than the Mythbuster's test vehicles. On the first, I'd guess that my gutless cruiser has a considerably lower power/weight ratio.

The critical question:

So, theoretically speaking, will these factors (and any others you can think of) serve to increase or decrease the critical speed at which is becomes more efficient to wind the windows up, and put the AC on?

Enquiring minds need to know ;-)

Andrew, who can't wait to see the contributions from this esteemed mythbusting crowd
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Reply By: Vivid Adventures - Monday, Feb 13, 2006 at 21:15

Monday, Feb 13, 2006 at 21:15
reference: Mythbusters Revisited II.
AnswerID: 154777

Reply By: BenSpoon - Monday, Feb 13, 2006 at 21:34

Monday, Feb 13, 2006 at 21:34
RAC reported on an investigation carried out a while back in their members magazine- at 80km/h, having a single window open is like having a mini parachute on your car and the official word is yes, it is more efficient to have your air con running at these speeds.
I have noticed I get 50km less to a main tank having air con on, whereas I get 75km less with a window open. It equals a poofteenth of a litre difference every kilometer, but I hit the air con now when on country roads.
AnswerID: 154790

Reply By: glenno(qld) - Monday, Feb 13, 2006 at 21:49

Monday, Feb 13, 2006 at 21:49
My ford falcon air con was turned on to 22 degrees in 1998 and i am yet to turn it off . I turn my house bedroom aircon on in november and it runs 7 days a week / 24 hrs a day and gets turned off around april . I dont drink , smoke , or hang around randy women so i can afford this little luxury .
AnswerID: 154796

Follow Up By: Member - Willie , Epping .Syd. - Tuesday, Feb 14, 2006 at 09:37

Tuesday, Feb 14, 2006 at 09:37
Hi Glenno ,
Are randy women more expensive than non randy ones ?
Willie .
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Follow Up By: pathfinder - Tuesday, Feb 14, 2006 at 13:44

Tuesday, Feb 14, 2006 at 13:44
onya glenno - doing wonders for the greenhouse effect...
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FollowupID: 408881

Follow Up By: glenno(qld) - Tuesday, Feb 14, 2006 at 15:38

Tuesday, Feb 14, 2006 at 15:38
but i have installed solar hot water and am installing water tanks so not all bad pathfinder
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FollowupID: 408900

Follow Up By: Bonz (Vic) - Tuesday, Feb 14, 2006 at 20:17

Tuesday, Feb 14, 2006 at 20:17
randy women are worth the extra cost
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Reply By: Member - Davoe (Widgiemooltha) - Monday, Feb 13, 2006 at 21:55

Monday, Feb 13, 2006 at 21:55
Rarely drive with the window down especially off road. My current vehicle is my first in ages with aircon and I operate it like a work vehicle - Aircon on all the time even in winter, just adjust the red/blue stick. It is fantastic in winter for dehumidifying the vehicle and stoping the windscreen fogging up. The day i worry if it uses more fuel or not will be the day i know I need a job that pays more
AnswerID: 154801

Reply By: Trekkie (Member - WA) - Monday, Feb 13, 2006 at 22:46

Monday, Feb 13, 2006 at 22:46
Not that you need an excuse to stop for a VB, but imagine how many exta VBs you need to have just because you are all hot and sticky. On the other hand, if you spent $5,000 a year on fuel and COULD save 5% by turning air con on and off, you might save $250 -------------- so what
AnswerID: 154813

Follow Up By: Vivid Adventures - Monday, Feb 13, 2006 at 22:55

Monday, Feb 13, 2006 at 22:55
you guys are no fun!

The question wasn't whether I should care ...

Perhaps I'll have to wait for Collyn's scientific analysis.

Fuel bill is more like twice that for what it's worth, and I had made a preliminary guess from experience (had to guess how much I had it on/off) that the air con was about 10% and 90 ks vs 110 ks another 10% (again FWIW).

Ciao for now
Andrew
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Follow Up By: Trekkie (Member - WA) - Monday, Feb 13, 2006 at 23:15

Monday, Feb 13, 2006 at 23:15
Hey Andrew if you fit a Fitch and Hiclone you will negate the fuel used by the Air Con
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Follow Up By: Vivid Adventures - Tuesday, Feb 14, 2006 at 00:31

Tuesday, Feb 14, 2006 at 00:31
yeah.

Been thinking that.

I'm making my own Fitch thingy at the moment, and don't know if I put the Hiclone on the Fitch or the Hitch on the Finclone.

Just trying to work out how to get the molecules lined up properly in my nano-technology machine.

Let me know the secret please.
Andrew.
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Follow Up By: Member - Willie , Epping .Syd. - Tuesday, Feb 14, 2006 at 09:39

Tuesday, Feb 14, 2006 at 09:39
Andrew ,
I think you have to factor in the medical costs for choked synuses after running the air-con constantly .
Willie .
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Follow Up By: Gerhardp1 - Tuesday, Feb 14, 2006 at 10:00

Tuesday, Feb 14, 2006 at 10:00
The extra load on the serpentine belt with the aircon on all the time will require that the belt be replaced more often, thereby negating the saving in fuel costs.
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Follow Up By: Bonz (Vic) - Tuesday, Feb 14, 2006 at 20:19

Tuesday, Feb 14, 2006 at 20:19
Two Hiclones on the trysty 3,0l Nissan and I can afford many things
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Reply By: warthog - Monday, Feb 13, 2006 at 23:22

Monday, Feb 13, 2006 at 23:22
In our patrol when on the dirt we always drive with the aircon on and the air set to fresh (not recirculate) in order to pressurise the cabin slightly and stop dirt getting sucked in the back doors. To drive with a front window down means a cabin full of dust as it gets sucked through the back doors, must create a bit of negative pressure in the cab.
AnswerID: 154818

Follow Up By: Vivid Adventures - Tuesday, Feb 14, 2006 at 00:35

Tuesday, Feb 14, 2006 at 00:35
Thanks warthog - everyone is very concerned about the dust in my car - thanks for that!

Of course off-road there is no question:

* AC on fresh air
unless
* driving in a dust storm, behind a road train or a patrol when I put it on recirc.

Ciao for now
Andrew
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Reply By: Lone Wolf - Tuesday, Feb 14, 2006 at 07:40

Tuesday, Feb 14, 2006 at 07:40
Just fart, with the windows up, and the AC in recirc mode. They will then toss the Jamie & Adam theory out of the quickly opening windows, and leave you in peace, forever after...
AnswerID: 154840

Reply By: Shaker - Tuesday, Feb 14, 2006 at 09:29

Tuesday, Feb 14, 2006 at 09:29
Ford Motor Company did extensive testing in the 1970s & found beyond doubt, that at highway speeds it is more efficient to have the air conditioning operating with the windows up, than to disturb the aerodynamics by winding the windows down.
I think that with the much greater aerodynamic efficiency that we have now, the findings by Ford would be even more relevant.
AnswerID: 154855

Follow Up By: Michael ( Moss Vale NSW) - Tuesday, Feb 14, 2006 at 11:26

Tuesday, Feb 14, 2006 at 11:26
The 1970 model ford was a cut down 20' shipping container, what aerodynamics??
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Follow Up By: Shaker - Tuesday, Feb 14, 2006 at 12:34

Tuesday, Feb 14, 2006 at 12:34
I don't recall saying it was 1970 model!
The test was done in the late 70s, early 80's.
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Reply By: Flash - Tuesday, Feb 14, 2006 at 12:32

Tuesday, Feb 14, 2006 at 12:32
This has to be a comedy, surely.
For the 0.00001 % difference in fuel economy at moderate speeds, surely passenger comfort is the desired result at the end of the day....
You don't buy a car worth 60 or 80 grand and then try to save 25 cents by sweating it out and breathing exhaust fumes, rather than riding in comfort with the A/C and NOT breathing in the diesel soot from the vehicle in front. Whether or not you catch the next green or red light will make more difference to your fuel consumption.
Now, if you REALLY want to save fuel, turn the A/C on when decelerating and off when accelerating- that would make a difference- and just accelerate a bit slower.
Please tell me this whole thread is a joke, please.
Cheers
AnswerID: 154888

Follow Up By: Vivid Adventures - Tuesday, Feb 14, 2006 at 12:53

Tuesday, Feb 14, 2006 at 12:53
I never said I REALLY wanted to save fuel.

You're off topic - answer the questions!
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Follow Up By: Bonz (Vic) - Tuesday, Feb 14, 2006 at 22:17

Tuesday, Feb 14, 2006 at 22:17
Its fair dinkum Flash, its about solving once and for all the age old questions, do you get wttere running in the rain or walking? Ans Walking etc etc etc
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Reply By: Longreach - Tuesday, Feb 14, 2006 at 15:21

Tuesday, Feb 14, 2006 at 15:21
I always thought the bigger the car (eg. 100 series) the less siginificant the effect of having the A/C running. With "the Sahara bar on the front, the tyres on the back, the roof rack and everyone's luggage on the roof, the whip antennae, etc." I'd have thought turning the A/C on would become an even smaller percentage of the already very big consumption (yes I too have a LC100).
Having said that, I think you should get a video camera, a couple of guys with little hair and large moustaches and do some serious testing. Drive the 100 series empty (yes you'll have to remove the Sahara bar for a serious test) for 1000km and A/C off. Then do it with A/C on. Then repeat it with all the gear. You will also need to blow something up. it would not be the same without the obligatory explosion. (Perhaps you should use a Range Rover !) Finally you should post the video on this forum (and perhaps sell it to SBS).
Looking forward to the results...
AnswerID: 154915

Follow Up By: Vivid Adventures - Tuesday, Feb 14, 2006 at 17:02

Tuesday, Feb 14, 2006 at 17:02
Wonderful thinking Longreach.

Ciao for now
Andrew going to charge up the batteries on the handycam.
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Follow Up By: Longreach - Tuesday, Feb 14, 2006 at 17:06

Tuesday, Feb 14, 2006 at 17:06
Don't forget to wear a beret during filming.

Cheers
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Reply By: Pterosaur - Tuesday, Feb 14, 2006 at 18:48

Tuesday, Feb 14, 2006 at 18:48
G'day Andrew,

from my last trip in the troopy (aerodynamics very similar to those of a brick)

fuel consumption with A/C 15.4l/100km (6.5k/litre) travelling < 100kph

fuel consumption without A/C 15.2l/100km (6.6 k/litre) travelling < 110kph

both datasets over approx. 4500km each

Fuel consumption without A/C 14.2l/100km (7.1 k/litre) travelling < 100kph

over 1350km.

Give the figures to the enquiring minds for further analysis (allow assumption that for 60% of time travelling without A/C, windows were down) - should stop them from dreaming up further demands for a while ! :D
AnswerID: 154956

Reply By: atoyot - Tuesday, Feb 14, 2006 at 21:25

Tuesday, Feb 14, 2006 at 21:25
Why do the V8 Supercars all run with the drivers side window down? I know they need fresh air and ventilation, but these guys are pulling up to 300k with the window down? Surely the loss of economy is from the extra drag of the window down, but maybe it makes little difference with the HP these guys have. Perhaps someone should tell them to have a window and turn the aircon on...

Andrew
AnswerID: 154995

Reply By: Member - Bware - Tuesday, Feb 14, 2006 at 21:44

Tuesday, Feb 14, 2006 at 21:44
I like fresh air ;)
AnswerID: 155001

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